Oort Clouds Around Other Stars Should be Visible in the Cosmic Microwave Background

The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA

For decades, scientists have theorized that beyond the edge of the Solar System, at a distance of up to 50,000 AU (0.79 ly) from the Sun, there lies a massive cloud of icy planetesimals known as the Oort Cloud. Named in honor of Dutch astronomer Jan Oort, this cloud is believed to be where long-term comets originate from. However, to date, no direct evidence has been provided to confirm the Oort Cloud’s existence.

This is due to the fact that the Oort Cloud is very difficult to observe, being rather far from the Sun and dispersed over a very large region of space. However, in a recent study, a team of astrophysicists from the University of Pennsylvania proposed a radical idea. Using maps of the Cosmic Microwave Background (CMB) created by the Planck mission and other telescopes, they believe that Oort Clouds around other stars can be detected.

The study – “Probing Oort clouds around Milky Way stars with CMB surveys“, which recently appeared online – was led by Eric J Baxter, a postdoctoral researcher from the Department of Physics and Astronomy at the University of Pennsylvania. He was joined by Pennsylvania professors Cullen H. Blake and Bhuvnesh Jain (Baxter’s primary mentor).

To recap, the Oort Cloud is a hypothetical region of space that is thought to extend from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun – though some estimates indicate it could reach as far as 100,000 to 200,000 AU (1.58 and 3.16 ly). Like the Kuiper Belt and the Scattered Disc, the Oort Cloud is a reservoir of trans-Neptunian objects, though it is over a thousands times more distant from our Sun as these other two.

This cloud is believed to have originated from a population of small, icy bodies within 50 AU of the Sun that were present when the Solar System was still young. Over time, it is theorized that orbital perturbations caused by the giant planets caused those objects that had highly-stable orbits to form the Kuiper Belt along the ecliptic plane, while those that had more eccentric and distant orbits formed the Oort Cloud.

According to Baxter and his colleagues, because the existence of the Oort Cloud played an important role in the formation of the Solar System, it is therefore logical to assume that other star systems have their own Oort Clouds – which they refer to as exo-Oort Clouds (EXOCs). As Dr. Baxter explained to Universe Today via email:

“One of the proposed mechanisms for the formation of the Oort cloud around our sun is that some of the objects in the protoplanetary disk of our solar system were ejected into very large, elliptical orbits by interactions with the giant planets.  The orbits of these objects were then affected by nearby stars and galactic tides, causing them to depart from orbits restricted to the plane of the solar system, and to form the now-spherical Oort cloud.  You could imagine that a similar process could occur around another star with giant planets, and we know that there are many stars out there that do have giant planets.”

As Baxter and his colleagues indicated in their study, detecting EXOCs is difficult, largely for the same reasons for why there is no direct evidence for the Solar System’s own Oort Cloud. For one, there is not a lot of material in the cloud, with estimates ranging from a few to twenty times the mass of the Earth. Second, these objects are very far away from our Sun, which means they do not reflect much light or have strong thermal emissions.

For this reason, Baxter and his team recommended using maps of the sky at the millimeter and submillimeter wavelengths to search for signs of Oort Clouds around other stars. Such maps already exist, thanks to missions like the Planck telescope which have mapped the Cosmic Microwave Background (CMB). As Baxter indicated:

“In our paper, we use maps of the sky at 545 GHz and 857 GHz that were generated from observations by the Planck satellite. Planck was pretty much designed *only* to map the CMB; the fact that we can use this telescope to study exo-Oort clouds and potentially processes connected to planet formation is pretty surprising!”

This is a rather revolutionary idea, as the detection of EXOCs was not part of the intended purpose of the Planck mission. By mapping the CMB, which is “relic radiation” left over from the Big Bang, astronomers have sought to learn more about how the Universe has evolved since the the early Universe – circa. 378,000 years after the Big Bang. However, their study does build on previous work led by Alan Stern (the principal investigator of the New Horizons mission).

All-sky data obtained by the ESA’s Planck mission, showing the different wavelenghts. Credit: ESA

In 1991, along with John Stocke (of the University of Colorado, Boulder) and Paul Weissmann (from NASA’s Jet Propulsion Laboratory), Stern conducted a study titled “An IRAS search for extra-solar Oort clouds“. In this study, they suggested using data from the Infrared Astronomical Satellite (IRAS) for the purpose of searching for EXOCs. However, whereas this study focused on certain wavelengths and 17 star systems, Baxter and his team relied on data for tens of thousands of systems and at a wider range of wavelengths.

Other current and future telescopes which Baxter and his team believe could be useful in this respect include the South Pole Telescope, located at the Amundsen–Scott South Pole Station in Antarctica; the Atacama Cosmology Telescope and the Simons Observatory in Chile; the Balloon-borne Large Aperture Submillimeter Telescope (BLAST) in Antarctica; the Green Bank Telescope in West Virgina, and others.

“Furthermore, the Gaia satellite has recently mapped out very accurately the positions and distances of stars in our galaxy,” Baxter added. “This makes choosing targets for exo-Oort cloud searches relatively straightforward. We used a combination of Gaia and Planck data in our analysis.”

To test their theory, Baxter and is team constructed a series of models for the thermal emission of exo-Oort clouds. “These models suggested that detecting exo-Oort clouds around nearby stars (or at least putting limits on their properties) was feasible given existing telescopes and observations,” he said. “In particular, the models suggested that data from the Planck satellite could potentially come close to detecting an exo-Oort cloud like our own around a nearby star.”

The relative sizes of the inner Solar System, Kuiper Belt and the Oort Cloud. (Credit: NASA, William Crochot)

In addition, Baxter and his team also detected a hint of a signal around some of the stars that they considered in their study – specifically in the Vega and Formalhaut systems. Using this data, they were able to place constraints on the possible existence of EXOCs at a distance of 10,000 to 100,000 AUs from these stars, which roughly coincides with the distance between our Sun and the Oort Cloud.

However, additional surveys will be needed before the existence any of EXOCs can be confirmed. These surveys will likely involve the James Webb Space Telescope, which is scheduled to launch in 2021. In the meantime, this study has some rather significant implications for astronomers, and not just because it involves the use of existing CMB maps for extra-solar studies. As Baxter put it:

“Just detecting an exo-Oort cloud would be really interesting, since as I mentioned above, we don’t have any direct evidence for the existence of our own Oort cloud. If you did get a detection of an exo-Oort cloud, it could in principle provide insights into processes connected to planet formation and the evolution of protoplanetary disks. For instance, imagine that we only detected exo-Oort clouds around stars that have giant planets. That would provide pretty convincing evidence that the formation of an Oort cloud is connected to giant planets, as suggested by popular theories of the formation of our own Oort cloud.”
As our knowledge of the Universe expands, scientists become increasingly interested in what our Solar System has in common with other star systems. This, in turn, helps us to learn more about the formation and evolution of our own system. It also provides possible hints as to how the Universe changed over time, and maybe even where life could be found someday.

Further Reading: arXiv

Now We Know When Stars Will Be Passing Through the Oort Cloud

To our Solar System, “close-encounters” with other stars happen regularly – the last occurring some 70,000 years ago and the next likely to take place 240,000 to 470,000 years from now. While this might sound like a “few and far between” kind of thing, it is quite regular in cosmological terms. Understanding when these encounters will happen is also important since they are known to cause disturbances in the Oort Cloud, sending comets towards Earth.

Thanks to a new study by Coryn Bailer-Jones, a researcher from the Max Planck Institute for Astronomy, astronomers now have refined estimates on when the next close-encounters will be happening. After consulting data from the ESA’s Gaia spacecraft, he concluded that over the course of the next 5 million years, that the Solar System can expect 16 close encounters, and one particularly close one!

For the sake of the study – which recently appeared in the journal Astronomy & Astrophysics under the title The Completeness-Corrected Rate of Stellar Encounters with the Sun From the First Gaia Data Release” – Dr. Bailer Jones used Gaia data to track the movements of more than 300,000 stars in our galaxy to see if they would ever pass close enough to the Solar System to cause a disturbance.

Artist’s impression of the ESA’s Gaia spacecraft. Credit: ESA/ATG medialab; background: ESO/S. Brunier

As noted, these types of disturbances have happened many times throughout the history of the Solar System. In order to dislodge icy objects from their orbit in the Oort Cloud – which extends out to about 15 trillion km (100,000 AU) from our Sun – and send them hurling into the inner Solar System, it is estimated that a star would need to pass within 60 trillion km (37 trillion mi; 400,000 AU) of our Sun.

While these close encounters pose no real risk to our Solar System, they have been known to increase comet activity. As Dr. Bailer-Jones explained to Universe Today via email:

“Their potential influence is to shake up the Oort cloud of comets surrounding our Sun, which could result in some being pushed into the inner solar system where is chance they could impact with the Earth. But the long-term probability of one such comet hitting the Earth is probably lower than the probability the Earth is hit by a near-Earth asteroid. So they don’t pose much more danger.”

One of the goals of the Gaia mission, which launched back in 2013, was to collect precise data on stellar positions and motions over the course of its five-year mission. After 14 months in space, the first catalogue was released, which contained information on more than a billion stars. This catalogue also contained the distances and motions across the sky of over two million stars.

By combining this new data with existing information, Dr. Bailer-Jones was able to calculate the motions of some 300,000 stars relative to the Sun over a five million year period. As he explained:

“I traced the orbits of stars observed by Gaia (in the so-called TGAS catalogue) backwards and forwards in time, to see when and how close they would come to the Sun. I then computed the so-called ‘completeness function’ of TGAS to find out what fraction of encounters would have been missed by the survey: TGAS doesn’t see fainter stars (and the very brightest stars are also omitted at present, for technical reasons), but using a simple model of the Galaxy I can estimate how many stars it is missing. Combining this with the actual number of encounters found, I could estimate the total rate of stellar encounters (i.e. including the ones not actually seen). This is necessarily a rather rough estimate, as it involves a number of assumptions, not least the model for what is not seen.”

From this, he was able to come up with a general estimate of the rate of stellar encounters over the past 5 million years, and for the next 5 million. He determined that the overall rate is about 550 stars per million years coming within 150 trillion km, and about 20 coming closer than 30 trillion km. This works out to about one potential close encounter every 50,000 years or so.

Dr. Bailor-Jones also determined that of the 300,000 stars he observed, 97 of them would pass within 150 trillion km (93 trillion mi; 1 million AU) of our Solar System, while 16 would come within 60 trillion km. While this would be close enough to disturb the Oort Cloud, only one star would get particularly close. That star is Gliese 710, a K-type yellow dwarf located about 63 light years from Earth which is about half the size of our Sun.

Stars speeding through the Galaxy. Credit: ESA

According to Dr. Bailer-Jones’ study, this star will pass by our Solar System in 1.3 million years, and at a distance of just 2.3 trillion km (1.4 trillion mi; 16 ,000AU). This will place it well within the Oort Cloud, and will likely turn many icy planetesimals into long-period comets that could head towards Earth. What’s more, Gliese 710 has a relatively slow velocity compared to other stars in our galaxy.

Whereas the average relative velocity of stars is estimated to be around 100.000 km/h (62,000 mph) at their closest approach, Gliese 710 will will have a speed of 50,000 km/h (31,000 mph). As a result, the star will have plenty of time to exert its gravitational influence on the Oort Cloud, which could potentially send many, many comets towards Earth and the inner Solar System.

Over the past few decades, this star has been well-documented by astronomers, and they were already pretty certain that it would experience a close encounter with our Solar System in the future. However, previous calculations indicated that it would pass within 3.1 to 13.6 trillion km (1.9 to 8.45 trillion mi; 20,722 to 90,910 AU) from our star system – and with a 90% certainty. Thanks to this most recent study, these estimates have been refined to 1.5–3.2 trillion km, with 2.3 trillion km being the most likely.

Again, while it might sound like these passes are on too large of a timescale to be of concern, in terms of the astronomical history, its a regular occurrence. And while not every close encounter is guaranteed to send comets hurling our way, understanding when and how these encounters have happened is intrinsic to understanding the history and evolution of our Solar System.

Understanding when a close encounters might happen next is also vital. Assuming we are still around when another  takes place, knowing when it is likely to happen could allow us to prepare for the worst – i.e. if a comets is set on a collision course with Earth! Failing that, humanity could use this information to prepare a scientific mission to study the comets that are sent our way.

The second release of Gaia data is scheduled for next April, and will contain information on an estimated 1 billion stars. That’s 20 times as many stars as the first catalogue, and about 1% the total number of stars within the Milky Way Galaxy. The second catalog will also include information on much more distant stars, will which allow for reconstructions of up to 25 million years into the past and future.

As Dr. Bailer-Jones indicated, the release of Gaia data has helped astronomers considerably. “[I]t greatly improves on what we had before, in both number of stars and precision,” he said. “But this is really just a taster of what will come in the second data release in April 2018, when we will provide parallaxes and proper motions for around one billion stars (500 times as many as in the first data release).”

With every new release, estimates on the movements of the galaxy’s stars (and the potential for close encounters) will be refined further. It will also help us to chart when major comet activity took place within the Solar System, and how this might have played a role in the evolution of the planets and life itself.

Further Reading: ESA

Colonizing the Outer Solar System

Colonizing The Outer Solar System


Okay, so this article is Colonizing the Outer Solar System, and is actually part 2 of our team up with Fraser Cain of Universe Today, who looked at colonizing the inner solar system. You might want jump over there now and watch that part first, if you are coming in from having seen part 1, welcome, it is great having you here.

Without further ado let us get started. There is no official demarcation between the inner and outer solar system but for today we will be beginning the outer solar system at the Asteroid Belt.

Artist concept of the asteroid belt. Credit: NASA
Artist concept of the asteroid belt. Credit: NASA

The Asteroid Belt is always of interest to us for colonization. We have talked about mining them before if you want the details on that but for today I’ll just remind everyone that there are very rich in metals, including precious metals like gold and platinum, and that provides all the motivation we need to colonize them. We have a lot of places to cover so we won’t repeat the details on that today.

You cannot terraform asteroids the way you could Venus or Mars so that you could walk around on them like Earth, but in every respect they have a lot going for them as a candidate. They’ve got plenty for rock and metal for construction, they have lots of the basic organic elements, and they even have some water. They also get a decent amount of sunlight, less than Mars let alone Earth, but still enough for use as a power source and to grow plants.

But they don’t have much gravity, which – pardon the pun – has its ups and downs. There just isn’t much mass in the Belt. The entire thing has only a small fraction of the mass of our moon, and over half of that is in the four biggest asteroids, essentially dwarf planets in their own right. The remainder is scattered over millions of asteroids. Even the biggest, Ceres, is only about 1% of 1% of Earth’s mass, has a surface gravity of 3% Earth-normal, and an escape velocity low enough most model rockets could get into orbit. And again, it is the biggest, most you could get away from by jumping hard and if you dropped an object on one it might take a few minutes to land.

Don't blink... an artist's conception of an asteroid blocking out a distant star. Image credit: NASA.
Don’t blink… an artist’s conception of an asteroid blocking out a distant star. Image credit: NASA.

You can still terraform one though, by definition too. The gentleman who coined the term, science fiction author Jack Williamson, who also coined the term genetic engineering, used it for a smaller asteroid just a few kilometers across, so any definition of terraforming has to include tiny asteroids too.

Of course in that story it’s like a small planet because they had artificial gravity, we don’t, if we want to fake gravity without having mass we need to spin stuff around. So if we want to terraform an asteroid we need to hollow it out and fill it with air and spin it around.

Of course you do not actually hollow out the asteroid and spin it, asteroids are loose balls of gravel and most would fly apart given any noticeable spin. Instead you would hollow it out and set a cylinder spinning inside it. Sort of like how a good thermos has an outside container and inside one with a layer of vacuum in between, we would spin the inner cylinder.

You wouldn’t have to work hard to hollow out an asteroid either, most aren’t big enough to have sufficient gravity and pressure to crush an empty beer can even at their center. So you can pull matter out from them very easily and shore up the sides with very thin metal walls or even ice. Or just have your cylinder set inside a second non-spinning outer skin or superstructure, like your washer or dryer.

You can then conduct your mining from the inside, shielded from space. You could ever pressurize that hollowed out area if your spinning living area was inside its own superstructure. No gravity, but warmth and air, and you could get away with just a little spin without tearing it apart, maybe enough for plants to grow to normally.

It should be noted that you can potentially colonize even the gas giants themselves, even though our focus today is mostly on their moons. That requires a lot more effort and technology then the sorts of colonies we are discussing today, Fraser and I decided to keep things near-future and fairly low tech, though he actually did an article on colonizing Jupiter itself last year that was my main source material back before got to talking and decided to do a video together.

Jupiter with Io and Ganymede taken by amateur astronomer Damian Peach. Credit: NASA / Damian Peach
Jupiter with Io and Ganymede taken by amateur astronomer Damian Peach. Credit: NASA / Damian Peach

Hydrogen is plentiful on Jupiter itself and floating refineries or ships that fly down to scoop it up might be quite useful, but again today we are more interested in its moons. The biggest problem with colonizing the moons of Jupiter is all the radiation the planet gives off.

Europa is best known as a place where the surface is covered with ice but beneath it is thought to be a vast subsurface ocean. It is the sixth largest moon coming right behind our own at number five and is one of the original four moons Galileo discovered back in 1610, almost two centuries before we even discovered Uranus, so it has always been a source of interest. However as we have discovered more planets and moons we have come to believe quite a few of them might also have subsurface oceans too.

Now what is neat about them is that water, liquid water, always leaves the door open to the possibility of life already existing there. We still know so little about how life originally evolved and what conditions permit that to occur that we cannot rule out places like Europa already having their own plants and animals swimming around under that ice.

They probably do not and obviously we wouldn’t want to colonize them, beyond research bases, if they did, but if they do not they become excellent places to colonize. You could have submarine cities in such places floating around in the sea or those buried in the surface ice layer, well shielded from radiation and debris. The water also geysers up to the surface in some places so you can start off near those, you don’t have to drill down through kilometers of ice on day one.

Water, and hydrogen, are also quite uncommon in the inner solar system so having access to a place like Europa where the escape velocity is only about a fifth of our own is quite handy for export. Now as we move on to talk about moons a lot it is important to note that when I say something has a fifth of the escape velocity of Earth that doesn’t mean it is fives time easier to get off of. Energy rises with the square of velocity so if you need to go five times faster you need to spend 5-squared or 25 times more energy, and even more if that place has tons of air creating friction and drag, atmospheres are hard to claw your way up through though they make landing easier too. But even ignoring air friction you can move 25 liters of water off of Europa for every liter you could export from Earth and even it is a very high in gravity compared to most moons and comets. Plus we probably don’t want to export lots of water, or anything else, off of Earth anyway.

Artist's concept of Trojan asteroids, small bodies that dominate our solar system. Credit: NASA
Artist’s concept of Trojan asteroids, small bodies that dominate our solar system. Credit: NASA

We should start by noting two things. First, the Asteroid Belt is not the only place you find asteroids, Jupiter’s Trojan Asteroids are nearly as numerous, and every planet, including Earth, has an equivalent to Jupiter’s Trojan Asteroids at its own Lagrange Points with the Sun. Though just as Jupiter dwarfs all the other planets so to does its collection of Lagrangian objects. They can quite big too, the largest 624 Hektor, is 400 km across, and has a size and shape similar to Pennsylvania.

And as these asteroids are at stable Lagrange Points, they orbit with Jupiter but always ahead and behind it, making transit to and from Jupiter much easier and making good waypoints.

Before we go out any further in the solar system we should probably address how you get the energy to stay alive. Mars is already quite cold compared to Earth, and the Asteroids and Jupiter even more so, but with thick insulation and some mirrors to bounce light in you can do fairly decently. Indeed, sunlight out by Jupiter is already down to just 4% of what Earth gets, meaning at Jovian distances it is about 50 W/m²

That might not sound like much but it is actually almost a third of what average illumination is on Earth, when you factor in atmospheric reflection, cloudy days, nighttime, and higher, colder latitudes. It is also a good deal brighter than the inside of most well-lit buildings, and is enough for decently robust photosynthesis to grow food. Especially with supplemental light from mirrors or LED growth lamps.

But once you get out to Saturn and further that becomes increasingly impractical and a serious issue, because while food growth does not show up on your electric bill it is what we use virtually all our energy for. Closer in to the sun we can use solar panels for power and we do not need any power to grow food. As we get further out we cannot use solar and we need to heat or cold habitats and supply lighting for food, so we need a lot more power even as our main source dries up.

So what are our options? Well the first is simple, build bigger mirrors. A mirror can be quite large and paper thin after all. Alternatively we can build those mirrors far away, closer to the sun, and and either focus them on the place we want illuminated or send an energy beam, microwaves perhaps or lasers, out to the destination to supply energy.

We also have the option of using fission, if we can find enough Uranium or Thorium. There is not a lot of either in the solar system, in the area of about one part per billion, but that does amount to hundreds of trillions of tons, and it should only take a few thousand tons a year to supply Earth’s entire electric grid. So we would be looking at millions of years worth of energy supply.

Of course fusion is even better, particularly since hydrogen becomes much more abundant as you get further from the Sun. We do not have fusion yet, but it is a technology we can plan around probably having inside our lifetimes, and while uranium and thorium might be counted in parts per billion, hydrogen is more plentiful than every other element combines, especially once you get far from the Sun and Inner Solar System.

So it is much better power source, an effectively unlimited one except on time scales of billions and trillion of years. Still, if we do not have it, we still have other options. Bigger mirrors, beaming energy outwards from closer to the Sun, and classic fission of Uranium and Thorium. Access to fusion is not absolutely necessary but if you have it you can unlock the outer solar system because you have your energy supply, a cheap and abundant fuel supply, and much faster and cheaper spaceships.

Of course hydrogen, plain old vanilla hydrogen with one proton, like the sun uses for fusion, is harder to fuse than deuterium and may be a lot longer developing, we also have fusion using Helium-3 which has some advantages over hydrogen, so that is worth keeping in mind as well as we proceed outward.

Since NASA's Cassini spacecraft arrived at Saturn, the planet's appearance has changed greatly. This view shows Saturn's northern hemisphere in 2016, as that part of the planet nears its northern hemisphere summer solstice in May 2017. Image credit: NASA/JPL-Caltech/Space Science Institute.
Since NASA’s Cassini spacecraft arrived at Saturn, the planet’s appearance has changed greatly. This view shows Saturn’s northern hemisphere in 2016, as that part of the planet nears its northern hemisphere summer solstice in May 2017. Image credit: NASA/JPL-Caltech/Space Science Institute.

Okay, let’s move on to Saturn, and again our focus is on its moons more than the planet itself. The biggest of those an the most interesting for colonization is Titan.

Titan is aptly named, this titanic moon contains more mass than than all of Saturn’s sixty or so other moons and by an entire order of magnitude at that. It is massive enough to hold an atmosphere, and one where the surface pressure is 45% higher than here on Earth. Even though Titan is much smaller than Earth, its atmosphere is about 20% more massive than our own. It’s almost all nitrogen too, even more than our own atmosphere, so while you would need a breather mask to supply oxygen and it is also super-cold, so you’d need a thick insulated suit, it doesn’t have to be a pressure suit like it would on Mars or almost anyplace else.

There’s no oxygen in the atmosphere, what little isn’t nitrogen is mostly methane and hydrogen, but there is plenty of oxygen in the ice on Titan which is quite abundant. So it has everything we need for life except energy and gravity. At 14% of earth normal it is probably too low for people to comfortably and safely adapt to, but we’ve already discussed ways of dealing with that. It is low enough that you could probably flap your arms and fly, if you had wing attached.

On the left is TALISE (Titan Lake In-situ Sampling Propelled Explorer), the ESA proposal. This would have it's own propulsion, in the form of paddlewheels. Credit: bisbos.com
On the left is TALISE (Titan Lake In-situ Sampling Propelled Explorer), the ESA proposal. This would have it’s own propulsion, in the form of paddlewheels. Credit: bisbos.com

It needs some source of energy though, and we discussed that. Obviously if you’ve got fusion you have all the hydrogen you need, but Titan is one of those places we would probably want to colonize early on if we could, it is something you need a lot of to terraform other places, and is also rich in a lot of the others things we want. So we often think of it as a low-tech colony since it is one we would want early on.

In an scenario like that it is very easy to imagine a lot of local transit between Titan and its smaller neighboring moons, which are more rocky and might be easier to dig fissile materials like Uranium and Thorium out of. You might have a dozen or so small outposts on neighboring moons mining fissile materials and other metals and a big central hub on Titan they delivered that too which also exported Nitrogen to other colonies in the solar system.

Moving back and forth between moons is pretty easy, especially since things landing on Titan can aerobrake quite easily, whereas Titan itself has a pretty strong gravity well and thick atmosphere to climb out of but is a good candidate for a space elevator, since it requires nothing more sophisticated than a Lunar Elevator on our own moon and has an abundant supply of the materials needed to make Zylon for instance, a material strong enough to make an elevator there and which we can mass manufacture right now.

Titan might be the largest and most useful of Saturn’s moons, but again it isn’t the only one and not all of the other are just rocks for mining. At last count it has over sixty and many of them quite large. One of those, Enceladus, Saturn’s sixth largest moon, is a lot like Jupiter’s Moon Europa, in that we believe it has a large and thick subsurface ocean. So just like Europa it is an interesting candidate for Colonization. So Titan might be the hub for Saturn but it wouldn’t be the only significant place to colonize.

Clouds tower into a twilight sky on Saturn. The planet’s glowing rings seem to bend at the horizon because of the dense air. (painting ©Michael Carroll)
Clouds tower into a twilight sky on Saturn. The planet’s glowing rings seem to bend at the horizon because of the dense air. (painting ©Michael Carroll)

While Saturn is best known for its amazing rings, they tend to be overlooked in colonization. Now those rings are almost all ice and in total mass about a quarter as much as Enceladus, which again is Saturn’s Sixth largest moon, which is itself not even a thousandth of the Mass of Titan.

In spite of that the rings are not a bad place to set up shop. Being mostly water, they are abundant in hydrogen for fusion fuel and have little mass individually makes them as easy to approach or leave as an asteroid. Just big icebergs in space really, and there are many moonlets in the rings that can be as large as half a kilometer across. So you can burrow down inside one for protection from radiation and impacts and possibly mine smaller ones for their ice to be brought to places where water is not abundant.

In total those rings, which are all frozen water, only mass about 2% of Earth’s oceans, and about as much as the entire Antarctic sheet. So it is a lot of fresh water that is very easy to access and move elsewhere, and ice mines in the rings of Saturn might be quite useful and make good homes. Living inside an iceball might not sound appealing but it is better than it sounds like and we will discuss that more when we reach the Kupier Belt.

Uranus and Neptune, the Solar System’s ice giant planets. Credit: Wikipedia Commons
Uranus and Neptune, the Solar System’s ice giant planets. Credit: Wikipedia Commons

But first we still have two more planets to look at, Uranus and Neptune.

Uranus, and Neptune, are sometimes known as Ice Giants instead of Gas Giants because it has a lot more water. It also has more ammonia and methane and all three get called ices in this context because they make up most of the solid matter when you get this far out in the solar system.

While Jupiter is over a thousand times the mass of Earth, Uranus weighs in at about 15 times the Earth and has only about double the escape velocity of Earth itself, the least of any of the gas giants, and it’s strange rotation, and its strange tilt contributes to it having much less wind than other giants. Additionally the gravity is just a little less than Earth’s in the atmosphere so we have the option for floating habitats again, though it would be a lot more like a submarine than a hot air balloon.

Like Venus, Uranus has very long days, at least in terms of places receiving continual sunlight, the poles get 42 years of perpetual sunlight then 42 of darkness. Sunlight being a relative term, the light is quite minimal especially inside the atmosphere. The low wind in many places makes it a good spot for gas extraction, such as Helium-3, and it’s a good planet to try to scoop gas from or even have permanent installations.

Now Uranus has a large collection of moons as well, useful and colonizable like the other moons we have looked at, but otherwise unremarkable beyond being named for characters from Shakespeare, rather than the more common mythological names. None have atmospheres though there is a possibility Oberon or Titania might have subsurface oceans.

Neptune makes for a brief entry, it is very similar to Uranus except it has the characteristically high winds of gas giants that Uranus’s skewed poles mitigate, meaning it has no advantages over Uranus and the disadvantages of high wind speeds everywhere and being even further from the Sun. It too has moons and one of them, Triton, is thought to have subsurface oceans as well. Triton also presumably has a good amount of nitrogen inside it since it often erupts geysers of nitrogen from its surface.

Neptune's largest moon Triton photographed on August 25, 1989 by Voyager 2. Credit: NASA
Neptune’s largest moon Triton photographed on August 25, 1989 by Voyager 2. Credit: NASA

Triton is one of the largest moons in the solar system, coming in seventh just after our Moon, number 5, and Europa at number 6. Meaning that were it not a moon it would probably qualify as a Dwarf Planet and it is often thought Pluto might be an escaped moon Neptune. So Triton might be one that didn’t escape, or didn’t avoid getting captured. In fact there are an awful lot of bodies in this general size range and composition wandering about in the outer regions of our solar system as we get out into the Kuiper Belt.

Pluto and its cohorts in the icy-asteroid-rich Kuiper Belt beyond the orbit of Neptune. Credit: NASA
Pluto and its cohorts in the icy-asteroid-rich Kuiper Belt beyond the orbit of Neptune. Credit: NASA

The Kuiper Belt is one of those things that has a claim on the somewhat arbitrary and hazy boundary marking the edge of the Solar System. It extends from out past Neptune to beyond Pluto and contains a good deal more mass than the asteroid Belt. It is where a lot of our comets come from and while there is plenty of rocks out there they tend to be covered in ice. In other words it is like our asteroid belt only there’s more of it and the one thing the belt is not very abundant in, water and hydrogen in general, is quite abundant out there. So if you have a power source life fusion they can be easily terraformed and are just as attractive as a source of minerals as the various asteroids and moons closer in.

Discovered in 2005, Makemake, a Kuiper Belt Object (KBO) has . Credit: NASA
Discovered in 2005, Makemake, a Kuiper Belt Object (KBO) has . Credit: NASA

We mentioned the idea of living inside hollowed out asteroids earlier and you can use the same trick for comets. Indeed you could shape them to be much bigger if you like, since they would be hollow and ice isn’t hard to move and shape especially in zero gravity. Same trick as before, you place a spinning cylinder inside it. Not all the objects entirely ice and indeed your average comet is more a frozen ball of mud then ice with rocky cores. We think a lot of near Earth Asteroids are just leftover comets. So they are probably pretty good homes if you have fusion, lots of fuel and raw materials for both life and construction.

This is probably your cheapest interstellar spacecraft too, in terms of effort anyway. People often talk about re-directing comets to Mars to bring it air and water, but you can just as easily re-direct it out of the solar system entirely. Comets tend to have highly eccentric orbits, so if you capture one when it is near the Sun you can accelerate it then, actually benefiting from the Oberth Effect, and drive it out of the solar system into deep space. If you have a fusion power source to live inside one then you also have an interstellar spaceship drive, so you just carve yourself a small colony inside the comet and head out into deep space.

You’ve got supplies that will last you many centuries at least, even if it were home to tens of thousand of people, and while we think of smaller asteroids and comets as tiny, that’s just in comparison to planets. These things tend to be the size of mountain so there is plenty of living space and a kilometer of dirty ice between you and space makes a great shield against even the kinds of radiation and collisions you can experience at relativistic speeds.

Artists' impression of the Kuiper belt and Oort cloud, showing both the origin and path of Halley's Comet. Image credit: NASA/JPL.
Artists’ impression of the Kuiper belt and Oort cloud. Credit: NASA/JPL

Now the Oort Cloud is much like the Kupier Belt but begins even further out and extends out probably an entire light year or more. We don’t have a firm idea of its exact dimensions or mass, but the current notion is that it has at least several Earth’s worth of mass, mostly in various icy bodies. These will be quite numerous, estimates usually assumes at least trillion icy bodies a kilometer across or bigger, and even more smaller ones. However the volume of space is so large that those kilometer wide bodies might each be a around a billion kilometers distant from neighbors, or about a light hour. So it is spread out quite thinly, and even the inner edge is about 10 light days away.

That means that from a practical standpoint there is no source of power out there, the sun is simply too diffuse for even massive collections of mirrors and solar panels to be of use. It also means light-speed messages home or to neighbors are quite delayed. So in terms of communication it is a lot more like pre-modern times in sparsely settled lands where talking with your nearest neighbors might require an hour long walk over to their farm, and any news from the big cities might take months to percolate out to you.

There’s probably uranium and thorium out there to be found, maybe a decent amount of it, so fission as a power source is not ruled out. If you have fusion instead though each of these kilometer wide icy bodies is like a giant tank of gasoline, and as with the Kupier Belt, ice makes a nice shield against impacts and radiation.

And while there might be trillions of kilometer wide chunks of ice out there, and many more smaller bodies, you would have quite a few larger ones too. There are almost certainly tons of planets in the Pluto size-range out these, and maybe even larger ones. Even after the Oort cloud you would still have a lot of these deep space rogue planets which could bridge the gap to another solar system’s Oort Cloud. So if you have fusion you have no shortage of energy, and could colonize trillions of these bodies. There probably is a decent amount of rock and metal out there too, but that could be your major import/export option shipping home ice and shipping out metals.

That’s the edge of the Solar System so that’s the end of this article. If you haven’t already read the other half, colonizing the inner Solar System, head on over now.

Does Our Galaxy Have a Habitable Zone?

I’ve got to say, you are one of the luckiest people I’ve ever met.

For starters, you are the descendant of an incomprehensible number of lifeforms who were successful, and survived long enough to find a partner, procreate, and have an offspring. Billions of years, and you are the result of an unbroken chain of success, surviving through global catastrophe after catastrophe. Nice going.

Not only that, but your lineage happened to be born on a planet, which was in just the right location around just the right kind of star. Not too hot, not too cold, just the right temperature where liquid water, and whatever else was necessary for life to get going. Again, I like your lucky streak.

Yup, you are pretty lucky to call this place home. Credit: NASA
Yup, you are pretty lucky to call this place home. Credit: NASA

In fact, you happened to be born into a Universe that has the right physical constants, like the force of gravity or the binding force of atoms, so that stars, planets and even the chemistry of life could happen at all.
But there’s another lottery you won, and you probably didn’t even know about it. You happened to be born on an unassuming, mostly harmless planet orbiting a G-type main sequence star in the habitable zone of the Milky Way.

Wait a second, even galaxies have habitable zones? Yep, and you’re in it right now.

The Milky Way is a big place, measuring up to 180,000 light years across. It contains 100 to 400 billion stars spread across this enormous volume.

We’re located about 27,000 light years away from the center of the Milky Way, and tens of thousands of light-years away from the outer rim.

Credit: ESA

The Milky Way has some really uninhabitable zones. Down near the center of the galaxy, the density of stars is much greater. And these stars are blasting out a combined radiation that would make it much more unlikely for life to evolve.

Radiation is bad for life. But it gets worse. There’s a huge cloud of comets around the Sun known as the Oort Cloud. Some of the greatest catastrophes in history happened when these comets were kicked into a collision course with the Earth by a passing star. Closer to the galactic core, these disruptions would happen much more often.

There’s another dangerous place you don’t want to be: the galaxy’s spiral arms. These are regions of increased density in the galaxy, where star formation is much more common. And newly forming stars blast out dangerous radiation.

Fortunately, we’re far away from the spiral arms, and we orbit the center of the Milky Way in a nice circular orbit, which means we don’t cross these spiral arms very often.

We stay nice and far away from the dangerous parts of the Milky Way, however, we’re still close enough to the action that our Solar System gathered the elements we needed for life.

The first stars in the Universe only had hydrogen, helium and a few other trace elements left over from the Big Bang. But when the largest stars detonated as supernovae, they seeded the surrounding regions with heavier elements like oxygen, carbon, even iron and gold.

Early stars were made almost entirely of hydrogen and helium. Credit: NASA/WMAP Science Team
Early stars were made almost entirely of hydrogen and helium. Credit: NASA/WMAP Science Team

Our solar nebula was seeded with the heavy elements from many generations of stars, giving us all the raw materials to help set evolution in motion.

If the Solar System was further out, we probably wouldn’t have gotten enough of those heavier elements. So, thanks multiple generations of dead stars.

According to astrobiologists the galactic habitable zone probably starts just outside the galactic bulge – about 13,000 light-years from the center, and ends about halfway out in the disk, 33,000 light-years from the center.

Remember, we’re 27,000 light-years from the center, so just inside that outer edge. Phew.

The Milky Way's habitable zone. Credit: NASA/Caltech
The Milky Way’s habitable zone. Credit: NASA/Caltech

Of course, not all astronomers believe in this Rare Earth hypothesis. In fact, just as we’re finding life on Earth wherever we find water, they believe that life is more robust and resilient. It could still survive and even thrive with more radiation, and less heavier elements.

Furthermore, we’re learning that solar systems might be able to migrate a significant distance from where they formed. Stars that started closer in where there were plenty of heavier elements might have drifted outward to the safer, calmer galactic suburbs, giving life a better chance at getting a foothold.

As always, we’ll need more data, more research to get an answer to this question.

Just when you thought you were already lucky, it turns out you were super duper extra lucky. Right Universe, right lineage, right solar system, right location in the Milky Way. You already won the greatest lottery in existence.

Planet 9 Search Turning Up Wealth Of New Objects

In 2014, Scott Sheppard of the Carnegie Institution for Science and Chadwick Trujillo of Northern Arizona University proposed an interesting idea. Noting the similarities in the orbits of distant Trans-Neptunian Objects (TNOs), they postulated that a massive object was likely influencing them. This was followed in 2016 by Konstantin Batygin and Michael E. Brown of Caltech suggesting that an undiscovered planet was the culprit.

Since that time, the hunt has been on for the infamous “Planet 9” in our Solar System. And while no direct evidence has been produced, astronomers believe they are getting closer to discerning its location. In a paper that was recently accepted by The Astronomical Journal, Sheppard and Trujillo present their latest discoveries, which they claim are further constraining the location of Planet 9.

For the sake of their study, Sheppard and Trujillo relied on information obtained by the Dark Energy Camera on the Victor Blanco 4-meter telescope in Chile and the Japanese Hyper Suprime-Camera on the 8-meter Subaru Telescope in Hawaii. With the help of David Tholen from the University of Hawaii, they have been conducting the largest deep-sky survey for objects beyond Neptune and the Kuiper Belt.

An illustration of the orbits of the new and previously known extremely distant Solar System objects. The clustering of most of their orbits indicates that they are likely be influenced by something massive and very distant, the proposed Planet X. Credit: Robin Dienel/Carnegie Science
An illustration of the orbits of the new and previously known extremely distant Solar System objects – showing the clustering in orbits that indicates that possible presence of Planet X. Credit: Robin Dienel/Carnegie Science

This survey is intended to find more objects that show the same clustering in their orbits, thus offering greater evidence that a massive planet exists in the outer Solar System. As Sheppard explained in a recent Carnegie press release:

“Objects found far beyond Neptune hold the key to unlocking our Solar System’s origins and evolution. Though we believe there are thousands of these small objects, we haven’t found very many of them yet, because they are so far away. The smaller objects can lead us to the much bigger planet we think exists out there. The more we discover, the better we will be able to understand what is going on in the outer Solar System.”

Their most recent discovery was a small collection of more extreme objects who’s peculiar orbits differ from the extreme and inner Oort cloud objects, in terms of both their eccentricities and semi-major axes. As with discoveries made using other instruments, these appear to indicate the presence of something massive effecting their orbits.

All of these objects have been submitted to the International Astronomical Union’s (IAU) Minor Planet Center for designation. They include 2014 SR349, an extreme TNO that has similar orbital characteristics as the previously-discovered extreme bodies that led Sheppard and Trujillo to infer the existence of a massive object in the region.

Another is 2014 FE72, an object who’s orbit is so extreme that it reaches about 3000 AUs from the Sun in a massively-elongated ellipse – something which can only be explained by the influence of a strong gravitational force beyond our Solar System. And in addition to being the first object observed at such a large distance, it is also the first distant Oort Cloud object found to orbit entirely beyond Neptune.

Artist's impression of Planet Nine as an ice giant eclipsing the central Milky Way, with a star-like Sun in the distance. Neptune's orbit is shown as a small ellipse around the Sun. The sky view and appearance are based on the conjectures of its co-proposer, Mike Brown.
Artist’s impression of Planet Nine as an ice giant eclipsing the central Milky Way, with a star-like Sun in the distance. Credit: ESO/Tomruen/nagualdesign

And then there’s  2013 FT28, which is similar but also different from the other extreme objects. For instance, 2013 FT28 shows similar clustering in terms of its semi-major axis, eccentricity, inclination, and argument of perihelion angle, but is different when it comes to its longitude of perihelion. This would seem to indicates that this particular clustering trend is less strong among the extreme TNOs.

Beyond the work of Sheppard and Trujillo, nearly 10 percent of the sky has now been explored by astronomers. Relying on the most advanced telescopes, they have revealed that there are several never-before-seen objects that orbit the Sun at extreme distances.

And as more distant objects with unexplained orbital parameters emerge, their interactions seem to fit with the idea of a massive distant planet that could pay a key role in the mechanics of the outer Solar System. However, as Sheppard has indicated, there really isn’t enough evidence yet to draw any conclusions.

“Right now we are dealing with very low-number statistics, so we don’t really understand what is happening in the outer Solar System,” he said. “Greater numbers of extreme trans-Neptunian objects must be found to fully determine the structure of our outer Solar System.”

Alas, we don’t yet know if Planet 9 is out there, and it will probably be many more years before confirmation can be made. But by looking to the visible objects that present a possible sign of its path, we are slowly getting closer to it. With all the news in exoplanet hunting of late, it is interesting to see that we can still go hunting in our own backyard!

Further Reading: The Astrophysical Journal Letters

It’s Finally Here! Comet Catalina Greets Dawn Skywatchers

If you love watching comets and live north of the equator, you’ve been holding your breath a l-o-n-g time for C/2013 US10 Catalina to make its northern debut. I’m thrilled to report the wait is over. The comet just passed perihelion on Nov. 15th and has begun its climb into morning twilight. 

Comet C/2013 US10 Catalina leaps into the morning sky in eastern Virgo this weekend at around magnitude +7. Comet positions are marked by small crosses every 5 days around 6 a.m. CST (12:00 Universal Time). Planet positions are shown for Nov. 21st. Stars to mag. +7. Source: Chris Marriott's SkyMap
Map showing the sky facing southeast around the start of dawn. Comet C/2013 US10 Catalina leaps into the morning sky in eastern Virgo beginning this weekend at around magnitude +7. Comet positions are marked by small crosses every 5 days around 6 a.m. CST (12:00 Universal Time) for mid-northern latitudes (Minneapolis, specifically). Planet positions are shown for Nov. 21st. Stars to mag. +7. Source: Chris Marriott’s SkyMap

The first post-perihelion photo, taken on Nov. 19th by astrophotographer Ajay Talwar from Devasthal Observatory high in the Indian Himalayas, show it as a starry dot with a hint of a tail only 1° above the eastern horizon at mid-twilight. Additional photos made on the following mornings show the comet inching up from the eastern horizon into better view. Estimates of its current brightness range from magnitude +6.8-7.0.

Sometimes black and white is better. This is the same chart as above. Credit: Chris Marriott's SkyMap
Sometimes black and white is better. This is the same chart as above but in a handier version for use at the telescope. Source: Chris Marriott’s SkyMap

Talwar, who teaches astrophotography classes and is a regular contributor to The World at Night (TWAN), drove 9 hours from his home to the Himalaya mountains, then climbed up the observatory dome to get enough horizon to photograph the comet. The window of opportunity was very narrow; Talwar had only 10 minutes to bag his images before the comet was overwhelmed by zodiacal light and twilight glow. When asked if it was visible in binoculars, he thought it would be but had too little time to check despite bringing a pair along.

The very first post-perihelion photo of Comet Catalina taken Nov. 19th from Devasthal Observatory. Prior to perihelion, the comet was only visible from the southern hemisphere. Copyright: Ajay Talwar
Ajay Talwar recorded the very first post-perihelion photo of Comet Catalina on Nov. 19th from Devasthal Observatory. Prior to perihelion, the comet was only visible from the southern hemisphere. Copyright: Ajay Talwar

A difficult object at the moment, once it frees itself from the horizon haze in about a week, Catalina should be easily visible in ordinary binoculars. Watch for it to gradually brighten through the end of the year, peaking around magnitude +5.5 — just barely naked eye — in late December and early January, when it will be well-placed high in the northeastern sky near the star Arcturus (see map). Matter of fact, on the first morning of the new year, it creeps only 1/2° southwest of the star for a splendid conjunction.

Even before perihelion, Comet Catalina was a beauty. This photo was taken on October 1, 2015. Credti: Jose Chambo
Even before perihelion, Comet Catalina was a beautiful thing. This photo was taken on October 1, 2015. Credit: Jose Chambo

Halloween 2013 was an auspicious one. That’s when Comet C/2013 US10 was first picked up by the Catalina Sky Survey. The “US10” part comes from initial observations that suggested it was an asteroid. Additional photos and observations instead revealed a fuzzy comet on a steeply tilted orbit headed for the inner Solar System after a long sojourn in the Oort Cloud.

Comet C/2013 US10 Catalina will slice through the plane of the Solar System at an angle of 149 never to return. Credit: JPL Horizons
Comet C/2013 US10 Catalina will slice through the plane of the Solar System at an angle of 149° never to return. It comes closest to Earth on Jan. 12, 2016. After that time, the comet will recede and fade. Credit: JPL Horizons

Its sunward journey has been nothing short of legendary, requiring several million years of inbound travel from the frigid fringe to the relative warmth of the inner Solar System. Catalina will pass closest to Earth on Jan. 12th at 66.9 million miles (107.7 million km) before buzzing off into interstellar space. Yes, interstellar. Perturbations by the planets have converted its orbit into a one-way ticket outta here.

Wow - check this out! Look to the east at the start of dawn on Dec. 7th to see a remarkable pairing of comet, Venus and the waning lunar crescent with earthshine. Source: Stellarium
Check this out! Look to the east at the start of dawn on Dec. 7th to see a remarkable pairing of comet, Venus and the waning lunar crescent with earthshine. Source: Stellarium

When using the maps above, keep in mind they show the comet’s changing position, but the constellations and planets can only be shown for the one date, Nov. 21st. Like the comet, they’ll also be slowly sliding upward in the coming days and mornings due to Earth’s revolution around the Sun; stars that are near the horizon on Nov. 21 at 5:30 or 6 a.m. will be considerably higher up in a darker sky by the same time in December. Adding the shift of the stars to that of the comet, Catalina gains about 1° of altitude per day in the coming two weeks.

When you go out to find Catalina in binoculars, note its location on the map and then use the stars as steppingstones, starting with a bright obvious one like Spica and “stepping” from there to the next until you arrive at the one closest to the comet.

I’m so looking forward to finding Catalina. Nothing like a potentially naked eye comet to warm up those cold December mornings. Mark your calendar for the morning of Dec. 7th, when this rare visitor will join Venus and the crescent Moon in the east at the start of morning twilight. See you in spirit at dawn!

Is Jupiter Our Friend Or Enemy?

Like me, you’re probably a little ego-geocentric about the importance of Earth. It’s where you were born, it’s where you keep all your stuff. It’s even where you’re going to die – I know, I know, not you Elon Musk, you’re going to “retire” on Mars, right after you nuke the snot out of it.

For the rest of us, Earth is the place. But in reality, when it comes to planets, this is somebody else’s racket. This is Jupiter’s Solar System, and we all sleep on its couch.

Jupiter accounts for 75% of the mass of the planets of the Solar System, nearly 318 times more massive than Earth, and isn’t just the name of everyone’s favorite secret princess. It’s the 1.9 × 10^27 kilogram gorilla in the room. Whatever Jupiter wants, Jupiter gets. Jupiter hungry? JUPITER HUNGRY.

What Jupiter apparently wants is to throw our stuff around the Solar System. Thanks to its immense gravity, Jupiter yanks material around in the asteroid belt, preventing the poor space rocks from ever forming up into anything larger than Ceres.

Jupiter gobbles up asteroids, comets, and spacecraft, and hurtles others on wayward trajectories. Who knows how much mayhem and destruction Jupiter has gotten into over the course of its 4.5 billion years in the Solar System.

Some scientists think we owe our existence to Jupiter’s protective gravity. It greedily vacuums up dangerous asteroids and comets in the Solar System.

Other scientists totally disagree and think that Jupiter is a bully, perturbing perfectly safe comets and asteroids into dangerous trajectories and flushing earth’s head in the toilet during recess.

Which is it? Is Jupiter our friend and protector, or evil enemy. We’ve already figured out how to dismantle you Jupiter, don’t make us put our plans into action.

Some of the most dangerous objects in the Solar System are long-period comets. These balls of rock and ice come from the deepest depths of the Oort cloud. Some event nudges these death missiles into trajectories that bring them into the inner Solar System, to shoot past the Sun and maybe, just maybe, smash into a planet and kill 99.99999% of the life on it.

The Solar System. Credit: NASA
The Solar System. Credit: NASA

There’s a pretty good chance some of the biggest extinctions in the history of the Earth were caused by impacts by long period comets.

As these comets make their way through the Solar System, they interact with Jupiter’s massive gravity, and get pushed this way and that. As we saw with Comet Shoemaker-Levy, some just get consumed entirely, like a tasty ice-rock sandwich.

The theory goes that Jupiter pushes these dangerous comets out of their murder orbits so they don’t smash into Earth and kill us all.

But a competing theory says that Jupiter actually diverts comets that would have completely missed our planet into deadly, Earth-killing trajectories.

Will the Sailor Scouts provide us any clues? Who can say?

Fragmentation of comets is common. Many sungrazers are broken up by thermal and tidal stresses during their perihelions. At top, an image of the comet Shoemaker-Levy 9 (May 1994) after a close approach with Jupiter which tore the comet into numerous fragments. An image taken by Andrew Catsaitis of components B and C of Comet 73P/Schwassmann–Wachmann 3 as seen together on 31 May 2006 (Credit: NASA/HST, Wikipedia, A.Catsaitis)
At top, an image of the comet Shoemaker-Levy 9 (May 1994) after a close approach with Jupiter which tore the comet into numerous fragments. An image taken by Andrew Catsaitis of components B and C of Comet 73P/Schwassmann–Wachmann 3 as seen together on 31 May 2006 (Credit: NASA/HST, Wikipedia, A.Catsaitis)

Here’s friend of the show, Dr. Kevin Grazier, a planetary scientist and scientific advisor for many of your favorite sci-fi TV shows and movies.

… [ see video for Interview with Dr. Grazier about Jupiter]

So which is it? Is Jupiter our friend or enemy? We’ll need to run more simulations and figure this out with more accuracy. And until then, it’s probably best if we just tremble in fear and worship Jupiter as a dark and capricious god until the evidence proves otherwise. It’s what Pascal would wager.

What are some other theories you’ve heard about and you’d like us to dig in further? Make some suggestions in the comments below.

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Solar System Guide

The Solar System. Image Credit: NASA

The Universe is a very big place, and we occupy a very small corner of it. Known as the Solar System, our stomping grounds are not only a tiny fraction of the Universe as we know it, but is also a very small part of our galactic neighborhood (aka. the Milky Way Galaxy). When it comes right down to it, our world is just a drop of water in an endless cosmic sea.

Nevertheless, the Solar System is still a very big place, and one which is filled with its fair share of mysteries. And in truth, it was only within the relatively recent past that we began to understand its true extent. And when it comes to exploring it, we’ve really only begun to scratch the surface.

Discovery:

With very few exceptions, few people or civilizations before the era of modern astronomy recognized the Solar System for what it was. In fact, the vast majority of astronomical systems posited that the Earth was a stationary object and that all known celestial objects revolved around it. In addition, they viewed it as being fundamentally different from other stellar objects, which they held to be ethereal or divine in nature.

Although there were some Greek, Arab and Asian astronomers during Antiquity and the Medieval period who believed that the universe was heliocentric in nature (i.e. that the Earth and other bodies revolved around the Sun) it was not until Nicolaus Copernicus developed his mathematically predictive model of a heliocentric system in the 16th century that it began to become widespread.

The first star party? Galileo shows of the sky in Saint Mark's square in Venice. Note the lack of adaptive optics. (Illustration in the Public Domain).
Galileo (1564 – 1642) would often show people how to use his telescope to view the sky in Saint Mark’s square in Venice. Note the lack of adaptive optics. Credit: Public Domain

During the 17th-century, scientists like Galileo Galilei, Johannes Kepler, and Isaac Newton developed an understanding of physics which led to the gradual acceptance that the Earth revolves round the Sun. The development of theories like gravity also led to the realization that the other planets are governed by the same physical laws as Earth.

The widespread use of the telescope also led to a revolution in astronomy. After Galileo discovered the moons of Jupiter in 1610, Christian Huygens would go on to discover that Saturn also had moons in 1655. In time, new planets would also be discovered (such as Uranus and Neptune), as well as comets (such as Halley’s Comet) and the Asteroids Belt.

By the 19th century, three observations made by three separate astronomers determined the true nature of the Solar System and its place the universe. The first was made in 1839 by German astronomer Friedrich Bessel, who successfully measured an apparent shift in the position of a star created by the Earth’s motion around the Sun (aka. stellar parallax). This not only confirmed the heliocentric model beyond a doubt, but revealed the vast distance between the Sun and the stars.

In 1859, Robert Bunsen and Gustav Kirchhoff (a German chemist and physicist) used the newly invented spectroscope to examined the spectral signature of the Sun. They discovered that it was composed of the same elements as existed on Earth, thus proving that Earth and the heavens were composed of the same elements.

With parallax technique, astronomers observe object at opposite ends of Earth's orbit around the Sun to precisely measure its distance. CREDIT: Alexandra Angelich, NRAO/AUI/NSF.
With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.

Then, Father Angelo Secchi  – an Italian astronomer and director at the Pontifical Gregorian University – compared the spectral signature of the Sun with those of other stars, and found them to be virtually identical. This demonstrated conclusively that our Sun was composed of the same materials as every other star in the universe.

Further apparent discrepancies in the orbits of the outer planets led American astronomer Percival Lowell to conclude that yet another planet, which he referred to as “Planet X“, must lie beyond Neptune. After his death, his Lowell Observatory conducted a search that ultimately led to Clyde Tombaugh’s discovery of Pluto in 1930.

Also in 1992, astronomers David C. Jewitt of the University of Hawaii and Jane Luu of the MIT discovered the Trans-Neptunian Object (TNO) known as (15760) 1992 QB1. This would prove to be the first of a new population, known as the Kuiper Belt, which had already been predicted by astronomers to exist at the edge of the Solar System.

Further investigation of the Kuiper Belt by the turn of the century would lead to additional discoveries. The discovery of Eris and other “plutoids” by Mike Brown, Chad Trujillo, David Rabinowitz and other astronomers would lead to the Great Planet Debate – where IAU policy and the convention for designating planets would be contested.

Structure and Composition:

At the core of the Solar System lies the Sun (a G2 main-sequence star) which is then surrounded by four terrestrial planets (the Inner Planets), the main Asteroid Belt, four gas giants (the Outer Planets), a massive field of small bodies that extends from 30 AU to 50 AU from the Sun (the Kuiper Belt). The system is then surrounded a spherical cloud of icy planetesimals (the Oort Cloud) that is believed to extend to a distance of 100,000 AU from the Sun into the Interstellar Medium.

The Sun contains 99.86% of the system’s known mass, and its gravity dominates the entire system. Most large objects in orbit around the Sun lie near the plane of Earth’s orbit (the ecliptic) and most planets and bodies rotate around it in the same direction (counter-clockwise when viewed from above Earth’s north pole). The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at greater angles to it.

It’s four largest orbiting bodies (the gas giants) account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System’s total mass.

Sun and Planets
The Sun and planets to scale. Credit: Illustration by Judy Schmidt, texture maps by Björn Jónsson

Astronomers sometimes informally divide this structure into separate regions. First, there is the Inner Solar System, which includes the four terrestrial planets and the Asteroid Belt. Beyond this, there’s the outer Solar System that includes the four gas giant planets. Meanwhile, there’s the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune (i.e. Trans-Neptunian Objects).

Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellites (or moons). In the case of the four giant planets, there are also planetary rings – thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent.

The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. The terrestrial planets of the Inner Solar System are composed primarily of silicate rock, iron and nickel. Beyond the Asteroid Belt, planets are composed mainly of gases (such as hydrogen, helium) and ices – like water, methane, ammonia, hydrogen sulfide and carbon dioxide.

Objects farther from the Sun are composed largely of materials with lower melting points. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (hence why they are sometimes referred to as “ice giants”) and the numerous small objects that lie beyond Neptune’s orbit.

Together, gases and ices are referred to as volatiles. The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, which lies roughly 5 AU from the Sun. Within the Kuiper Belt, objects and planetesimals are composed mainly of these materials and rock.

Formation and Evolution:

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud composed of hydrogen, helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System (known as the pre-solar nebula) collapsed, conservation of angular momentum caused it to rotate faster.

The center, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a hot, dense protostar at the center. The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies.

Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets (Jupiter, Saturn, Uranus, and Neptune) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the frost line).

The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved.

At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.

The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute
The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute

The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium. This will occur roughly 5 billion years from now and mark the end of the Sun’s main-sequence life. At this time, the core of the Sun will collapse, and the energy output will be much greater than at present.

The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. The expanding Sun is expected to vaporize Mercury and Venus and render Earth uninhabitable as the habitable zone moves out to the orbit of Mars. Eventually, the core will be hot enough for helium fusion and the Sun will burn helium for a time, after which nuclear reactions in the core will start to dwindle.

At this point, the Sun’s outer layers will move away into space, leaving a white dwarf – an extraordinarily dense object that will have half the original mass of the Sun, but will be the size of Earth. The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.

Inner Solar System:

In the inner Solar System, we find the “Inner Planets” – Mercury, Venus, Earth, and Mars – which are so named because they orbit closest to the Sun. In addition to their proximity, these planets have a number of key differences that set them apart from planets elsewhere in the Solar System.

For starters, the inner planets are rocky and terrestrial, composed mostly of silicates and metals, whereas the outer planets are gas giants. The inner planets are also much more closely spaced than their outer Solar System counterparts. In fact, the radius of the entire region is less than the distance between the orbits of Jupiter and Saturn.

Generally, inner planets are smaller and denser than their counterparts, and have few to no moons or rings circling them. The outer planets, meanwhile, often have dozens of satellites and rings composed of particles of ice and rock.

The terrestrial inner planets are composed largely of refractory minerals such as the silicates, which form their crusts and mantles, and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather. All of them have impact craters and tectonic surface features as well, such as rift valleys and volcanoes.

Of the inner planets, Mercury is the closest to our Sun and the smallest of the terrestrial planets. Its magnetic field is only about 1% that of Earth’s, and it’s very thin atmosphere means that it is hot during the day (up to 430°C) and freezing at night (as low as -187 °C) because the atmosphere can neither keep heat in or out. It has no moons of its own and is comprised mostly of iron and nickel. Mercury is one of the densest planets in the Solar System.

Venus, which is about the same size as Earth, has a thick toxic atmosphere that traps heat, making it the hottest planet in the Solar System. This atmosphere is composed of 96% carbon dioxide, along with nitrogen and a few other gases. Dense clouds within Venus’ atmosphere are composed of sulphuric acid and other corrosive compounds, with very little water. Much of Venus’ surface is marked with volcanoes and deep canyons – the biggest of which is over 6400 km (4,000 mi) long.

Earth is the third inner planet and the one we know best. Of the four terrestrial planets, Earth is the largest, and the only one that currently has liquid water, which is necessary for life as we know it. Earth’s atmosphere protects the planet from dangerous radiation and helps keep valuable sunlight and warmth in, which is also essential for life to survive.

Like the other terrestrial planets, Earth has a rocky surface with mountains and canyons, and a heavy metal core. Earth’s atmosphere contains water vapor, which helps to moderate daily temperatures. Like Mercury, the Earth has an internal magnetic field. And our Moon, the only one we have, is comprised of a mixture of various rocks and minerals.

Mars, as it appears today, Credit: NASA
Mars, as it appears today, Credit: NASA

Mars is the fourth and final inner planet, and is also known as the “Red Planet” due to the oxidization of iron-rich materials that form the planet’s surface. Mars also has some of the most interesting terrain features of any of the terrestrial planets. These include the largest mountain in the Solar System (Olympus Mons) which rises some 21,229 m (69,649 ft) above the surface, and a giant canyon called Valles Marineris – which is 4000 km (2500 mi) long and reaches depths of up to 7 km (4 mi).

Much of Mars’ surface is very old and filled with craters, but there are geologically newer areas of the planet as well. At the Martian poles are polar ice caps that shrink in size during the Martian spring and summer. Mars is less dense than Earth and has a smaller magnetic field, which is indicative of a solid core, rather than a liquid one.

Mars’ thin atmosphere has led some astronomers to believe that the surface water that once existed there might have actually taken liquid form, but has since evaporated into space. The planet has two small moons called Phobos and Deimos.

Outer Solar System:

The outer planets (sometimes called Jovian planets or gas giants) are huge planets swaddled in gas that have rings and plenty of moons. Despite their size, only two of them are visible without telescopes: Jupiter and Saturn. Uranus and Neptune were the first planets discovered since antiquity, and showed astronomers that the solar system was bigger than previously thought.

The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute
The outer planets of our Solar System at approximately relative sizes. From left, Jupiter, Saturn, Uranus and Neptune. Credit: Lunar and Planetary Institute

Jupiter is the largest planet in our Solar System and spins very rapidly (10 Earth hours) relative to its orbit of the sun (12 Earth years). Its thick atmosphere is mostly made up of hydrogen and helium, perhaps surrounding a terrestrial core that is about Earth’s size. The planet has dozens of moons, some faint rings and a Great Red Spot – a raging storm that has happening for the past 400 years at least.

Saturn is best known for its prominent ring system – seven known rings with well-defined divisions and gaps between them. How the rings got there is one subject under investigation. It also has dozens of moons. Its atmosphere is mostly hydrogen and helium, and it also rotates quickly (10.7 Earth hours) relative to its time to circle the Sun (29 Earth years).

Uranus was first discovered by William Herschel in 1781. The planet’s day takes about 17 Earth hours and one orbit around the Sun takes 84 Earth years. Its mass contains water, methane, ammonia, hydrogen and helium surrounding a rocky core. It has dozens of moons and a faint ring system. The only spacecraft to visit this planet was the Voyager 2 spacecraft in 1986.

Neptune is a distant planet that contains water, ammmonia, methane, hydrogen and helium and a possible Earth-sized core. It has more than a dozen moons and six rings. NASA’s Voyager 2 spacecraft also visited this planet and its system by 1989 during its transit of the outer Solar System.

How many moons are there in the Solar System? Image credit: NASA
How many moons are there in the Solar System? Image credit: NASA

Trans-Neptunian Region:

There have been more than a thousand objects discovered in the Kuiper Belt, and it’s theorized that there are as many as 100,000 objects larger than 100 km in diameter. Given to their small size and extreme distance from Earth, the chemical makeup of KBOs is very difficult to determine.

However, spectrographic studies conducted of the region since its discovery have generally indicated that its members are primarily composed of ices: a mixture of light hydrocarbons (such as methane), ammonia, and water ice – a composition they share with comets. Initial studies also confirmed a broad range of colors among KBOs, ranging from neutral grey to deep red.

This suggests that their surfaces are composed of a wide range of compounds, from dirty ices to hydrocarbons. In 1996, Robert H. Brown et al. obtained spectroscopic data on the KBO 1993 SC, revealing its surface composition to be markedly similar to that of Pluto (as well as Neptune’s moon Triton) in that it possessed large amounts of methane ice.

Water ice has been detected in several KBOs, including 1996 TO66, 38628 Huya and 20000 Varuna. In 2004, Mike Brown et al. determined the existence of crystalline water ice and ammonia hydrate on one of the largest known KBOs, 50000 Quaoar. Both of these substances would have been destroyed over the age of the Solar System, suggesting that Quaoar had been recently resurfaced, either by internal tectonic activity or by meteorite impacts.

Keeping Pluto company out in the Kuiper belt are many other objects worthy of mention. Quaoar, Makemake, Haumea, Orcus and Eris are all large icy bodies in the Belt and several of them even have moons of their own. These are all tremendously far away, and yet, very much within reach.

Oort Cloud and Farthest Regions:

The Oort Cloud is thought to extend from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun, though some estimates place the outer edge as far as 100,000 and 200,000 AU (1.58 and 3.16 ly). The Cloud is thought to be comprised of two regions – a spherical outer Oort Cloud of 20,000 – 50,000 AU (0.32 – 0.79 ly), and disc-shaped inner Oort (or Hills) Cloud of 2,000 – 20,000 AU (0.03 – 0.32 ly).

The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi), and billions that measure 20 kilometers (12 mi) in diameter. Its total mass is not known, but – assuming that Halley’s Comet is a typical representation of outer Oort Cloud objects – it has the combined mass of roughly 3×1025 kilograms (6.6×1025 pounds), or five Earths.

The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA

Based on the analyses of past comets, the vast majority of Oort Cloud objects are composed of icy volatiles – such as water, methane, ethane, carbon monoxide, hydrogen cyanide, and ammonia. The appearance of asteroids thought to be originating from the Oort Cloud has also prompted theoretical research that suggests that the population consists of 1-2% asteroids.

Earlier estimates placed its mass up to 380 Earth masses, but improved knowledge of the size distribution of long-period comets has led to lower estimates. The mass of the inner Oort Cloud, meanwhile, has yet to be characterized. The contents of both Kuiper Belt and the Oort Cloud are known as Trans-Neptunian Objects (TNOs), because the objects of both regions have orbits that that are further from the Sun than Neptune’s orbit.

Exploration:

Our knowledge of the Solar System also benefited immensely from the advent of robotic spacecraft, satellites, and robotic landers. Beginning in the mid-20th century, in what was known as “The Space Age“, manned and robotic spacecraft began exploring planets, asteroids and comets in the Inner and Outer Solar System.

All planets in the Solar System have now been visited to varying degrees by spacecraft launched from Earth. Through these unmanned missions, humans have been able to get close-up photographs of all the planets. In the case of landers and rovers, tests have been performed on the soils and atmospheres of some.

Sputnik 1
Photograph of a Russian technician putting the finishing touches on Sputnik 1, humanity’s first artificial satellite. Credit: NASA/Asif A. Siddiqi

The first artificial object sent into space was the Soviet satellite Sputnik 1, which was launched in space in 1957, successfully orbited the Earth for months, and collected information on the density of the upper atmosphere and the ionosphere. The American probe Explorer 6, launched in 1959, was the first satellite to capture images of the Earth from space.

Robotic spacecraft conducting flybys also revealed considerable information about the planet’s atmospheres, geological and surface features. The first successful probe to fly by another planet was the Soviet Luna 1 probe, which sped past the Moon in 1959. The Mariner program resulted in multiple successful planetary flybys, consisting of the Mariner 2 mission past Venus in 1962, the Mariner 4 mission past Mars in 1965, and the Mariner 10 mission past Mercury in 1974.

By the 1970’s, probes were being dispatched to the outer planets as well, beginning with the Pioneer 10 mission which flew past Jupiter in 1973 and the Pioneer 11 visit to Saturn in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980-1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989.

Launched on January 19th, 2006, the New Horizons probe is the first man-made spacecraft to explore the Kuiper Belt. This unmanned mission flew by Pluto in July 2015. Should it prove feasible, the mission will also be extended to observe a number of other Kuiper Belt Objects (KBOs) in the coming years.

Orbiters, rovers, and landers began being deployed to other planets in the Solar System by the 1960’s. The first was the Soviet Luna 10 satellite, which was sent into lunar orbit in 1966. This was followed in 1971 with the deployment of the Mariner 9 space probe, which orbited Mars, and the Soviet Venera 9 which orbited Venus in 1975.

The Galileo probe became the first artificial satellite to orbit an outer planet when it reached Jupiter in 1995, followed by the CassiniHuygens probe orbiting Saturn in 2004. Mercury and Vesta were explored by 2011 by the MESSENGER and Dawn probes, respectively, with Dawn establishing orbit around the asteroid/dwarf planet Ceres in 2015.

The first probe to land on another Solar System body was the Soviet Luna 2 probe, which impacted the Moon in 1959. Since then, probes have landed on or impacted on the surfaces of Venus in 1966 (Venera 3), Mars in 1971 (Mars 3 and Viking 1 in 1976), the asteroid 433 Eros in 2001 (NEAR Shoemaker), and Saturn’s moon Titan (Huygens) and the comet Tempel 1 (Deep Impact) in 2005.

Curiosity Rover snapped this self portrait mosaic with the MAHLI camera while sitting on flat sedimentary rocks at the “John Klein” outcrop where the robot conducted historic first sample drilling inside the Yellowknife Bay basin, on Feb. 8 (Sol 182) at lower left in front of rover. The photo mosaic was stitched from raw images snapped on Sol 177, or Feb 3, 2013, by the robotic arm camera - accounting for foreground camera distortion. Credit: NASA/JPL-Caltech/MSSS/Marco Di Lorenzo/KenKremer (kenkremer.com).
Curiosity Rover self portrait mosaic, taken with the MAHLI camera while sitting on flat sedimentary rocks at the “John Klein” outcrop in Feb. 2013. Credit: NASA/JPL-Caltech/MSSS/Marco Di Lorenzo/KenKremer

To date, only two worlds in the Solar System, the Moon and Mars, have been visited by mobile rovers. The first robotic rover to land on another planet was the Soviet Lunokhod 1, which landed on the Moon in 1970. The first to visit another planet was Sojourner, which traveled 500 meters across the surface of Mars in 1997, followed by Spirit (2004), Opportunity (2004), and Curiosity (2012).

Manned missions into space began in earnest in the 1950’s, and was a major focal point for both the United States and Soviet Union during the “Space Race“. For the Soviets, this took the form of the Vostok program, which involved sending manned space capsules into orbit.

The first mission – Vostok 1 – took place on April 12th, 1961, and was piloted by Soviet cosmonaut Yuri Gagarin (the first human being to go into space). On June 6th, 1963, the Soviets also sent the first woman – Valentina Tereshvoka – into space as part of the Vostok 6 mission.

In the US, Project Mercury was initiated with the same goal of placing a crewed capsule into orbit. On May 5th, 1961, astronaut Alan Shepard went into space aboard the Freedom 7 mission and became the first American (and second human) to go into space.

After the Vostok and Mercury programs were completed, the focus of both nations and space programs shifted towards the development of two and three-person spacecraft, as well as the development of long-duration spaceflights and extra-vehicular activity (EVA).

Bootprint in the moon dust from Apollo 11. Credit: NASA
Bootprint in the moon dust from Apollo 11. Credit: NASA

This took the form of the Voshkod and Gemini programs in the Soviet Union and US, respectively. For the Soviets, this involved developing a two to three-person capsule, whereas the Gemini program focused on developing the support and expertise needed for an eventual manned mission to the Moon.

These latter efforts culminated on July 21st, 1969 with the Apollo 11 mission, when astronauts Neil Armstrong and Buzz Aldrin became the first men to walk on the Moon. As part of the Apollo program, five more Moon landings would take place through 1972, and the program itself resulted in many scientific packages being deployed on the Lunar surface, and samples of moon rocks being returned to Earth.

After the Moon Landing took place, the focus of the US and Soviet space programs then began to shift to the development of space stations and reusable spacecraft. For the Soviets, this resulted in the first crewed orbital space stations dedicated to scientific research and military reconnaissance – known as the Salyut and Almaz space stations.

The first orbital space station to host more than one crew was NASA’s Skylab, which successfully held three crews from 1973 to 1974. The first true human settlement in space was the Soviet space station Mir, which was continuously occupied for close to ten years, from 1989 to 1999. It was decommissioned in 2001, and its successor, the International Space Station, has maintained a continuous human presence in space since then.

Space Shuttle Columbia launching on its maiden voyage on April 12th, 1981. Credit: NASA
Space Shuttle Columbia launching on its maiden voyage on April 12th, 1981. Credit: NASA

The United States’ Space Shuttle, which debuted in 1981, became the only reusable spacecraft to successfully make multiple orbital flights. The five shuttles that were built (Atlantis, Endeavour, Discovery, Challenger, Columbia and Enterprise) flew a total of 121 missions before being decommissioned in 2011.

During their history of service, two of the craft were destroyed in accidents. These included the Space Shuttle Challenger – which exploded upon take-off on Jan. 28th, 1986 – and the Space Shuttle Columbia which disintegrated during re-entry on Feb. 1st, 2003.

In 2004, then-U.S. President George W. Bush announced the Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a manned mission to Mars. These goals have since been maintained by the Obama administration, and now include plans for an Asteroid Redirect mission, where a robotic craft will tow an asteroid closer to Earth so a manned mission can be mounted to it.

All the information gained from manned and robotic missions about the geological phenomena of other planets – such as mountains and craters – as well as their seasonal, meteorological phenomena (i.e. clouds, dust storms and ice caps) have led to the realization that other planets experience much the same phenomena as Earth. In addition, it has also helped scientists to learn much about the history of the Solar System and its formation.

As our exploration of the Inner and Outer Solar System has improved and expanded, our conventions for categorizing planets has also changed. Our current model of the Solar System includes eight planets (four terrestrial, four gas giants), four dwarf planets, and a growing number of Trans-Neptunian Objects that have yet to be designated. It also contains and is surrounded by countless asteroids and planetesimals.

Given its sheer size, composition and complexity, researching our Solar System in full detail would take an entire lifetime. Obviously, no one has that kind of time to dedicate to the topic, so we have decided to compile the many articles we have about it here on Universe Today in one simple page of links for your convenience.

There are thousands of facts about the solar system in the links below. Enjoy your research.

The Solar System:

Theories about the Solar System:

Moons:

Anything EXTREME!:

Solar System Stuffs:

What is the Oort Cloud?

The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA

For thousands of years, astronomers have watched comets travel close to Earth and light up the night sky. In time, these observations led to a number of paradoxes. For instance, where were these comets all coming from? And if their surface material vaporizes as they approach the Sun (thus forming their famous halos), they must formed farther away, where they would have existed there for most of their lifespans.

In time, these observations led to the theory that far beyond the Sun and planets, there exists a large cloud of icy material and rock where most of these comets come from. This existence of this cloud, which is known as the Oort Cloud (after its principal theoretical founder), remains unproven. But from the many short and long-period comets that are believed to have come from there, astronomers have learned a great deal about it structure and composition.

Definition:

The Oort Cloud is a theoretical spherical cloud of predominantly icy planetesimals that is believed to surround the Sun at a distance of up to around 100,000 AU (2 ly). This places it in interstellar space, beyond the Sun’s Heliosphere where it defines the cosmological boundary between the Solar System and the region of the Sun’s gravitational dominance.

Like the Kuiper Belt and the Scattered Disc, the Oort Cloud is a reservoir of trans-Neptunian objects, though it is over a thousands times more distant from our Sun as these other two. The idea of a cloud of icy infinitesimals was first proposed in 1932 by Estonian astronomer Ernst Öpik, who postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.

In 1950, the concept was resurrected by Jan Oort, who independently hypothesized its existence to explain the behavior of long-term comets. Although it has not yet been proven through direct observation, the existence of the Oort Cloud is widely accepted in the scientific community.

Structure and Composition:

The Oort Cloud is thought to extend from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun, though some estimates place the outer edge as far as 100,000 and 200,000 AU (1.58 and 3.16 ly). The Cloud is thought to be comprised of two regions – a spherical outer Oort Cloud of 20,000 – 50,000 AU (0.32 – 0.79 ly), and disc-shaped inner Oort (or Hills) Cloud of 2,000 – 20,000 AU (0.03 – 0.32 ly).

The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi), and billions that measure 20 kilometers (12 mi) in diameter. Its total mass is not known, but – assuming that Halley’s Comet is a typical representation of outer Oort Cloud objects – it has the combined mass of roughly 3×1025 kilograms (6.6×1025 pounds), or five Earths.

Based on the analyses of past comets, the vast majority of Oort Cloud objects are composed of icy volatiles – such as water, methane, ethane, carbon monoxide, hydrogen cyanide, and ammonia. The appearance of asteroids thought to be originating from the Oort Cloud has also prompted theoretical research that suggests that the population consists of 1-2% asteroids.

Earlier estimates placed its mass up to 380 Earth masses, but improved knowledge of the size distribution of long-period comets has led to lower estimates. The mass of the inner Oort Cloud, meanwhile, has yet to be characterized. The contents of both Kuiper Belt and the Oort Cloud are known as Trans-Neptunian Objects (TNOs), because the objects of both regions have orbits that that are further from the Sun than Neptune’s orbit.

A belt of comets called the Oort Cloud is theorized to encircle the Solar system (image credit: NASA/JPL).
A belt of comets called the Oort Cloud is theorized to encircle the Solar system (image credit: NASA/JPL).

Origin:

The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago. The most widely accepted hypothesis is that the Oort cloud’s objects initially coalesced much closer to the Sun as part of the same process that formed the planets and minor planets, but that gravitational interaction with young gas giants such as Jupiter ejected them into extremely long elliptic or parabolic orbits.

Recent research by NASA suggests that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart. It is also suggested that many – possibly the majority – of Oort cloud objects were not formed in close proximity to the Sun.

Alessandro Morbidelli of the Observatoire de la Cote d’Azur has conducted simulations on the evolution of the Oort cloud from the beginnings of the Solar System to the present. These simulations indicate that gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This is offered as an explanation for why the outer Oort Cloud is nearly spherical in shape while the Hills cloud, which is bound more strongly to the Sun, has not acquired a spherical shape.

A comparison of the Solar System and its Oort Cloud. 70,000 years ago, Scholz's Star and companion passed along the outer boundaries of our Solar System (Credit: NASA, Michael Osadciw/University of Rochester)
A comparison of the Solar System and its Oort Cloud. 70,000 years ago, Scholz’s Star and companion passed along the outer boundaries of our Solar System. Credit: NASA, Michael Osadciw/University of Rochester

Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud’s formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.

Comets:

Comets are thought to have two points of origin within the Solar System. They start as infinitesimals in the Oort Cloud and then become comets when passing stars knock some of them out of their orbits, sending into a long-term orbit that take them into the inner solar system and out again.

Short-period comets have orbits that last up to two hundred years while the orbits of long-period comets can last for thousands of years. Whereas short-period comets are believed to have emerged from either the Kuiper Belt or the scattered disc, the accepted hypothesis is that long-period comets originate in the Oort Cloud. However, there are some exceptions to this rule.

For example, there are two main varieties of short-period comet: Jupiter-family comets and Halley-family comets. Halley-family comets, named for their prototype (Halley’s Comet) are unusual in that although they are short in period, they are believed to have originated from the Oort cloud. Based on their orbits, it is suggested they were once long-period comets that were captured by the gravity of a gas giant and sent into the inner Solar System.

Evolution of a comet as it orbits the sun. Credit: Laboratory for Atmospheric and Space Sciences/ NASA
Evolution of a comet as it orbits the sun. Credit: Laboratory for Atmospheric and Space Sciences/ NASA

Exploration:

Because the Oort Cloud is so much farther out than the Kuiper Belt, the region remained unexplored and largely undocumented. Space probes have yet to reach the area of the Oort cloud, and Voyager 1 – the fastest and farthest of the interplanetary space probes currently exiting the Solar System – is not likely to provide any information on it.

At its current speed, Voyager 1 will reach the Oort cloud in about 300 years, and will will take about 30,000 years to pass through it. However, by around 2025, the probe’s radioisotope thermoelectric generators will no longer supply enough power to operate any of its scientific instruments. The other four probes currently escaping the Solar System – Voyager 2, Pioneer 10 and 11, and New Horizons – will also be non-functional when they reach the Oort cloud.

Exploring the Oort Cloud presents numerous difficulties, most of which arise from the fact that it is incredible distant from Earth. By the time a robotic probe could actually reach it and begin exploring the area in earnest, centuries will have passed here on Earth. Not only would those who had sent it out in the first place be long dead, but humanity will have most likely invented far more sophisticated probes or even manned craft in the meantime.

Still, studies can be (and are) conducted by examining the comets that it periodically spits out, and long-range observatories are likely to make some interesting discoveries from this region of space in the coming years. It’s a big cloud. Who knows what we might find lurking in there?

We have many interesting articles about the Oort Cloud and Solar System for Universe Today. Here’s an article about how big the Solar System is, and one on the diameter of the Solar System. And here’s all you need to know about Halley’s Comet and Beyond Pluto.

You might also want to check out this article from NASA on the Oort Cloud and one from the University of Michigan on the origin of comets.

Do not forget to take a look at the podcast from Astronomy Cast. Episode 64: Pluto and the Icy Outer Solar System and Episode 292: The Oort Cloud.

Reference:
NASA Solar System Exploration: Kuiper Belt & Oort Cloud

Weekly Space Hangout – February 20, 2015 – Charles Black from SEN

Host: Fraser Cain (@fcain)

Guests:
Ramin Skibba (@raminskibba)
Dave Dickinson (@astroguyz / www.astroguyz.com)

Special Guest: Charles Black (@charlesblack / sen.com/charles-black)
Continue reading “Weekly Space Hangout – February 20, 2015 – Charles Black from SEN”