When the Apollo astronauts returned to Earth, they came bearing 380.96 kilograms (839.87 lb) of Moon rocks. From the study of these samples, scientists learned a great deal about the Moon’s composition, as well as its history of formation and evolution. For example, the fact that some of these rocks were magnetized revealed that roughly 3 billion years ago, the Moon had a magnetic field.
Much like Earth, this field would have been the result of a dynamo effect in the Moon’s core. But until recently, scientists have been unable to explain how the Moon could maintain such a dynamo effect for so long. But thanks to a new study by a team of scientists from the Astromaterials Research and Exploration Science (ARES) Division at NASA’s Johnson Space Center, we might finally have a answer.
To recap, the Earth’s magnetic core is an integral part of what keeps our planet habitable. Believed to be the result of a liquid outer core that rotates in the opposite direction as the planet, this field protects the surface from much of the Sun’s radiation. It also ensures that our atmosphere is not slowly stripped away by solar wind, which is what happened with Mars.
For the sake of their study, which was recently published in the journal Earth and Planetary Science Letters, the ARES team sought to determine how a molten, churning core could generate a magnetic field on the Moon. While scientists have understood how the Moon’s core could have powered such a field in the past, they have been unclear as to how it could have been maintained it for such a long time.
Towards this end, the ARES team considered multiple lines of geochemical and geophysical evidence to put constraints on the core’s composition. As Kevin Righter, the lead of the JSC’s high pressure experimental petrology lab and the lead author of the study, explained in a NASA press release:
“Our work ties together physical and chemical constraints and helps us understand how the moon acquired and maintained its magnetic field – a difficult problem to tackle for any inner solar system body. We created several synthetic core compositions based on the latest geochemical data from the moon, and equilibrated them at the pressures and temperatures of the lunar interior.”
Specifically, the ARES scientists conducted simulations of how the core would have evolved over time, based on varying levels of nickel, sulfur and carbon content. This consisted of preparing powders or iron, nickel, sulfur and carbon and mixing them in the proper proportions – based on recent analyses of Apollo rock samples.
Once these mixtures were prepared, they subjected them to heat and pressure conditions consistent with what exists at the Moon’s core. They also varied these temperatures and pressures based on the possibility that the Moon underwent changes in temperature during its early and later history – i.e. hotter during its early history and cooler later on.
What they found was that a lunar core composed of iron/nickel that had a small amount of sulfur and carbon – specifically 0.5% sulfur and 0.375% carbon by weight – fit the bill. Such a core would have a high melting point and would have likely started crystallizing early in the Moon’s history, thus providing the necessary heat to drive the dynamo and power a lunar magnetic field.
This field would have eventually died out after heat flow led the core to cool, thus arresting the dynamo effect. Not only do these results provide an explanation for all the paleomagnetic and seismic data we currently have on the Moon, it is also consistent with everything we know about the Moon’s geochemical and geophysical makeup.
Prior to this, core models tended to place the Moon’s sulfur content much higher. This would mean that it had a much lower melting point, and would have meant crystallization could not have occurred until much more recently in its history. Other theories have been proposed, ranging from sheer forces to impacts providing the necessary heat to power a dynamo.
However, the ARES team’s study provides a much simpler explanation, and one which happens to fit with all that we know about the Moon. Naturally, additional studies will be needed before there is any certainty on the issue. No doubt, this will first require that human beings establish a permanent outpost on the Moon to conduct research.
But it appears that for the time being, one of the deeper mysteries of the Earth-Moon system might be resolved at last.
For centuries, scientists have been attempting to explain how the Moon formed. Whereas some have argued that it formed from material lost by Earth due to centrifugal force, others asserted that a preformed Moon was captured by Earth’s gravity. In recent decades, the most widely-accepted theory has been the Giant-impact hypothesis, which states that the Moon formed after the Earth was struck by a Mars-sized object (named Theia) 4.5 billion years ago.
According to a new study by an international team of researchers, the key to proving which theory is correct may come from the first nuclear tests conducted here on Earth, some 70 years ago. After examining samples of radioactive glass obtained from the Trinity test site in New Mexico (where the first atomic bomb was detonated), they determined that samples of Moon rocks showed a similar depletion of volatile elements.
For decades, scientists have been of the belief that the Moon, Earth’s only natural satellite, was four and a half billion years old. According to this theory, the Moon was created from a fiery cataclysm produced by a collision between the Earth with a Mars-sized object (named Theia) roughly 100 million years after the formation of primordial Earth.
But according to a new study by researchers from UCLA (who re-examined some of the Apollo Moon Rocks), these estimates may have been off by about 40 to 140 million years. Far from simply adjusting our notions of the Moon’s proper age, these findings are also critical to our understanding of the Solar System and the formation and evolution of its rocky planets.
These fragments were of a compound known as zircon, a type of silicate mineral that contains trace amounts of radioactive elements (like uranium, thorium, and lutetium). As Kevin McKeegan, a UCLA professor of geochemistry and cosmochemistry and a co-author of the study, explained, “Zircons are nature’s best clocks. They are the best mineral in preserving geological history and revealing where they originated.”
By examining the radioactive decay of these elements, and correcting for cosmic ray exposure, the research team was able to get highly precise estimates of the zircon fragments ages. Using one of UCLA’s mass spectrometers, they were able to measure the rate at which the deposits of uranium in the zircon turned into lead, and the deposits of lutetium turned into hafnium.
In the end, their data indicated that the Moon formed about 4.51 billion years ago, which places its birth within the first 60 million years of the Solar System or so. Previously, dating Moon rocks proved difficult, mainly because most of them contained fragments of many different kinds of rocks, and these samples were determined to be tainted by the effects of multiple impacts.
However, Barboni and her team were able to examine eight zircons that were in good condition. More importantly, these silicate deposits are believed to have formed shortly after the collision between Earth and Theia, when the Moon was still an unsolidified mass covered in oceans of magma. As these oceans gradually cooled, the Moon’s body became differentiated between its crust, mantle and core.
Because zircon minerals were formed during the initial magma ocean, uranium-lead dating reaches all the way back to a time before the Moon became a solidified mass. As Edward Young, a UCLA professor of geochemistry and cosmochemistry and a co-author of the study, put it, “Mélanie was very clever in figuring out the Moon’s real age dates back to its pre-history before it solidified, not to its solidification.”
These findings have not only determined the age of the Moon with a high degree of accuracy (and for the first time), it also has implications for our understanding of when and how rocky planes formed within the Solar System. By placing accurate dates on when certain bodies formed, we are able to understand the context in which they formed, which also helps to determine what mechanisms were involved.
And this was just the first revelation produced by the research team, which hopes to continue studying the zircon fragments to see what they can learn about the Moon’s early history.
Science fiction has told us again and again, we belong out there, among the stars. But before we can build that vast galactic empire, we’ve got to learn how to just survive in space. Fortunately, we happen to live in a Solar System with many worlds, large and small that we can use to become a spacefaring civilization.
This is half of an epic two-part article that I’m doing with Isaac Arthur, who runs an amazing YouTube channel all about futurism, often about the exploration and colonization of space. Make sure you subscribe to his channel.
This article is about colonizing the inner Solar System, from tiny Mercury, the smallest planet, out to Mars, the focus of so much attention by Elon Musk and SpaceX. In the other article, Isaac will talk about what it’ll take to colonize the outer Solar System, and harness its icy riches. You can read these articles in either order, just read them both.
At the time I’m writing this, humanity’s colonization efforts of the Solar System are purely on Earth. We’ve exploited every part of the planet, from the South Pole to the North, from huge continents to the smallest islands. There are few places we haven’t fully colonized yet, and we’ll get to that.
But when it comes to space, we’ve only taken the shortest, most tentative steps. There have been a few temporarily inhabited space stations, like Mir, Skylab and the Chinese Tiangong Stations.
Our first and only true colonization of space is the International Space Station, built in collaboration with NASA, ESA, the Russian Space Agency and other countries. It has been permanently inhabited since November 2nd, 2000. Needless to say, we’ve got our work cut out for us.
Before we talk about the places and ways humans could colonize the rest of the Solar System, it’s important to talk about what it takes to get from place to place.
Just to get from the surface of Earth into orbit around our planet, you need to be going about 10 km/s sideways. This is orbit, and the only way we can do it today is with rockets. Once you’ve gotten into Low Earth Orbit, or LEO, you can use more propellant to get to other worlds.
If you want to travel to Mars, you’ll need an additional 3.6 km/s in velocity to escape Earth gravity and travel to the Red Planet. If you want to go to Mercury, you’ll need another 5.5 km/s.
And if you wanted to escape the Solar System entirely, you’d need another 8.8 km/s. We’re always going to want a bigger rocket.
The most efficient way to transfer from world to world is via the Hohmann Transfer. This is where you raise your orbit and drift out until you cross paths with your destination. Then you need to slow down, somehow, to go into orbit.
One of our primary goals of exploring and colonizing the Solar System will be to gather together the resources that will make future colonization and travel easier. We need water for drinking, and to split it apart for oxygen to breathe. We can also turn this water into rocket fuel. Unfortunately, in the inner Solar System, water is a tough resource to get and will be highly valued.
We need solid ground. To build our bases, to mine our resources, to grow our food, and to protect us from the dangers of space radiation. The more gravity we can get the better, since low gravity softens our bones, weakens our muscles, and harms us in ways we don’t fully understand.
Each world and place we colonize will have advantages and disadvantages. Let’s be honest, Earth is the best place in the Solar System, it’s got everything we could ever want and need. Everywhere else is going to be brutally difficult to colonize and make self-sustaining.
We do have one huge advantage, though. Earth is still here, we can return whenever we like. The discoveries made on our home planet will continue to be useful to humanity in space through communications, and even 3D printing. Once manufacturing is sophisticated enough, a discovery made on one world could be mass produced half a solar system away with the right raw ingredients.
We will learn how to make what we need, wherever we are, and how to transport it from place to place, just like we’ve always done.
Mercury is the closest planet from the Sun, and one of the most difficult places that we might attempt the colonize. Because it’s so close to the Sun, it receives an enormous amount of energy. During the day, temperatures can reach 427 C, but without an atmosphere to trap the heat, night time temperatures dip down to -173 C. There’s essentially no atmosphere, 38% the gravity of Earth, and a single solar day on Mercury lasts 176 Earth days.
Mercury does have some advantages, though. It has an average density almost as high as Earth, but because of its smaller size, it actually means it has a higher percentage of metal than Earth. Mercury will be incredibly rich in metals and minerals that future colonists will need across the Solar System.
With the lower gravity and no atmosphere, it’ll be far easier to get that material up into orbit and into transfer trajectories to other worlds.
But with the punishing conditions on the planet, how can we live there? Although the surface of Mercury is either scorching or freezing, NASA’s MESSENGER spacecraft turned up regions of the planet which are in eternal shadow near the poles. In fact, these areas seem to have water ice, which is amazing for anywhere this close to the Sun.
You could imagine future habitats huddled into those craters, pulling in solar power from just over the crater rim, using the reservoirs of water ice for air, fuel and water.
High powered solar robots could scour the surface of Mercury, gathering rare metals and other minerals to be sent off world. Because it’s bathed in the solar winds, Mercury will have large deposits of Helium-3, useful for future fusion reactors.
Over time, more and more of the raw materials of Mercury will find their way to the resource hungry colonies spread across the Solar System.
It also appears there are lava tubes scattered across Mercury, hollows carved out by lava flows millions of years ago. With work, these could be turned into safe, underground habitats, protected from the radiation, high temperatures and hard vacuum on the surface.
With enough engineering ability, future colonists will be able to create habitats on the surface, wherever they like, using a mushroom-shaped heat shield to protect a colony built on stilts to keep it off the sun-baked surface.
Mercury is smaller than Mars, but is a good deal denser, so it has about the same gravity, 38% of Earth’s. Now that might turn out to be just fine, but if we need more, we have the option of using centrifugal force to increase it. Space Stations can generate artificial gravity by spinning, but you can combine normal gravity with spin-gravity to create a stronger field than either would have.
So our mushroom habitat’s stalk could have an interior spinning section with higher gravity for those living inside it. You get a big mirror over it, shielding you from solar radiation and heat, you have stilts holding it off the ground, like roots, that minimize heat transfer from the warmer areas of ground outside the shield, and if you need it you have got a spinning section inside the stalk. A mushroom habitat.
Venus is the second planet in the Solar System, and it’s the evil twin of Earth. Even though it has roughly the same size, mass and surface gravity of our planet, it’s way too close to the Sun. The thick atmosphere acts like a blanket, trapping the intense heat, pushing temperatures at the surface to 462 C.
Everywhere on the planet is 462 C, so there’s no place to go that’s cooler. The pure carbon dioxide atmosphere is 90 times thicker than Earth, which is equivalent to being a kilometer beneath the ocean on Earth.
In the beginning, colonizing the surface of Venus defies our ability. How do you survive and stay cool in a thick poisonous atmosphere, hot enough to melt lead? You get above it.
One of the most amazing qualities of Venus is that if you get into the high atmosphere, about 52.5 kilometers up, the air pressure and temperature are similar to Earth. Assuming you can get above the poisonous clouds of sulphuric acid, you could walk outside a floating colony in regular clothes, without a pressure suit. You’d need a source of breathable air, though.
Even better, breathable air is a lifting gas in the cloud tops of Venus. You could imagine a future colony, filled with breathable air, floating around Venus. Because the gravity on Venus is roughly the same as Earth, humans wouldn’t suffer any of the side effects of microgravity. In fact, it might be the only place in the entire Solar System other than Earth where we don’t need to account for low gravity.
Now the day on Venus is incredibly long, 243 earth days, so if you stay over the same place the whole time it would be light for four months then dark for four months. Not ideal for solar power on a first glance, but Venus turns so slowly that even at the equator you could stay ahead of the sunset at a fast walk.
So if you have floating colonies it would take very little effort to stay constantly on the light side or dark side or near the twilight zone of the terminator. You are essentially living inside a blimp, so it may as well be mobile. And on the day side it would only take a few solar panels and some propellers to stay ahead. And since it is so close to the Sun, there’s plenty of solar power. What could you do with it?
The atmosphere itself would probably serve as a source of raw materials. Carbon is the basis for all life on Earth. We’ll need it for food and building materials in space. Floating factories could process the thick atmosphere of Venus, to extract carbon, oxygen, and other elements.
Heat resistant robots could be lowered down to the surface to gather minerals and then retrieved before they’re cooked to death.
Venus does have a high gravity, so launching rockets up into space back out of Venus’ gravity well will be expensive.
Over longer periods of time, future colonists might construct large solar shades to shield themselves from the scorching heat, and eventually, even start cooling the planet itself.
The next planet from the Sun is Earth, the best planet in the Solar System. One of the biggest advantages of our colonization efforts will be to get heavy industry off our planet and into space. Why pollute our atmosphere and rivers when there’s so much more space… in space.
Over time, more and more of the resource gathering will happen off world, with orbital power generation, asteroid mining, and zero gravity manufacturing. Earth’s huge gravity well means that it’s best to bring materials down to Earth, not carry them up to space.
However, the normal gravity, atmosphere and established industry of Earth will allow us to manufacture the lighter high tech goods that the rest of the Solar System will need for their own colonization efforts.
But we haven’t completely colonized Earth itself. Although we’ve spread across the land, we know very little about the deep ocean. Future colonies under the oceans will help us learn more about self-sufficient colonies, in extreme environments. The oceans on Earth will be similar to the oceans on Europa or Enceladus, and the lessons we learn here will teach us to live out there.
As we return to space, we’ll colonize the region around our planet. We’ll construct bigger orbital colonies in Low Earth Orbit, building on our lessons from the International Space Station.
One of the biggest steps we need to take, is understanding how to overcome the debilitating effects of microgravity: the softened bones, weakened muscles and more. We need to perfect techniques for generating artificial gravity where there is none.
The best technique we have is rotating spacecraft to generate artificial gravity. Just like we saw in 2001, and The Martian, by rotating all or a portion of a spacecraft, you can generated an outward centrifugal force that mimics the acceleration of gravity. The larger the radius of the space station, the more comfortable and natural the rotation feels.
Low Earth Orbit also keeps a space station within the Earth’s protective magnetosphere, limiting the amount of harmful radiation that future space colonists will experience.
Other orbits are useful too, including geostationary orbit, which is about 36,000 kilometers above the surface of the Earth. Here spacecraft orbit the Earth at exactly the same rate as the rotation of Earth, which means that stations appear in fixed positions above our planet, useful for communication.
Geostationary orbit is higher up in Earth’s gravity well, which means these stations will serve a low-velocity jumping off points to reach other places in the Solar System. They’re also outside the Earth’s atmospheric drag, and don’t require any orbital boosting to keep them in place.
By perfecting orbital colonies around Earth, we’ll develop technologies for surviving in deep space, anywhere in the Solar System. The same general technology will work anywhere, whether we’re in orbit around the Moon, or out past Pluto.
When the technology is advanced enough, we might learn to build space elevators to carry material and up down from Earth’s gravity well. We could also build launch loops, electromagnetic railguns that launch material into space. These launch systems would also be able to loft supplies into transfer trajectories from world to world throughout the Solar System.
Earth orbit, close to the homeworld gives us the perfect place to develop and perfect the technologies we need to become a true spacefaring civilization. Not only that, but we’ve got the Moon.
The Moon, of course, is the Earth’s only natural satellite, which orbits us at an average distance of about 400,000 kilometers. Almost ten times further than geostationary orbit.
The Moon takes a surprising amount of velocity to reach from Low Earth Orbit. It’s close, but expensive to reach, thrust speaking.
But that fact that it’s close makes the Moon an ideal place to colonize. It’s close to Earth, but it’s not Earth. It’s airless, bathed in harmful radiation and has very low gravity. It’s the place that humanity will learn to survive in the harsh environment of space.
But it still does have some resources we can exploit. The lunar regolith, the pulverized rocky surface of the Moon, can be used as concrete to make structures. Spacecraft have identified large deposits of water at the Moon’s poles, in its permanently shadowed craters. As with Mercury, these would make ideal locations for colonies.
Our spacecraft have also captured images of openings to underground lava tubes on the surface of the Moon. Some of these could be gigantic, even kilometers high. You could fit massive cities inside some of these lava tubes, with room to spare.
Helium-3 from the Sun rains down on the surface of the Moon, deposited by the Sun’s solar wind, which could be mined from the surface and provide a source of fuel for lunar fusion reactors. This abundance of helium could be exported to other places in the Solar System.
The far side of the Moon is permanently shadowed from Earth-based radio signals, and would make an ideal location for a giant radio observatory. Telescopes of massive size could be built in the much lower lunar gravity.
We talked briefly about an Earth-based space elevator, but an elevator on the Moon makes even more sense. With the lower gravity, you can lift material off the surface and into lunar orbit using cables made of materials we can manufacture today, such as Zylon or Kevlar.
One of the greatest threats on the Moon is the dusty regolith itself. Without any kind of weathering on the surface, these dust particles are razor sharp, and they get into everything. Lunar colonists will need very strict protocols to keep the lunar dust out of their machinery, and especially out of their lungs and eyes, otherwise it could cause permanent damage.
Although the vast majority of asteroids in the Solar System are located in the main asteroid belt, there are still many asteroids orbiting closer to Earth. These are known as the Near Earth Asteroids, and they’ve been the cause of many of Earth’s great extinction events.
These asteroids are dangerous to our planet, but they’re also an incredible resource, located close to our homeworld.
The amount of velocity it takes to get to some of these asteroids is very low, which means travel to and from these asteroids takes little energy. Their low gravity means that extracting resources from their surface won’t take a tremendous amount of energy.
And once the orbits of these asteroids are fully understood, future colonists will be able to change the orbits using thrusters. In fact, the same system they use to launch minerals off the surface would also push the asteroids into safer orbits.
These asteroids could be hollowed out, and set rotating to provide artificial gravity. Then they could be slowly moved into safe, useful orbits, to act as space stations, resupply points, and permanent colonies.
There are also gravitationally stable points at the Sun-Earth L4 and L5 Lagrange Points. These asteroid colonies could be parked there, giving us more locations to live in the Solar System.
The future of humanity will include the colonization of Mars, the fourth planet from the Sun. On the surface, Mars has a lot going for it. A day on Mars is only a little longer than a day on Earth. It receives sunlight, unfiltered through the thin Martian atmosphere. There are deposits of water ice at the poles, and under the surface across the planet.
Martian ice will be precious, harvested from the planet and used for breathable air, rocket fuel and water for the colonists to drink and grow their food. The Martian regolith can be used to grow food. It does have have toxic perchlorates in it, but that can just be washed out.
The lower gravity on Mars makes it another ideal place for a space elevator, ferrying goods up and down from the surface of the planet.
Unlike the Moon, Mars has a weathered surface. Although the planet’s red dust will get everywhere, it won’t be toxic and dangerous as it is on the Moon.
Like the Moon, Mars has lava tubes, and these could be used as pre-dug colony sites, where human Martians can live underground, protected from the hostile environment.
Mars has two big problems that must be overcome. First, the gravity on Mars is only a third that of Earth’s, and we don’t know the long term impact of this on the human body. It might be that humans just can’t mature properly in the womb in low gravity.
Researchers have proposed that Mars colonists might need to spend large parts of their day on rotating centrifuges, to simulate Earth gravity. Or maybe humans will only be allowed to spend a few years on the surface of Mars before they have to return to a high gravity environment.
The second big challenge is the radiation from the Sun and interstellar cosmic rays. Without a protective magnetosphere, Martian colonists will be vulnerable to a much higher dose of radiation. But then, this is the same challenge that colonists will face anywhere in the entire Solar System.
That radiation will cause an increased risk of cancer, and could cause mental health issues, with dementia-like symptoms. The best solution for dealing with radiation is to block it with rock, soil or water. And Martian colonists, like all Solar System colonists will need to spend much of their lives underground or in tunnels carved out of rock.
In addition to Mars itself, the Red Planet has two small moons, Phobos and Deimos. These will serve as ideal places for small colonies. They’ll have the same low gravity as asteroid colonies, but they’ll be just above the gravity well of Mars. Ferries will travel to and from the Martian moons, delivering fresh supplies and sending Martian goods out to the rest of the Solar System.
We’re not certain yet, but there are good indicators these moons might have ice inside them, if so that is an excellent source of fuel and could make initial trips to Mars much easier by allowing us to send a first expedition to those moons, who then begin producing fuel to be used to land on Mars and to leave Mars and return home.
According to Elon Musk, if a Martian colony can reach a million inhabitants, it’ll be self-sufficient from Earth or any other world. At that point, we would have a true, Solar System civilization.
Now, continue on to the other half of this article, written by Isaac Arthur, where he talks about what it will take to colonize the outer Solar System. Where water ice is plentiful but solar power is feeble. Where travel times and energy require new technologies and techniques to survive and thrive.
Since time immemorial, people have been staring up at the Moon with awe and wonder. For as long as there has been life on this planet, the Moon has been orbiting it. And as time went on, scholars and astronomers began to observe it regularly and calculate its orbit. In so doing, they learned some rather interesting things about its behavior.
For example, the Moon has an orbital period that is the same as its rotational period. In essence, it is tidally locked to the Earth, which means that it always presents the same face to us as it orbits around our planet. And during the course of its orbit, it also appears larger and smaller in the sky, which is due to the fact it is sometimes closer than at other times.
For starters, the Moon follows an elliptical path around the Earth – with an average eccentricity of 0.0549 – which means that its orbit is not perfectly circular. Its average orbital distance is 384,748 km, which ranges from 364,397 km at its closest, to 406,731 km at its most distant.
This non-circular orbit causes variations in the Moon’s angular speed and apparent size as it moves towards and away from an observer on Earth. When it’s full and at its closest point to Earth (perigee), the Moon can look over 10% bigger, and 30% brighter than when it’s at a more distant point in its orbit (apogee).
The mean inclination of the Moon’s orbit to the ecliptic plane (i.e. the apparent path of the Sun through the sky) is 5.145°. Because of this inclination, the moon is above the horizon at the North and South Pole for almost two weeks every month, even though the Sun is below the horizon for six months out of the year.
The Moon’s sidereal orbital period and rotational period are the same – 27.3 days. This phenomena, known as synchronous rotation, is what allows for the same hemisphere to be facing Earth all the time. Hence why the far side is colloquially referred to as the “Dark Side”, but this name is misleading. As the Moon orbits Earth, different parts are in sunlight or darkness at different times and neither side is permanently dark or illuminated.
Because Earth is moving as well – rotating on its axis as it orbits the Sun – the Moon appears to orbit us every 29.53 days. This is known as its synodic period, which is the amount of time it takes for the Moon to reappear in the same place in the sky. During a synodic period, the Moon will go through changes in its appearance, which are known as “phases“.
These changes in appearance are due to the Moon receiving more or less illumination (from our perspective). A full cycle of these phases is known as a Lunar Cycle, which comes down to the Moon’s orbit around the Earth, and our mutual orbit around the Sun. When the Sun, the Moon and Earth are perfectly lined up, the angle between the Sun and the Moon is 0-degrees.
At this point, the side of the Moon facing the Sun is fully illuminated, and the side facing the Earth is enshrouded in darkness. We call this a New Moon. After this, the phase of the Moon changes, because the angle between the Moon and the Sun is increasing from our perspective. A week after a New Moon, and the Moon and Sun are separated by 90-degrees, which effects what we will see.
And then, when the Moon and Sun are on opposite sides of the Earth, they’re at 180-degrees – which corresponds to a Full Moon. The period in which a Moon will go from a New Moon to a Full Moon and back again is also known as “Lunar Month”. One of these lasts 28 days, and encompasses what are known as “waxing” and “waning” Moons. During the former period, the Moon brightens and its angle relative to the Sun and Earth increases.
When the Moon is in between the Earth and the Sun, the side of the Moon facing away from the Earth is fully illuminated, and the side we can see is shrouded in darkness. As the Moon orbits the Earth, the angle between the Moon and the Sun increases. At this point, the angle between the Moon and Sun is 0 degrees, which gradually increases over the next two weeks. This is what astronomers call a waxing moon.
After the first week, the angle between the Moon and the Sun is 90-degrees and continues to increase to 180-degrees, when the Sun and Moon are on opposite sides of the Earth. When the Moon starts to decrease its angle again, going from 180-degrees back down to 0-degrees, astronomers say that it’s a waning moon. In other words, when the Moon is waning, it will have less and less illumination every night until it’s a New Moon.
When the Moon is no longer full, but it hasn’t reached a quarter moon – i.e. when it’s half illuminated from our perspective – we say that it’s a Waning Gibbous Moon. This is the exact reverse of a Waxing Gibbous Moon, when the Moon is increasing in brightness from a New Moon to a Full Moon.
This is followed by a Third Quarter (or last quarter) Moon. During this period, 50% of the Moon’s disc will be illuminated (left side in the northern hemisphere, and the right in the southern), which is the opposite of how it would appear during a First Quarter. These phases are often referred to as a “Half Moon”, since half the disc is illuminated at the time.
Finally, a Waning Crescent is when the Moon appears as a sliver in the night sky, where between 49–1% of one side is illuminated after a Full Moon (again, left in the northern hemisphere, right in the southern). This is the opposite of a Waxing Crescent, when 1-49% of the other wide is illuminated before it reaches a Full Moon.
Future of the Moon’s Orbit:
Currently, the Moon’s is slowly drifting away from the Earth, at a rate of about 1 to 2 cm per year. This is directly related to the fact that here on Earth, the day’s are getting longer – by a rate of 1/500th of a second every century. In fact, astronomers have estimated that roughly 620 million years ago, a day was only 21 hours long, and the Moon was between 6,200 – 12,400 km closer.
Now, the days are 24 hours long and getting longer, and the Moon is already at a average distance of 384,400 km. Eventually, the Earth and the Moon will be tidally locked to each other, so the same side of the Earth will always face the Moon, just like the same side of the Moon always presents the same face to the Earth. But this won’t happen for billions of years from now.
For as long as human beings have been staring up at the night sky, the Moon has been a part of our world. And over the course of the roughly 4.5 billion years that it has been our only natural satellite, the relationship between it and our planet has changed. As time goes on, it will continue to change; but to us, it will still be the Moon.
Have you ever heard that it’s possible to buy property on the Moon? Perhaps someone has told you that, thanks to certain loopholes in the legal code, it is possible to purchase your very own parcel of lunar land. And in truth, many celebrities have reportedly bought into this scheme, hoping to snatch up their share of land before private companies or nations do.
Despite the fact that there may be several companies willing to oblige you, the reality is that international treaties say that no nation owns the Moon. These treaties also establish that the Moon is there for the good of all humans, and so it’s impossible for any state to own any lunar land. But does that mean private ownership is impossible too? The short answer is yes.
The long answer is, it’s complicated. At present, there are multiple nations hoping to build outposts and settlements on the Moon in the coming decades. The ESA hopes to build a “international village” between 2020 and 2030 and NASA has plans for its own for a Moon base.
Because of this, a lot of attention has been focused lately on the existing legal framework for the Moon and other celestial bodies. Let’s take a look at the history of “space law”, shall we?
Outer Space Treaty:
On Jan. 27th, 1967, the United States, United Kingdom, and the Soviet Union sat down together to work out a treaty on the exploration and use of outer space. With the Soviets and Americans locked in the Space Race, there was fear on all sides that any power that managed to put resources into orbit, or get to the Moon first, might have an edge on the others – and use these resources for evil!
The treaty is overseen the United Nations Office for Outer Space Affairs (UNOOSA). It’s a big document, with lots of articles, subsections, and legalese. But the most relevant clause is Article II of the treaty, where it states:
“Outer space, including the moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means.”
“Loophole” in the Treaty:
Despite clearly saying that Outer Space is the property of all humanity, and can only be used for the good of all, the language is specific to national ownership. As a result, there is no legal consensus on whether or not the treaty’s prohibition are also valid as far as private appropriation is concerned.
However, Article II addresses only the issue of national ownership, and contains no specific language about the rights of private individuals or bodies in owning anything in outer space. Because of this, there are some who have argued that property rights should be recognized on the basis of jurisdiction rather than territorial sovereignty.
Looking to Article VI though, it states that governments are responsible for the actions of any party therein. So it is clear that the spirit of the treaty is meant to apply to all entities, be they public or private. As it states:
“States Parties to the Treaty shall bear international responsibility for national activities in outer space, including the moon and other celestial bodies, whether such activities are carried on by governmental agencies or by non-governmental entities, and for assuring that national activities are carried out in conformity with the provisions set forth in the present Treaty. The activities of non-governmental entities in outer space, including the moon and other celestial bodies, shall require authorization and continuing supervision by the appropriate State Party to the Treaty.”
In other words, any person, organization or company operating in space is answerable to their respective government. But since no specific mention is made of private ownership, there are those who claim that this represents a “loophole” in the treaty which allows them to claim and sell land on the Moon at this time. Because of this ambiguity, there have been attempts to augment the Outer Space Treaty.
The Moon Treaty:
On Dec. 18th, 1979, members of the United Nations presented an agreement which was meant to be a follow-up to the Outer Space Treaty and close its supposed loopholes. Known as the “Agreement Governing the Activities of States on the Moon and Other Celestial Bodies” – aka. “The Moon Treaty” or “Moon Agreement” – this treaty intended to establish a legal framework for the use of the Moon and other celestial bodies.
Much like the Outer Space Treaty, the agreement established that the Moon should be used for the benefit of all humanity and not for the sake of any individual state. The treaty banned weapons testing, declared that any scientific research must be open and shared with the international community, and that nations and individuals and organizations could not claim anything.
In practice, the treaty failed because it has not been ratified by any state that engages in crewed space exploration or has domestic launch capability. This includes the United States, the larger members of the ESA, Russia, China, Japan and India. Though it expressly forbids both national and private ownership of land on the Moon, or the use thereof for non-scientific, non-universal purposes, the treaty effectively has no teeth.
Bottom line, there is nothing that expressly forbids companies from owning land on the Moon. However, with no way to claim that land, anyone attempting to sell land to prospective buyers is basically selling snake oil. Any documentation that claims you own land on the Moon is unenforceable, and no nation on the planet that has signed either the Outer Space Treaty or the Moon Treaty will recognize it.
Then again, if you were able to fly up to the Moon and build a settlement there, it would be pretty difficult for anyone to stop you. But don’t expect that to the be the last word on the issue. With multiple space agencies looking to create “international villages” and companies hoping to create a tourist industry, you could expect some serious legal battles down the road!
But of course, this is all academic. With no atmosphere to speak of, temperatures reaching incredible highs and lows – ranging from 100 °C (212 °F) to -173 °C (-279.4 °F) – its low gravity (16.5 % that of Earth), and all that harsh Moon dust, nobody outside of trained astronauts (or the clinically insane) should want to spend a significant amount of time there!
In the coming decades, the world’s largest space agencies all have some rather big plans. Between NASA, the European Space Agency (ESA), Roscosmos, the Indian Space Research Organisation (ISRO), or the China National Space Administration (CNSA), there are plans to return to the Moon, crewed missions to Mars, and crewed missions to Near-Earth Objects (NEOs).
In all cases, geological studies are going to be a major aspect of the mission. For this reason, the ESA recently unveiled a new training program known as the Pangaea course, a study program which focuses on identifying planetary geological features. This program showcases just how important planetary geologists will be to future missions.
Pangaea takes its name from the super-continent that that existed during the late Paleozoic and early Mesozoic eras (300 to 175 million years ago). Due to convection in Earth’s mantle, this continent eventually broke up, giving rise to the seven continents that we are familiar with today.
Francesco Sauro – a field geologist, explorer and the designer of the course – explained the purpose of Pangaea in an ESA press release:
“This Pangaea course – named after the ancient supercontinent – will help astronauts to find interesting rock samples as well as to assess the most likely places to find traces of life on other planets. We created a course that enables astronauts on future missions to other planetary bodies to spot the best areas for exploration and the most scientifically interesting rocks to take samples for further analysis by the scientists back on Earth.”
This first part of the course will take place this week, where astronaut trainer Matthias Maurer and astronauts Luca Parmitano and Pedro Duque will be learning from a panel of planetary geology experts. These lessons will include how to recognize certain types of rock, how to draw landscapes, and the exploration of a canyon that has sedimentary features similar to the ones observed in the Murray Buttes region, which was recently imaged by the Curiosity rover.
The geology panel will include such luminaries as Matteo Messironi (a geologist working on the Rosetta and ExoMars missions), Harald Hiesinger (an expert in lunar geology), Anna Maria Fioretti (a meteorite expert), and Nicolas Mangold (a Mars expert currently working with NASA’s Curiosity team).
Once this phase of the course is complete, a series of field trips will follow to locations that were chosen because their geological features resemble those of other planets. This will include the town of Bressanone in northeastern Italy, which lies a few kilometers outside of the Brenner Pass (the part of the Alps that lies between Italy and Austria).
This past summer, the latest program involved a team of six international astronauts spending two weeks in a cave network in Sardinia, Italy. In this environment, 800-meters (2625 ft) beneath the surface, the team carried out a series of research and exploration activities designed to recreate aspects of a space expedition.
As the teams explore the caves of Sardinia, they encountered caverns, underground lakes and examples of strange microscopic life – all things they could encounter in extra-terrestrial environments. While doing this, they also get the change to test out new technologies and methods for research and experiments.
In a way that is similar to expeditions aboard the ISS, the program was designed to teach an international team of astronauts how to address the challenges of living and working in confined spaces. These include limited privacy, less equipment for hygiene and comfort, difficult conditions, variable temperatures and humidity, and extremely difficult emergency evacuation procedures.
Above all, the program attempts to foster teamwork, communication skills, decision-making, problem-solving, and leadership. This program is now an integral part of the ESA’s astronaut training and is conducted once a year. And as project leader Loredana Bessone explained, the Pangaea course fits with the aims of the CAVES program quite well.
“Pangaea complements our CAVES underground training,” she said. “CAVES focuses on team behaviour and operational aspects of a space mission, whereas Pangaea focuses on developing knowledge and skills for planetary geology and astrobiology.”
From all of these efforts, it is clear that the ESA, NASA and other space agencies want to make sure that future generations of astronauts are trained to conduct field geology and will be able to identify targets for scientific research. But of course, understanding the importance planetary geology in space exploration is not exactly a new phenomenon.
In fact, the study of planetary geology is rooted in the Apollo era, when it became a field separate from other fields of geological research. And geology experts played a very pivotal role when it came to selecting the landing sites of the Apollo missions. As Emily Lakdawalla, the Senior Editor of The Planetary Society (and a geologist herself), told Universe Today in a phone interview:
“The Apollo astronauts received training in field geology before they went to the Moon. Jim Head at Brown University, who was my advisor, was one person who provided that training. Before there were missions, the Lunar Orbiter program returned photos that geologists used to map the surface of the Moon and find good landing sites.”
This tradition is being carried on today with instruments like the Mars Global Surveyor. Before the Spirit and Opportunity rovers were deployed to the Martian surface, NASA scientists studied images taken by this orbiter to determine which potential landing sites would prove to be the valuable for conducting research.
And thanks to the experience gained by the Apollo missions and improvements made in both technology and instrumentation, the process has become much more sophisticated. Compared to the Apollo-era, today’s NASA mission planners have much more detailed information to go on.
“These days, the orbiter photos have such high resolutions that its just like having aerial photographs, which is something Earth geologists have always used as a tool to scope out an area before going to study it,” Lakdawalla said. “With these photos, we can map out an area in detail before we send a rover, and determine where the most high-value samples will be.”
Looking ahead, everything that’s learned from sending astronauts to the Moon – and from the study of the lunar rocks they brought back – is going to play a vital role when it comes time to explore Mars, go back to the Moon, and investigate NEOs. As Lakdawalla explained, in each case, the purpose of the geological studies will be a bit different.
“The goal in obtaining samples from the Moon was about understanding the chronology of the Moon. The timescale we have developed for the Moon are anchored in the Apollo samples. But we think that the samples have been sampling one major impact – the Imbrium impact. The next Moon samples will attempt to sample other time periods so we can determine if our time scales are correct.”
“On Mars, the questions is, ‘what are the history of water on Mars’. You try to find rocks from orbit that will answer that questions – rocks that have either been altered by water or formed in water. And that is how you select your landing zone.”
And with future missions to NEOs, astronauts will be tasked with examining geological samples which date back to the formation of the Solar System. From this, we are likely to get a better understanding of how our Solar System formed and evolved over the many billion years it has existed.
Clearly, it is a good time to be a geologist, as their expertise will be called upon for future missions to space. Hope they like tang!
The Moon is great and all, but I wish it was closer. Close enough that I could see all kinds of detail on its surface without a telescope or a pair of binoculars. Close enough that I could just reach up and grab enough cheese for a lifetime of grilled cheese sandwiches.
Sure, there would be all kinds of horrible problems with having the Moon that much closer. Intense tides, a total lack of good dark nights for stargazing, and something else… Oh right, the total destruction of life on Earth. On second thought the Moon can stay right where it is, thank you very much.
The Earth’s Moon is located an average distance of 384,400 kilometers away. I say average because the Moon actually follows an elliptical orbit. At its closest point, it’s only 362,600 km, and at its furthest point, it’s 405,400 kilometers.
Still, that’s so far that it takes light a little over a second to reach the Moon, traveling almost 300,000 km/s. The Moon is far.
But what if the Moon was much closer? How close could it get and still be the Moon?
Once again, I need to remind you that this is purely theoretical. The Moon isn’t getting closer to us, in fact, it’s getting further. The Moon is slowly drifting away from us at a distance of almost 4 centimeters per year.
Let’s go back to the beginning, when the young Earth collided with a Mars-sized planet billions of years ago. This catastrophic encounter completely resurfaced planet Earth, and kicked up a massive amount of debris into orbit. Well, a Moon’s worth of debris, which collected together by mutual gravity into the roughly spherical Moon we recognize today.
Shortly after its formation, the Moon was much closer, and the Earth was spinning more rapidly. A day on Earth was only 6 hours long, and the Moon took just 17 days to orbit the Earth.
The Earth’s gravity stopped the Moon’s relative rotation, and the Moon’s gravity has been slowing the Earth’s rotation. To maintain the overall angular momentum of the system, the Moon has been drifting away to compensate.
This conservation of momentum is very important because it works both ways. As long as a moon takes longer than a day to orbit its planet, you’re going to see this same effect. The planet’s rotation slows, and the moon drifts further to compensate.
But if you have a scenario where the moon orbits faster than the planet rotates, you have the exact opposite situation. The moon makes the planet rotate more quickly, and it drifts closer to compensate. This can’t end well.
Once you get close enough, gravity becomes a harsh mistress.
There’s a point in all gravitational interactions called the Roche Limit. This is the point at which an object held together by gravity (like the Moon), gets close enough to another celestial body that it gets torn apart.
The exact point depends on the mass, size and density of the two objects. For example, the Roche Limit between the Earth and the Moon is about 9,500 kilometers, assuming the Moon is a solid ball. In other words, if the Moon gets within 9,500 kilometers or so, of the Earth, the gravity of the Earth overwhelms the gravity holding the Moon together.
The Moon would be torn apart, and turned into a ring. And then the pieces of the ring would continue to orbit the Earth until they all came crashing down. When that happened, it would be a series of very bad days for anyone living on Earth.
If an average comet got within about 18,000 km of Earth, it would get torn to pieces. While the Sun can, and does, tear apart comets from about 1.3 million km away.
This sounds purely theoretical, but this is actually going to happen over at Mars. Its largest moon Phobos orbits more quickly than a Martian day, which means that it’s drifting closer and closer to the planet. In a few million years, it’ll cross the Roche Limit, tear into a ring, and then all the pieces of the former Phobos will crash down onto Mars. We did a whole article on this.
Now you might be wondering, wait a second. I’m a separate object from the Earth, why don’t I get torn apart since I’m definitely within the Earth’s Roche Limit.
You do have gravity holding you together, but it’s insignificant compared to the chemical bonds holding you together. This is why physicists consider gravity to actually be a pretty weak force compared to all the other forces of the Universe.
You’d need to go somewhere with really intense gravity, like a black hole, for the Roche Limit to overcome the forces holding you together.
So that’s it. Bring the Moon within 9,500 kilometers or so and it would no longer be a Moon. It would be torn apart into a ring, a Halo ring, if you will, capable of wiping out all life on a planet infected by the flood. All the moons we see in the Solar System are are least at the Roche Limit or beyond, otherwise they would have broken up long ago… and probably did.
For millennia, human beings stared up at the night sky and were held in awe by the Moon. To many ancient cultures, it represented a deity, and its cycles were accorded divine significance. By the time of Classical Antiquity and the Middle Ages, the Moon was considered to be a heavenly body that orbited Earth, much like the other known planets of the day (Mercury, Venus, Mars, Jupiter, and Saturn).
However, our understanding of moons was revolutionized when in 1610, astronomer Galileo Galilei pointed his telescope to Jupiter and noticed ” four wandering stars” around Jupiter. From this point onward, astronomers have come to understand that planets other than Earth can have their own moons – in some cases, several dozen or more. So just how many moons are there in the Solar System?
In truth, answering that question requires a bit of a clarification first. If we are talking about confirmed moons that orbit any of the planets of the Solar System (i.e. those that are consistent with the definition adopted by the IAU in 2006), then we can say that there are currently 173 known moons. If, however, we open the floor to dwarf planets that have objects orbiting them, the number climbs to 182.
However, over 200 minor-planet moons have also been observed in the Solar System (as of Jan. 2012). This includes the 76 known objects in the asteroid belt with satellites, four Jupiter Trojans, 39 near-Earth objects (two with two satellites each), 14 Mars-crossers, and 84 natural satellites of Trans-Neptunian Objects. And some 150 additional small bodies have been observed within the rings of Saturn. If we include all these, then we can say that the Solar System has 545 known satellites.
Inner Solar System:
The planets of the Inner Solar system – Mercury, Venus, Earth and Mars – are all terrestrial planets, which means that they are composed of silicate rock and minerals that are differentiated between a metallic core and a silicate mantle and crust. For a number of reasons, few satellites exist within this region of the Solar System.
All told, only three natural satellites exist orbiting planetary bodies in the Inner Solar System – Earth and Mars. While scientist theorize that there were moons around Mercury and Venus in the past, it is believed that these moons impacted on the surface a long time ago. The reason for this sparseness of satellites has a lot to do with the gravitational influence of the Sun.
Both Mercury and Venus are too close to the Sun (and in Mercury’s case, too weak in terms of its own gravitational pull) to have grabbed onto a passing object, or held onto rings of debris in orbit that could have coalesced to form a satellite over time. Earth and Mars were able to retain satellites, but mainly because they are the outermost of the Inner planets.
Earth has only the one natural satellite, which we are familiar with – the Moon. With a mean radius of 1737 km and a mass of 7.3477 x 10²² kg, the Moon is 0.273 times the size of Earth and 0.0123 as massive, which is quite large for a satellite. It is also the second densest moon in our Solar System (after Io), with a mean density of 3.3464 g/cm³.
Several theories have been proposed for the formation of the Moon. The prevailing hypothesis today is that the Earth-Moon system formed as a result of an impact between the newly-formed proto-Earth and a Mars-sized object (named Theia) roughly 4.5 billion years ago. This impact would have blasted material from both objects into orbit, where it eventually accreted to form the Moon.
Mars, meanwhile, has two moons – Phobos and Deimos. Like our own Moon, both of the Martian moons are tidally locked to Mars, so they always present the same face to the planet. Compared to our Moon, they are rough and asteroid-like in appearance, and also much smaller. Hence the prevailing theory that they were once asteroids that were kicked out of the Main Belt by Jupiter’s gravity, and were then acquired by Mars.
The larger moon is Phobos, whose name comes from the Greek word which means “fear” (i.e. phobia). Phobos measures just 22.7 km across and has an orbit that places it closer to Mars than Deimos. Compared to Earth’s own Moon — which orbits at a distance of 384,403 km away from our planet — Phobos orbits at an average distance of only 9,377 km above Mars.
Mars’ second moon is Deimos, which takes its name from the Greek word for panic. It is even smaller, measuring just 12.6 km across, and is also less irregular in shape. Its orbit places it much farther away from Mars, at a distance of 23,460 km, which means that Deimos takes 30.35 hours to complete an orbit around Mars.
These three moons are the sum total of moons to be found within the Inner Solar System (at least, by the conventional definition). But looking further abroad, we see that this is really just the tip of the iceberg. To think we once believed that the Moon was the only one of its kind!
Outer Solar System:
Beyond the Asteroid Belt (and Frost Line), things become quite different. In this region of the Solar System, every planet has a substantial system of Moons; in the case of Jupiter and Saturn, reaching perhaps even into the hundreds. So far, a total of 170 moons have been confirmed orbiting the Outer Planets, while several hundred more orbit minor bodies and asteroids.
Due to its immense size, mass, and gravitational pull, Jupiter has the most satellites of any planet in the Solar System. At present, the Jovian system includes 67 known moons, though it is estimated that it may have up to 200 moons and moonlets (the majority of which are yet to been confirmed and classified).
The four largest Jovian moons are known as the Galilean Moons (named after their discoverer, Galileo Galilei). They include: Io, the most volcanically active body in our Solar System; Europa, which is suspected of having a massive subsurface ocean; Ganymede, the largest moon in our Solar System; and Callisto, which is also thought to have a subsurface ocean and features some of the oldest surface material in the Solar System.
Then there’s the Inner Group (or Amalthea group), which is made up of four small moons that have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree. This groups includes the moons of Metis, Adrastea, Amalthea, and Thebe. Along with a number of as-yet-unseen inner moonlets, these moons replenish and maintain Jupiter’s faint ring system.
Jupiter also has an array of Irregular Satellites, which are substantially smaller and have more distant and eccentric orbits than the others. These moons are broken down into families that have similarities in orbit and composition, and are believed to be largely the result of collisions from large objects that were captured by Jupiter’s gravity.
Similar to Jupiter, it is estimated that Saturn has at least 150 moons and moonlets, but only 53 of these moons have been given official names. Of these, 34 are less than 10 km in diameter and another 14 are between 10 and 50 km in diameter. However, some of its inner and outer moons are rather large, ranging from 250 to over 5000 km.
Traditionally, most of Saturn’s moons have been named after the Titans of Greek mythology, and are grouped based on their size, orbits, and proximity to Saturn. The innermost moons and regular moons all have small orbital inclinations and eccentricities and prograde orbits. Meanwhile, the irregular moons in the outermost regions have orbital radii of millions of kilometers, orbital periods lasting several years, and move in retrograde orbits.
The Inner Large Moons, which orbit within the E Ring, includes the larger satellites MimasEnceladus, Tethys, and Dione. These moons are all composed primarily of water ice, and are believed to be differentiated into a rocky core and an icy mantle and crust. The Large Outer Moons, which orbit outside of the Saturn’s E Ring, are similar in composition to the Inner Moons – i.e. composed primarily of water ice and rock.
At 5150 km in diameter, and 1,350×1020 kg in mass, Titan is Saturn’s largest moon and comprises more than 96% of the mass in orbit around the planet. Titan is also the only large moon to have its own atmosphere, which is cold, dense, and composed primarily of nitrogen with a small fraction of methane. Scientists have also noted the presence of polycyclic aromatic hydrocarbons in the upper atmosphere, as well as methane ice crystals.
The surface of Titan, which is difficult to observe due to persistent atmospheric haze, shows only a few impact craters, evidence of cryo-volcanoes, and longitudinal dune fields that were apparently shaped by tidal winds. Titan is also the only body in the Solar System beside Earth with bodies of liquid on its surface, in the form of methane–ethane lakes in Titan’s north and south polar regions.
Uranus has 27 known satellites, which are divided into the categories of larger moons, inner moons, and irregular moons (similar to other gas giants). The largest moons of Uranus are, in order of size, Miranda, Ariel, Umbriel, Oberon and Titania. These moons range in diameter and mass from 472 km and 6.7 × 1019 kg for Miranda to 1578 km and 3.5 × 1021 kg for Titania. Each of these moons is particularly dark, with low bond and geometric albedos. Ariel is the brightest while Umbriel is the darkest.
All of the large moons of Uranus are believed to have formed in the accretion disc, which existed around Uranus for some time after its formation, or resulted from the large impact suffered by Uranus early in its history. Each one is comprised of roughly equal amounts of rock and ice, except for Miranda which is made primarily of ice.
The ice component may include ammonia and carbon dioxide, while the rocky material is believed to be composed of carbonaceous material, including organic compounds (similar to asteroids and comets). Their compositions are believed to be differentiated, with an icy mantle surrounding a rocky core.
Neptune has 14 known satellites, all but one of which are named after Greek and Roman deities of the sea (except for S/2004 N 1, which is currently unnamed). These moons are divided into two groups – the regular and irregular moons – based on their orbit and proximity to Neptune. Neptune’s Regular Moons – Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N 1, and Proteus – are those that are closest to the planet and which follow circular, prograde orbits that lie in the planet’s equatorial plane.
Neptune’s irregular moons consist of the planet’s remaining satellites (including Triton). They generally follow inclined eccentric and often retrograde orbits far from Neptune. The only exception is Triton, which orbits close to the planet, following a circular orbit, though retrograde and inclined.
In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Neso and Psamathe – a group that includes both prograde and retrograde objects. With the exception of Triton and Nereid, Neptune’s irregular moons are similar to those of other giant planets and are believed to have been gravitationally captured by Neptune.
With a mean diameter of around 2700 km ( mi) and a mass of 214080 ± 520 x 1017 kg, Triton is the largest of Neptune’s moons, and the only one large enough to achieve hydrostatic equilibrium (i.e. is spherical in shape). At a distance of 354,759 km from Neptune, it also sits between the planet’s inner and outer moons.
These moons make up the lion’s share of natural satellites found in the Solar System. However, thanks to ongoing exploration and improvements made in our instrumentation, satellites are being discovered in orbit around minor bodies as well.
Dwarf Planets and Other Bodies:
As already noted, there are several dwarf planets, TNOs, and other bodies in the Solar System that also have their own moons. These consist mainly of the natural satellites that have been confirmed orbiting Pluto, Eris, Haumea and Makemake. With five orbiting satellites, Pluto has the most confirmed moons (though that may change with further observation).
The largest, and closest in orbit to Pluto, is Charon. This moon was first identified in 1978 by astronomer James Christy using photographic plates from the United States Naval Observatory (USNO) in Washington, D.C. Beyond Charon lies the four other circumbinary moons – Styx, Nix, Kerberos, and Hydra, respectively.
Nix and Hydra were discovered simultaneously in 2005 by the Pluto Companion Search Team using the Hubble Space Telescope. The same team discovered Kerberos in 2011. The fifth and final satellite, Styx, was discovered by the New Horizons spacecraft in 2012 while capturing images of Pluto and Charon.
Charon, Styx and Kerberos are all massive enough to have collapsed into a spheroid shape under their own gravity. Nix and Hydra, meanwhile, are oblong in shape. The Pluto-Charon system is unusual, since it is one of the few systems in the Solar System whose barycenter lies above the primary’s surface. In short, Pluto and Charon orbit each other, causing some scientists to claim that it is a “double-dwarf system” instead of a dwarf planet and an orbiting moon.
In addition, it is unusual in that each body is tidally locked to the other. Charon and Pluto always present the same face to each other; and from any position on either body, the other is always at the same position in the sky, or always obscured. This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its common center of gravity.
In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers. This would seem indicate that Pluto does have a subsurface ocean that is warm in temperature, and that the core is geologically active. Pluto’s moons are believed to have been formed by a collision between Pluto and a similar-sized body early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.
Coming in second is Haumea, which has two known moons – Hi’iaka and Namaka – which are named after the daughters of the Hawaiian goddess. Both were discovered in 2005 by Brown’s team while conducting observations of Haumea at the W.M. Keck Observatory. Hi’iaka, which was initially nicknamed “Rudolph” by the Caltech team, was discovered January 26th, 2005.
It is the outer and – at roughly 310 km in diameter – the larger and brighter of the two, and orbits Haumea in a nearly circular path every 49 days. Infrared observations indicate that its surface is almost entirely covered by pure crystalline water ice. Because of this, Brown and his team have speculated that the moon is a fragment of Haumea that broke off during a collision.
Namaka, the smaller and innermost of the two, was discovered on June 30th, 2005, and nicknamed “Blitzen”. It is a tenth the mass of Hi‘iaka and orbits Haumea in 18 days in a highly elliptical orbit. Both moons circle Haumea is highly eccentric orbits. No estimates have been made yet as to their mass.
Eris has one moon called Dysnomia, which is named after the daughter of Eris in Greek mythology, which was first observed on September 10th, 2005 – a few months after the discovery of Eris. The moon was spotted by a team using the Keck telescopes in Hawaii, who were busy carrying out observations of the four brightest TNOs (Pluto, Makemake, Haumea, and Eris) at the time.
In April of 2016, observations using the Hubble Space Telescope‘s Wide Field Camera 3 revealed that Makemake had a natural satellite – which was designated S/2015 (136472) 1 (nicknamed MK 2 by the discovery team). It is estimated to be 175 km (110 mi) km in diameter and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake.
Largest and Smallest Moons:
The title for largest moon in the Solar System goes to Ganymede, which measures 5262.4 kilometers (3270 mi) in diameter. This not only makes it larger than Earth’s Moon, but larger even than the planet Mercury – though it has only half of Mercury’s mass. As for the smallest satellite, that is a tie between S/2003 J 9 and S/2003 J 12. These two satellites, both of which orbit Jupiter, measure about 1 km (0.6 mi) in diameter.
An important thing to note when discussing the number of known moons in the Solar System is that the key word here is “known”. With every passing year, more satellites are being confirmed, and the vast majority of those we now know about were only discovered in the past few decades. As our exploration efforts continue, and our instruments improve, we may find that there are hundreds more lurking around out there!
In 1610, Italian astronomer Galileo Galilei looked up at the heavens using a telescope of his making. And what he saw would forever revolutionize the field of astronomy, our understanding of the Universe, and our place in it. Centuries later, Galileo’s is still held in such high esteem; not only for the groundbreaking research he conducted, but because of his immense ingenuity in developing his own research tools.
And at the center of it all is Galileo’s famous telescope, which still inspires curiosity centuries later. How exactly did he invent it. How exactly was it an improvement on then-current designs? What exactly did he see with it when he looked up at the night sky? And what has become of it today? Luckily, all of these are questions we are able to answer.
Galileo’s telescope was the prototype of the modern day refractor telescope. As you can see from this diagram below, which is taken from Galileo’s own work – Sidereus Nuncius (“The Starry Messenger”) – it was a simple arrangement of lenses that first began with optician’s glass fixed to either end of a hollow cylinder.
Galileo had no diagrams to work from, and instead relied on his own system of trial and error to achieve the proper placement of the lenses. In Galileo’s telescope the objective lens was convex and the eye lens was concave (today’s telescopes make use of two convex lenses). Galileo knew that light from an object placed at a distance from a convex lens created an identical image on the opposite side of the lens.
He also knew that if he used a concave lens, the object would appear on the same side of the lens where the object was located. If moved at a distance, it appeared larger than the object. It took a lot of work and different arrangements to get the lens the proper sizes and distances apart, but Galileo’s telescope remained the most powerful and accurately built for a great many years.
History of Galileo’s Telescope:
Naturally, Galileo’s telescope had some historical antecedents. In the late summer of 1608, a new invention was all the rage in Europe – the spyglass. These low power telescopes were likely made by almost all advanced opticians, but the very first was credited to Hans Lippershey of Holland. These primitive telescopes only magnified the view a few times over.
Much like our modern times, the manufacturers were quickly trying to corner the market with their invention. But Galileo Galilei’s friends convinced his own government to wait – sure that he could improve the design. When Galileo heard of this new optical instrument he set about engineering and making improved versions, with higher magnification.
Galileo’s telescope was similar to how a pair of opera glasses work – a simple arrangement of glass lenses to magnify objects. His first versions only improved the view to the eighth power, but Galileo’s telescope steadily improved. Within a few years, he began grinding his own lenses and changing his arrays. Galileo’s telescope was now capable of magnifying normal vision by a factor of 10, but it had a very narrow field of view.
However, this limited ability didn’t stop Galileo from using his telescope to make some amazing observations of the heavens. And what he saw, and recorded for posterity, was nothing short of game-changing.
What Galileo Observed:
One fine Fall evening, Galileo pointed his telescope towards the one thing that people thought was perfectly smooth and as polished as a gemstone – the Moon. Imagine his surprise when found that it, in his own words, was “uneven, rough, full of cavities and prominences.” Galileo’s telescope had its flaws, such as a narrow field of view that could only show about one quarter of the lunar disk without repositioning.
Nevertheless, a revolution in astronomy had begun! Months passed, and Galileo’s telescope improved. On January 7th, 1610, he turned his new 30 power telescope towards Jupiter, and found three small, bright “stars” near the planet. One was off to the west, the other two were to the east, and all three were in a straight line. The following evening, Galileo once again took a look at Jupiter, and found that all three of the “stars” were now west of the planet – still in a straight line!
And there were more discoveries awaiting Galileo’s telescope: the appearance of bumps next to the planet Saturn (the edges of Saturn’s rings), spots on the Sun’s surface (aka. Sunspots), and seeing Venus change from a full disk to a slender crescent. Galileo Galilei published all of these findings in a small book titled Sidereus Nuncius (“The Starry Messenger”) in 1610.
While Galileo was not the first astronomer to point a telescope towards the heavens, he was the first to do so scientifically and methodically. Not only that, but the comprehensive notes he took on his observations, and the publication of his discoveries, would have a revolutionary impact on astronomy and many other fields of science.
Galileo’s Telescope Today:
Today, over 400 years later, Galileo’s Telescope still survives under the constant care of the Istituto e Museo di Storia della Scienza (renamed the Museo Galileo in 2010) in Italy. The Museum holds exhibitions on Galileo’s telescope and the observations he made with it. The displays consist of these rare and precious instruments – including the objective lens created by the master and the only two existing telescopes built by Galileo himself.
Thanks to Galileo’s careful record keeping, craftsmen around the world have recreated Galileo’s telescope for museums and replicas are now sold for amateurs and collectors as well. Despite the fact that astronomers now have telescopes of immense power at their disposal, many still prefer to go the DIY route, just like Galileo!
Few scientists and astronomers have had the same impact Galileo had. Even fewer are regarded as pioneers in the sciences, or revolutionary thinkers who forever changed humanity’s perception of the heavens and their place within it. Little wonder then why his most prized instrument is kept so well preserved, and is still the subject of study over four centuries later.
We have written many interesting articles on Galileo here at Universe Today. Here’s