Is Time To Go Back to Uranus and Neptune? Revisiting Ice Giants of the Solar System

We've Got To Go Back!


I look forward to all the future missions that NASA is going to be sending out in the Solar System. Here, check this out. You can use NASA’s website to show you all the future missions. Here’s everything planned for the future, here’s everything going to Mars.

Now, let’s look and see what missions are planned for the outer planets of the Solar System, especially Uranus and Neptune. Oh, that’s so sad… there’s nothing.

Uranus, seen by Voyager 2. Image credit: NASA/JPL

It’s been decades since humanity had an up close look at Uranus and Neptune. For Uranus, it was Voyager 2, which swept through the system in 1986. We got just a few tantalizing photographs of the ice giant planet and it’s moons.

Mosaic of the four highest-resolution images of Ariel taken by the Voyager 2 space probe during its 1986 flyby of Uranus. Credit: NASA/JPL

What’s that?

Oberon, as imaged by the Voyager 2 probe during its flyby on Jan. 24, 1986. Credit: NASA

What’s going on there?

Color composite of the Uranian satellite Miranda, taken by Voyager 2 on Jan. 24, 1986, from a distance of 147,000 km (91,000 mi). Credit: NASA/JPL

What are those strange features? Sorry, insufficient data.

And then Voyager 2 did the same, zipping past Neptune in 1989.

Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL

Check this out.

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

What’s going here on Triton? Wouldn’t you like to know more? Well, too bad! You can’t it’s done, that’s all you get.

Don’t get me wrong, I’m glad we’ve studied all these other worlds. I’m glad we’ve had orbiters at Mercury, Venus, everything at Mars, Jupiter, and especially Saturn. We’ve seen Ceres and Vesta, and the Moon up close. We even got a flyby of Pluto and Charon.

It’s time to go back to Uranus and Neptune, this time to stay.

And I’m not the only one who feels this way.

Scientists at NASA recently published a report called the Ice Giant Mission Study, and it’s all about various missions that could be sent to explore Uranus, Neptune and their fascinating moons.

The team of scientists who worked on the study considered a range of potential missions to the ice giants, and in the end settled on four potential missions; three that could go to Uranus, and one headed for Neptune. Each of them would cost roughly $2 billion.

Uranus is closer, easier to get to, and the obvious first destination of a targeted mission. For Uranus, NASA considered three probes.

The first idea is a flyby mission, which will sweep past Uranus gathering as much science as it can. This is what Voyager 2 did, and more recently what NASA’s New Horizons did at Pluto. In addition, it would have a separate probe, like the Cassini and Galileo missions, that would detach and go into the atmosphere to sample the composition below the cloudtops. The mission would be heavy and require an Atlas V rocket with the same configuration that sent Curiosity to Mars. The flight time would take 10 years.

NASA’s Curiosity Mars Science Laboratory (MSL) rover blasts off for Mars atop a stunningly beautiful Atlas V rocket. Credit: Ken Kremer – kenkremer.com

The main science goal of this mission would be to study the composition of Uranus. It would make some other measurements of the system as it passed through, but it would just be a glimpse. Better than Voyager, but nothing like Cassini’s decade plus observations of Saturn.

I like where this is going, but I’m going to hold out for something better.

The next idea is an orbiter. Now we’re talking! It would have all the same instruments as the flyby and the detachable probe. But because it would be an orbiter, it would require much more propellant. It would have triple the launch mass of the flyby mission, which means a heavier Atlas V rocket. And a slightly longer flight time; 12 years instead of 10 for the flyby.

Because it would remain at Uranus for at least 3 years, it would be able to do an extensive analysis of the planet and its rings and moons. But because of the atmospheric probe, it wouldn’t have enough mass for more instruments. It would have more time at Uranus, but not a much better set of tools to study it with.

Okay, let’s keep going. The next idea is an orbiter, but without the detachable probe. Instead, it’ll have the full suite of 15 scientific instruments, to study Uranus from every angle. We’re talking visible, doppler, infrared, ultraviolet, thermal, dust, and a fancy wide angle camera to give us those sweet planetary pictures we like to see.

Study Uranus? Yes please. But while we’re at it, let’s also sent a spacecraft to Neptune.

The labeled ring arcs of Neptune as seen in newly processed data. The image spans 26 exposures combined into a equivalent 95 minute exposure, and the ring trace and an image of the occulted planet Neptune is added for reference. (Credit: M. Showalter/SETI Institute).

As part of the Ice Giants Study, the researchers looked at what kind of missions would be possible. In this case, they settled on a single recommended mission. A huge orbiter with an additional atmospheric probe. This mission would be almost twice as massive as the heaviest Uranus mission, so it would need a Delta IV Heavy rocket to even get out to Neptune.

As it approached Neptune, the mission would release an atmospheric probe to descend beneath the cloudtops and sample what’s down there. The orbiter would then spend an additional 2 years in the environment of Neptune, studying the planet and its moons and rings. It would give us a chance to see its fascinating moon Triton up close, which seems to be a captured Kuiper Belt Object.

Unfortunately there’s no perfect grand tour trajectory available to us any more, where a single spacecraft could visit all the large planets in the Solar System. Missions to Uranus and Neptune will have to be separate, however, if NASA’s Space Launch System gets going, it could carry probes for both destinations and launch them together.

The goal of these missions is the science. We want to understand the ice giants of the outer Solar System, which are quite different from both the inner terrestrial planets and the gas giants Jupiter and Saturn.

The Solar System. Credit: NASA

The gas giants are mostly hydrogen and helium, like the Sun. But the ice giants are 65% water and other ices made from methane and ammonia. But it’s not like they’re big blobs of water, or even frozen water. Because of their huge gravity, the ice giants crush this material with enormous pressure and temperature.

What happens when you crush water under this much pressure? It would all depend on the temperature and pressure. There could be different types of ice down there. At one level, it could be an electrically conductive soup of hydrogen and oxygen, and then further down, you might get crystallized oxygen with hydrogen ions running through it.

Hailstones made of diamond could form out of the carbon-rich methane and fall down through the layers of the planets, settling within a molten carbon core. What I’m saying is, it could be pretty strange down there.

We know that ice giants are common in the galaxy, in fact, they’ve made up the majority of the extrasolar planets discovered so far. By better understanding the ones we have right here in our own Solar System, we can get a sense of the distant extrasolar planets turning up. We’ll be better able to distinguish between the super earths and mini-neptunes.

Artist’s impression of the Milky Way’s 100 billion exoplanets. Credit: NASA, ESA, and M. Kornmesser (ESO)

Another big question is how these planets formed in the first place. In their current models, most planetary astronomers think these planets had very short time windows to form. They needed to have massive enough cores to scoop up all that material before the newly forming Sun’s solar wind blasted it all out into space. And yet, why are these kinds of planets so common in the Universe?

The NASA mission planners developed a total of 12 science objectives for these missions, focusing on the composition of the planets and their atmospheres. And if there’s time, they’d like to know about how heat moves around, their constellations of rings and moons. They’d especially like to investigate Neptune’s moons Triton, which looks like a captured Kuiper Belt Object, as it orbits in the reverse direction from all the other moons in the Solar System.

In terms of science, the two worlds are very similar. But because Neptune has Triton. If I had to choose, I’d go with a Neptune mission.

Neptune and its large moon Triton as seen by Voyager 2 on August 28th, 1989. (Credit: NASA).

Are you excited? I’m excited. Here’s the bad news. According to NASA, the best launch windows for these missions would be 2029 or 2034. And that’s just the launch time, the flight time is an additional decade or more on top of that. In other words, the first photos from a Uranus flyby could happen in 2039 or 2035, while orbiters could arrive at either planet in the 2040s. I’m sure my future grandchildren will enjoy watching these missions arrive.

But then, we have to keep everything in perspective. NASA’s Cassini mission was under development in the 1980s. It didn’t launch until 1997, and it didn’t get to Saturn until 2004. It’s been almost 20 years since that launch, and almost 40 years since they started working on it.

I guess we need to be more patient. I can be patient.

What About a Mission to Titan?

What About a Mission to Titan?


As you probably know, NASA recently announced plans to send a mission to Jupiter’s moon Europa. If all goes well, the Europa Clipper will blast off for the world in the 2020s, and orbit the icy moon to discover all its secrets.

And that’s great and all, I like Europa just fine. But you know where I’d really like us to go next? Titan.

Titan, as you probably know, is the largest moon orbiting Saturn. In fact, it’s the second largest moon in the Solar System after Jupiter’s Ganymede. It measures 5,190 kilometers across, almost half the diameter of the Earth. This place is big.

It orbits Saturn every 15 hours and 22 days, and like many large moons in the Solar System, it’s tidally locked to its planet, always showing Saturn one side.

Titan image taken by Cassini on Oct. 7, 2013 (Credit: NASA/JPL-Caltech/Space Science Institute)

Before NASA’s Voyager spacecraft arrived in 1980, astronomers actually thought that Titan was the biggest moon in the Solar System. But Voyager showed that it actually has a thick atmosphere, that extends well into space, making the true size of the moon hard to judge.

This atmosphere is one of the most interesting features of Titan. In fact, it’s the only moon in the entire Solar System with a significant atmosphere. If you could stand on the surface, you would experience about 1.45 times the atmospheric pressure on Earth. In other words, you wouldn’t need a pressure suit to wander around the surface of Titan.

You would, however, need a coat. Titan is incredibly cold, with an average temperature of almost -180 Celsius. For you Fahrenheit people that’s -292 F. The coldest ground temperature ever measured on Earth is almost -90 C, so way way colder.

You would also need some way to breathe, since Titan’s atmosphere is almost entirely nitrogen, with trace amounts of methane and hydrogen. It’s thick and poisonous, but not murderous, like Venus.

Titan has only been explored a couple of times, and we’ve actually only landed on it once.

The first spacecraft to visit Titan was NASA’s Pioneer 11, which flew past Saturn and its moons in 1979. This flyby was followed by NASA’s Voyager 1 in 1980 and then Voyager 2 in 1981. Voyager 1 was given a special trajectory that would take it as close as possible to Titan to give us a close up view of the world.

Saturn’s moon Titan lies under a thick blanket of orange haze in this Voyager 1 picture. Credit: NASA

Voyager was able to measure its atmosphere, and helped scientists calculate Titan’s size and mass. It also got a hint of darker regions which would later turn out to be oceans of liquid hydrocarbons.

The true age of Titan exploration began with NASA’s Cassini spacecraft, which arrived at Saturn on July 4, 2004. Cassini made its first flyby of Titan on October 26, 2004, getting to within 1,200 kilometers or 750 miles of the planet. But this was just the beginning. By the end of its mission later this year, Cassini will have made 125 flybys of Titan, mapping the world in incredible detail.

Cassini saw that Titan actually has a very complicated hydrological system, but instead of liquid water, it has weather of hydrocarbons. The skies are dotted with methane clouds, which can rain and fill oceans of nearly pure methane.

And we know all about this because of Cassini’s Huygen’s lander, which detached from the spacecraft and landed on the surface of Titan on January 14, 2005. Here’s an amazing timelapse that shows the view from Huygens as it passed down through the atmosphere of Titan, and landed on its surface.

Huygens landed on a flat plain, surrounded by “rocks”, frozen globules of water ice. This was lucky, but the probe was also built to float if it happened to land on liquid instead.

It lasted for about 90 minutes on the surface of Titan, sending data back to Earth before it went dark, wrapping up the most distant landing humanity has ever accomplished in the Solar System.

Although we know quite a bit about Titan, there are still so many mysteries. The first big one is the cycle of liquid. Across Titan there are these vast oceans of liquid methane, which evaporate to create methane clouds. These rain, creating mists and even rivers.

This false-color mosaic of Saturn’s largest moon Titan, obtained by Cassini’s visual and infrared mapping spectrometer, shows what scientists interpret as an icy volcano. Credit: NASA/JPL/University of Arizona

Is it volcanic? There are regions of Titan that definitely look like there have been volcanoes recently. Maybe they’re cryovolcanoes, where the tidal interactions with Saturn cause water to well up from beneath crust and erupt onto the surface.

Is there life there? This is perhaps the most intriguing possibility of all. The methane rich system has the precursor chemicals that life on Earth probably used to get started billions of years ago. There’s probably heated regions beneath the surface and liquid water which could sustain life. But there could also be life as we don’t understand it, using methane and ammonia as a solvent instead of water.

To get a better answer to these questions, we’ve got to return to Titan. We’ve got to land, rove around, sail the oceans and swim beneath their waves.

Now you know all about this history of the exploration of Titan. It’s time to look at serious ideas for returning to Titan and exploring it again, especially its oceans.

Planetary scientists have been excited about the exploration of Titan for a while now, and a few preliminary proposals have been suggested, to study the moon from the air, the land, and the seas.

The spacecraft, balloon, and lander of the Titan Saturn System Mission. Credit: NASA Jet Propulsion Laboratory

First up, there’s the Titan Saturn System Mission, a mission proposed in 2009, for a late 2020s arrival at Titan. This spacecraft would consist of a lander and a balloon that would float about in the atmosphere, and study the world from above. Over the course of its mission, the balloon would circumnavigate Titan once from an altitude of 10km, taking incredibly high resolution images. The lander would touch down in one of Titan’s oceans and float about on top of the liquid methane, sampling its chemicals.

As we stand right now, this mission is in the preliminary stages, and may never launch.

The Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR) concept for an aerial explorer for Titan. Credit: Mike Malaska

In 2012, Dr. Jason Barnes and his team from the University of Idaho proposed sending a robotic aircraft to Titan, which would fly around in the atmosphere photographing its surface. Titan is actually one of the best places in the entire Solar System to fly an airplane. It has a thicker atmosphere and lower gravity, and unlike the balloon concept, an airplane is free to go wherever it needs powered by a radioactive thermal generator.

Although the mission would only cost about $750 million or so, NASA hasn’t pushed it beyond the conceptual stage yet.

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

An even cooler plan would put a boat down in one of Titan’s oceans. In 2012, a team of Spanish engineers presented their idea for how a Titan boat would work, using propellers to put-put about across Titan’s seas. They called their mission the Titan Lake In-Situ Sampling Propelled Explorer, or TALISE.

Propellers are fine, but it turns out you could even have a sailboat on Titan. The methane seas have much less density and viscosity than water, which means that you’d only experience about 26% the friction of Earth. Cassini measured windspeeds of about 3.3 m/s across Titan, which half the average windspeed of Earth. But this would be plenty of wind to power a sail when you consider Titan’s thicker atmosphere.

And here’s my favorite idea. A submarine. This 6-meter vessel would float on Titan’s Kraken Mare sea, studying the chemistry of the oceans, measuring currents and tides, and mapping out the sea floor.

It would be capable of diving down beneath the waves for periods, studying interesting regions up close, and then returning to the surface to communicate its findings back to Earth. This mission is in the conceptual stage right now, but it was recently chosen by NASA’s Innovative Advanced Concepts Group for further study. If all goes well, the submarine would travel to Titan by 2038 when there’s a good planetary alignment.

Okay? Are you convinced? Let’s go back to Titan. Let’s explore it from the air, crawl around on the surface and dive beneath its waves. It’s one of the most interesting places in the entire Solar System, and we’ve only scratched the surface.

If I’ve done my job right, you’re as excited about a mission to Titan as I am. Let’s go back, let’s sail and submarine around that place. Let me know your thoughts in the comments.

If it Wasn’t Already Strange Enough, now Saturn’s Hexagon Storm is Changing Color

Ever since the Voyager 2 made its historic flyby of Saturn, astronomers have been aware of the persistent hexagonal storm around the gas giant’s north pole. This a six-sided jetstream has been a constant source of fascination, due to its sheer size and immense power. Measuring some 13,800 km (8,600 mi) across, this weather system is greater in size than planet Earth.

And thanks to the latest data to be provided by the Cassini space probe, which entered orbit around Saturn in 2009, it seems that this storm is even stranger than previously thought. Based on images snapped between 2012 and 2016, the storm appears to have undergone a change in color, from a bluish haze to a golden-brown hue.

The reasons for this change remain something of a mystery, but scientists theorize that it may be the result of seasonal changes due to the approaching summer solstice (which will take place in May of 2017). Specifically, they believe that the change is being driven by an increase in the production of photochemical hazes in the atmosphere, which is due to increased exposure to sunlight.

 Natural color images taken by NASA's Cassini wide-angle camera, showing the changing appearance of Saturn's north polar region between 2012 and 2016.. Credit: NASA/JPL-Caltech/Space Science Institute/Hampton University
Natural color images taken by NASA’s Cassini wide-angle camera, showing the changing appearance of Saturn’s north polar region between 2012 and 2016.. Credit: NASA/JPL-Caltech/Space Science Institute/Hampton University

This reasoning is based in part on past observations of seasonal change on Saturn. Like Earth, Saturn experiences seasons because its axis is tilted relative to its orbital plane (26.73°). But since its orbital period is almost 30 years, these seasons last for seven years.

Between November 1995 and August 2009, the hexagonal storm also underwent some serious changes, which coincided with Saturn going from its Autumnal to its Spring Equinox. During this period, the north polar atmosphere became clear of aerosols produced by photochemical reactions, which was also attributed to the fact that the northern polar region was receiving less in the way of sunlight.

However, since that time, the polar atmosphere has been exposed to continuous sunlight, and this has coincided with aerosols being produced inside the hexagon, making the polar atmosphere appear hazy. As Linda J. Spilker, the Cassini mission’s project scientist, told Universe Today via email:

“We have seen dramatic changes in the color inside Saturn’s north polar hexagon in the last 4 years.  That color change is probably the result of changing seasons at Saturn, as Saturn moves toward northern summer solstice in May 2017. As more sunlight shines on the hexagon, more haze particles are produced and this haze gives the hexagon a more golden color.”
This diagram shows the main events of Saturn's year, and where in the Saturnian year the Voyager 1 and Cassini missions occurred. Credit: Ralph Lorenz
Diagram showing he main events of Saturn’s year, and where in the Saturnian year the Voyager 1 and Cassini missions occurred. Credit: Ralph Lorenz

All of this has helped scientists to test theoretical models of Saturn’s atmosphere. In the past, it has been speculated that this six-sided storm acts as a barrier that prevents outside haze particles from entering. The previous differences in color – the planet’s atmosphere being golden while the polar storm was darker and bluish – certainly seemed to bear this out.

The fact that it is now changing color and starting to look more like the rest of the atmosphere could mean that the chemical composition of the polar region is now changing and becoming more like the rest of the planet. Other effects, which include changes in atmospheric circulation (which are in turn the result of seasonally shifting solar heating patterns) might also be influencing the winds in the polar regions.

Needless to say, the giant planets of the Solar System have always been a source of fascination for scientists and astronomers. And if these latest images are any indication, it is that we still have much to learn about the dynamics of their atmospheres.

“It is very exciting to see this transformation in Saturn’s hexagon color with changing seasons,” said Spilker. “With Saturn seasons over 7 years long, these new results show us that it is certainly worth the wait.”

 R. G. French (Wellesley College) et al., NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
The seasons on Saturn, visualized with images taken by the Hubble Heritage Team. Credit: R. G. French (Wellesley College) et al./NASA/ESA/Hubble Heritage Team (STScI/AURA)

It also shows that Cassini, which has been in operation since 1997, is still able to provide new insights into Saturn and its system of moons. In recent weeks, this included information about seasonal variations on Titan, Saturn’s largest moon. By April 22nd, 2017, the probe will commence its final 22 orbits of Saturn. Barring any mission extensions, it is scheduled enter into Saturn’s atmosphere (thus ending its mission) on Sept. 15th, 2017.

Further Reading: NASA/JPL/Caltech

What Is The Surface of Neptune Like?

Neptune Hurricanes

As a gas giant (or ice giant), Neptune has no solid surface. In fact, the blue-green disc we have all seen in photographs over the years is actually a bit of an illusion. What we see is actually the tops of some very deep gas clouds, which in turn give way to water and other melted ices that lie over an approximately Earth-size core made of silicate rock and a nickel-iron mix. If a person were to attempt to stand on Neptune, they would sink through the gaseous layers.

As they descended, they would experience increased temperatures and pressures until they finally touched down on the solid core itself. That being said, Neptune does have a surface of sorts, (as with the other gas and ice giants) which is defined by astronomers as being the point in the atmosphere where the pressure reaches one bar. Because of this, Neptune’s surface is one of the most active and dynamic places in entire the Solar System.

Continue reading “What Is The Surface of Neptune Like?”

How Long Does It Take to Get to Jupiter?

We’re always talking about Pluto, or Saturn or Mars. But nobody ever seems to talk about Jupiter any more. Why is that? I mean, it’s the largest planet in the Solar System. 318 times the mass of the Earth has got to count for something, right? Right?

 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

Jupiter is one of the most important places in the Solar System. The planet itself is impressive; with ancient cyclonic storms larger than the Earth, or a magnetosphere so powerful it defies comprehension.

One of the most compelling reasons to visit Jupiter is because of its moons. Europa, Callisto and Ganymede might all contain vast oceans of liquid water underneath icy shells. And as you probably know, wherever we find liquid water on Earth, we find life.

And so, the icy moons of Jupiter are probably the best place to look for life in the entire Solar System.

And yet, as I record this video in early 2016, there are no spacecraft at Jupiter or its moons. In fact, there haven’t been any there for years. The last spacecraft to visit Jupiter was NASA’s New Horizons in 2007. Mars is buzzing with orbiters and rovers, we just got close up pictures of Pluto! and yet we haven’t seen Jupiter close up in almost 10 years. What’s going on?

Part of the problem is that Jupiter is really far away, and it takes a long time to get there.

How long? Let’s take a look at all the spacecraft that have ever made this journey.

The first spacecraft to ever cross the gulf from the Earth to Jupiter was NASA’s Pioneer 10. It launched on March 3, 1972 and reached on December 3, 1973. That’s a total of 640 days of flight time.

But Pioneer 10 was just flying by, on its way to explore the outer Solar System. It came within 130,000 km of the planet, took the first close up pictures ever taken of Jupiter, and then continued on into deep space for another 11 years before NASA lost contact.

Pioneer 11 took off a year later, and arrived a year later. It made the journey in 606 days, making a much closer flyby, getting within 21,000 kilometers of Jupiter, and visiting Saturn too.

Next came the Voyager spacecraft. Voyager 1 took only 546 days, arriving on March 5, 1979, and Voyager 2 took 688 days.

So, if you’re going to do a flyby, you’ll need about 550-650 days to make the journey.

But if you actually want to slow down and go into orbit around Jupiter, you’ll need to take a much slower journey. The only spacecraft to ever stick around Jupiter was NASA’s Galileo spacecraft, which launched on October 18, 1989.

Instead of taking the direct path to Jupiter, it made two gravitational assisting flybys of Earth and one of Venus to pick up speed, finally arriving at Jupiter on December 8, 1995. That’s a total of 2,242 days.

So why did Galileo take so much longer to get to Jupiter? It’s because you need to be going slow enough that when you reach Jupiter, you can actually enter orbit around the planet, and not just speed on past.

And now, after this long period of Jupiterlessness, we’re about to have another spacecraft arrive at the massive planet and go into orbit. NASA’s Juno spacecraft was launched back on August 5, 2011 and it’s been buzzing around the inner Solar System, building up the velocity to make the journey to Jupiter.

 NASA's Juno spacecraft launched on August 6, 2011 and should arrive at Jupiter on July 4, 2016. Credit: NASA / JPL

NASA’s Juno spacecraft launched on August 6, 2011 and should arrive at Jupiter on July 4, 2016. Credit: NASA / JPL

It did a flyby of Earth back in 2013, and if everything goes well, Juno will make its orbital insertion into the Jovian system on July 4, 2016. Total flight time: 1,795 days.

Once again, we’ll have a spacecraft observing Jupiter and its moon.s

This is just the beginning. There are several more missions to Jupiter in the works. The European Space Agency will be launching the Jupiter Icy Moons Mission in 2022, which will take nearly 8 years to reach Jupiter by 2030.

NASA’s Europa Multiple-Flyby Mission [Editor’s note: formerly known as the Europa Clipper] will probably launch in the same timeframe, and spend its time orbiting Europa, trying to get a better understand the environment on Europa. It probably won’t be able to detect any life down there, beneath the ice, but it’ll figure out exactly where the ocean starts.

So, how long does it take to get to Jupiter? Around 600 days if you want to just do a flyby and aren’t planning to stick around, or about 2,000 days if you want to actually get into orbit.

10 Interesting Facts About Neptune

Neptune is a truly fascinating world. But as it is, there is much that people don’t know about it. Perhaps it is because Neptune is the most distant planet from our Sun, or because so few exploratory missions have ventured that far out into our Solar System. But regardless of the reason, Neptune is a gas (and ice) giant that is full of wonder!

Below, we have compiled a list of 10 interesting facts about this planet. Some of them, you might already know. But others are sure to surprise and maybe even astound you. Enjoy!

Continue reading “10 Interesting Facts About Neptune”

Uranus’ “Sprightly” Moon Ariel

The outer Solar System has enough mysteries and potential discoveries to keep scientists busy for decades. Case in point, Uranus and it’s system of moons. Since the beginning of the Space Age, only one space probe has ever passed by this planet and its system of moons. And yet, that which has been gleaned from this one mission, and over a century and a half of Earth- (and space-) based observation, has been enough to pique the interest of many generations of scientists.

For instance, just about all detailed knowledge of Uranus’ 27 known moons – including the “sprightly” moon Ariel – has been derived from information obtained by the Voyager 2 probe. Nevertheless, this single flyby revealed that Ariel is composed of equal parts ice and rock, a cratered and geologically active surface, and a seasonal cycle that is both extreme and very unusual (at least by our standards!)

Discovery and Naming:

Ariel was discovered on October 24th, 1851, by English astronomer William Lassel, who also discovered the larger moon of Umbriel on the same day. While William Herschel, who discovered Uranus’ two largest moons of Oberon and Titania in 1787, claimed to have observed four other moons in Uranus’ orbit, those claims have since been concluded to be erroneous.

A montage of Uranus's moons. Image credit: NASA
A montage of Uranus’s major moons. Image credit: NASA

As with all of Uranus’ moons, Ariel was named after a character from Alexander Pope’s The Rape of the Lock and Shakespeare’s The Tempest. In this case, Ariel refers to a spirit of the air who initiates the great storm in The Tempest and a sylph who protects the female protagonist in The Rape of the Lock. The names of all four then-known satellites of Uranus were suggested by John Herschel in 1852 at the request of Lassell.

Size, Mass and Orbit:

With a mean radius of 578.9 ± 0.6 km and a mass of 1.353 ± 0.120 × 1021 kg, Ariel is equivalent in size to 0.0908 Earths and 0.000226 times as massive. Ariel’s orbit of Uranus is almost circular, with an average distance (semi-major axis) of 191,020 km – making it the second closest of Uranus’ five major moons (behind Miranda). It has a very small orbital eccentricity (0.0012) and is inclined very little relative to Uranus’ equator (0.260°).

With an average orbital velocity of 5.51 km/s, Ariel takes 2.52 days to complete a single orbit of Uranus. Like most moons in the outer Solar System, Ariel’s rotation is synchronous with its orbit. This means that the moon is tidally locked with Uranus, with one face constantly pointed towards the planet.

Ariel orbits and rotates within Uranus’ equatorial plane, which means it rotates perpendicular to the Sun. This means that its northern and southern hemispheres face either directly towards the Sun or away from it at the solstices, which results in an extreme seasonal cycle of permanent day or night for a period of 42 years.

Size comparison between Earth, the Moon, and Ariel. Credit: NASA/JPL/USGS/Tom Reding
Size comparison between Earth, the Moon, and Ariel. Credit: NASA/JPL/USGS/Tom Reding

Ariel’s orbit lies completely inside the Uranian magnetosphere, which means that its trailing hemisphere is regularly struck by magnetospheric plasma co-rotating with the planet. This bombardment is believed to be the cause of the darkening of the trailing hemispheres (see below), which has been observed for all Uranian moons (with the exception of Oberon).

Currently Ariel is not involved in any orbital resonance with other Uranian satellites. In the past, however, it may have been in a 5:3 resonance with Miranda, which could have been partially responsible for the heating of that moon, and 4:1 resonance with Titania, from which it later escaped.

Composition and Surface Features:

Ariel is the fourth largest of Uranus’ moons, but is believed to be the third most-massive. Its average density of 1.66 g/cm3 indicates that it is roughly composed of equal parts water ice and rock/carbonaceous material, including heavy organic compounds. Based on spectrographic analysis of the surface, the leading hemisphere of Ariel has been revealed to be richer in water ice than its trailing hemisphere.

The cause of this is currently unknown, but it may be related to bombardment by charged particles from Uranus’s magnetosphere, which is stronger on the trailing hemisphere. The interaction of energetic particles and water ice causes sublimation and the decomposition of methane trapped in the ice (as clathrate hydrate), darkening the methanogenic and other organic molecules and leaving behind a dark, carbon-rich residue (aka. tholins).

The highest-resolution Voyager 2 color image of Ariel. Canyons with floors covered by smooth plains are visible at lower right. The bright crater Laica is at lower left. Credit: NASA/JPL
The highest-resolution Voyager 2 color image of Ariel, showing canyons with floors covered by smooth plains (lower right) and the bright Laica crater (lower left). Credit: NASA/JPL

Based on its size, estimates of its ice/rock distribution, and the possibility of salt or ammonia in its interior, Ariel’s interior is thought to be differentiated between a rocky core and an icy mantle. If true, the radius of the core would account for 64% of the moon’s radius (372 km) and 52% of its mass. And while the presence of water ice and ammonia could mean Ariel harbors an interior ocean at it’s core-mantle boundary, the existence of such an ocean is considered unlikely.

Infrared spectroscopy has also identified concentrations of carbon dioxide (CO²) on Ariel’s surface, particularly on its trailing hemisphere. In fact, Ariel shows the highest concentrations of CO² on of any Uranian satellite, and was the first moon to have this compound discovered on its surface.

Though the precise reason for this is unknown, it is possible that it is produced from carbonates or organic material that have been exposed to Uranus’ magnetosphere or solar ultraviolet radiation – due to the asymmetry between the leading and trailing hemispheres. Another explanation is outgassing, where primordial CO² trapped in Ariel’s interior ice escaped thanks to past geological activity.

The observed surface of Ariel can be divided into three terrain types: cratered terrain, ridged terrain and plains. Other features include chasmata (canyons), fault scarps (cliffs), dorsa (ridges) and graben (troughs or trenches). Impact craters are the most common feature on Ariel, particularly in the south pole, which is the moon’s oldest and most geographically extensive region.

False-color map of Ariel. The prominent noncircular crater below and left of center is Yangoor. Part of it was erased during formation of ridged terrain via extensional tectonics. Credit: NASA/JPL/USGS
False-color map of Ariel, showing the prominent Yangoor crater (left of center) and patches of ridged terrain (far left). Credit: USGS

Compared to the other moons of Uranus, Ariel appears to be fairly evenly-cratered. The surface density of the craters, which is significantly lower than those of Oberon and Umbriel, suggest that they do not date to the early history of the Solar System. This means that Ariel must have been completely resurfaced at some point in its history, most likely in the past when the planet had a more eccentric orbit and was therefore more geologically active.

The largest crater observed on Ariel, Yangoor, is only 78 km across, and shows signs of subsequent deformation. All large craters on Ariel have flat floors and central peaks, and few are surrounded by bright ejecta deposits. Many craters are polygonal, indicating that their appearance was influenced by the crust’s preexisting structure. In the cratered plains there are a few large (about 100 km in diameter) light patches that may be degraded impact craters.

The cratered terrain is intersected by a network of scarps, canyons and narrow ridges, most of which occur in Ariel’s mid-southern latitudes. Known as chasmata, these canyons were probably graben that formed due to extensional faulting triggered by global tension stresses – which in turn are believed to have been caused by water and/or liquid ammonia freezing in the interior.

These chasmata are typically 15–50 km wide and are mainly oriented in an east- or northeasterly direction. The widest graben have grooves running along the crests of their convex floors (known as valles). The longest canyon is Kachina Chasma, which is over 620 km long.

was taken Jan. 24, 1986, from a distance of 130,000 km (80,000 mi). The complexity of Ariel's surface indicates that a variety of geologic processes have occurred. Credit: NASA/JPL
Image of Ariel, taken on Jan. 24, 1986, from a distance of 130,000 km (80,000 mi) showing the complexity of Ariel’s surface. Credit: NASA/JPL

The ridged terrain on Ariel, which is the second most-common type, consists of bands of ridges and troughs hundreds of kilometers long. These ridges are found bordering cratered terrain and cutting it into polygons. Within each band (25-70 km wide) individual ridges and troughs have been observed that are up to 200 km long and 10-35 km apart. Here too, these features are believed to be a modified form of graben or the result of geological stresses.

The youngest type of terrain observed on Ariel are its plains, which consists of relatively low-lying smooth areas. Due to the varying levels of cratering found in these areas, the plains are believed to have formed over a long period of time. They  are found on the floors of canyons and in a few irregular depressions in the middle of the cratered terrain.

The most likely origin for the plains is through cryovolcanism, since their geometry resembles that of shield volcanoes on Earth, and their topographic margins suggests the eruption of viscous liquid – possibly liquid ammonia. The canyons must therefore have formed at a time when endogenic resurfacing was still taking place on Ariel.

Uranus and Ariel
Ariel’s transit of Uranus, which was captured by the Hubble Space Telescope on July 26th, 2008. Credit: NASA, ESA, L. Sromovsky (University of Wisconsin, Madison), H. Hammel (Space Science Institute), and K. Rages (SETI)

Ariel is the most reflective of Uranus’s moons, with a Bond albedo of about 23%. The surface of Ariel is generally neutral in color, but there appears to be an asymmetry where the trailing hemisphere is slightly redder. The cause of this, is believed to be interaction between Ariel’s trailing hemisphere and radiation from Uranus’ magnetosphere and Solar ultraviolet radiation, which converts organic compounds in the ice into tholins.

Like all of Uranus’ major moons, Ariel is thought to have formed in the Uranunian accretion disc; which existed around Uranus for some time after its formation, or resulted from a large impact suffered by Uranus early in its history.

Exploration:

Due to its proximity to Uranus’ glare, Ariel is difficult to view by amateur astronomers. However, since the 19th century, Ariel has been observed many times by ground-based on space-based instruments. For example, on July 26th, 2006, the Hubble Space Telescope captured a rare transit made by Ariel of Uranus, which cast a shadow that could be seen on the Uranian cloud tops. Another transit, in 2008, was recorded by the European Southern Observatory.

It was not until the 1980s that images were obtained by the first and only orbiter to ever pass through the Uranus’ system. This was the Voyager 2 space probe, which photographed the moon during its January 1986 flyby.  The probe’s closest approach was at a distance of 127,000 km (79,000 mi) – significantly less than the distances to all other Uranian moons except Miranda.

Voyager 2. Credit: NASA
Artist’s impression of the Voyager 2 space probe. Credit: NASA

The images acquired covered only about 40% of the surface, but only 35% was captured with the quality required for geological mapping and crater counting. This was partly due to the fact that the flyby coincided with the southern summer solstice, where the southern hemisphere was pointed towards the Sun and the northern hemisphere was completely concealed by darkness.

No missions have taken place to study Uranus’ system of moons since and none are currently planned. However, the possibility of sending the Cassini spacecraft to Uranus was evaluated during its mission extension planning phase in April of 2008. It was determined that it would take about twenty years for Cassini to get to the Uranian system after departing Saturn. However, this proposal and the ultimate fate of the mission remain undecided at this time.

All in all, Uranus’ moon Ariel is in good company. Like it’s fellow Uranians, its axial tilt is almost the exact same as Uranus’, it is composed of almost equal parts ice and rock, it is geologically active, and its orbit leads to an extreme seasonal cycle. However, Ariel stands alone when its to its brightness and its youthful surface. Unfortunately, this bright and youthful appearance has not made it an easier to observe.

Alas, as with all Uranian moons, exploration of this moon is still in its infancy and there is much we do not know about it. One can only hope another deep-space mission, like a modified Cassini flyby, takes place in the coming years and finishes the job started by Voyager 2!

We have many interesting articles on Ariel and Uranus’ moons here at Universe Today. Here’s one about Ariel’s 2006 transit of Uranus, its 2008 transit, and one which answers the all-important question How Many Moons Does Uranus Have?

For more information, check out NASA’s Solar System Exploration page on Ariel, and The Planetary Society’s Voyager 2 Ariel image catalog.

Sources:

 

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

Neptune’s Moon Triton

The planets of the outer Solar System are known for being strange, as are their many moons. This is especially true of Triton, Neptune’s largest moon. In addition to being the seventh-largest moon in the Solar System, it is also the only major moon that has a retrograde orbit – i.e. it revolves in the direction opposite to the planet’s rotation. This suggests that Triton did not form in orbit around Neptune, but is a cosmic visitor that passed by one day and decided to stay.

And like most moons in the outer Solar System, Triton is believed to be composed of an icy surface and a rocky core. But unlike most Solar moons, Triton is one of the few that is known to be geologically active. This results in cryovolcanism, where geysers periodically break through the crust and turn the surface Triton into what is sure to be a psychedelic experience!

Discovery and Naming:

Triton was discovered by British astronomer William Lassell on October 10th, 1846, just 17 days after the discovery of Neptune by German astronomer Johann Gottfried Galle. After learning about the discovery, John Herschel – the son of famed English astronomer William Herschel, who discovered many of Saturn’s and Uranus’ moons – wrote to Lassell and recommended he observe Neptune to see if it had any moons as well.

New Horizons image of Neptune and its largest moon, Triton. June 23, 2010. Credit: NASA
New Horizons image of Neptune and its largest moon, Triton, taken by the LORRI instrument on June 23, 2010. Credit: NASA

Lassell did so and discovered Neptune’s largest moon eight days later. Thirty-four years later, French astronomer Camille Flammarion named the moon Triton – after the Greek sea god and son of Poseidon (the equivalent of the Roman god Neptune) – in his 1880 book Astronomie Populaire. It would be several decades before the name caught on however. Until the discovery of the second moon Nereid in 1949, Triton was commonly known simply as “the satellite of Neptune”.

Size, Mass and Orbit:

At 2.14 × 1022 kg, and with a diameter of approx. 2,700 kilometers (1,680 miles) km, Triton is the largest moon in the Neptunian system – comprising more than 99.5% of all the mass known to orbit the planet. In addition to being the seventh-largest moon in the Solar System, it is also more massive than all known moons in the Solar System smaller than itself combined.

With no axial tilt and an eccentricity of virtually zero, the moon orbits Neptune at a distance of 354,760 km (220,438 miles). At this distance, Triton is the farthest satellite of Neptune, and orbits the planet every 5.87685 Earth days. Unlike other moons of its size, Triton has a retrograde orbit around its host planet.

Most of the outer irregular moons of Jupiter and Saturn have retrograde orbits, as do some of Uranus’s outer moons. However, these moons are all much more distant from their primaries, and are rather small in comparison. Triton also has a synchronous orbit with Neptune, which means it keeps one face aimed towards the planet at all times.

As Neptune orbits the Sun, Triton’s polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other, moves into the sunlight. Such changes were observed in April of 2010 by astronomers using the European Southern Observatory’s Very Large Telescope.

Another all-important aspect of Triton’s orbit is that it is decaying. Scientists estimate that in approximately 3.6 billion years, it will pass below Neptune’s Roche limit and will be torn apart.

Composition:

Triton has a radius, density (2.061 g/cm3), temperature and chemical composition similar to thatof Pluto. Because of this, and the fact that it circles Neptune in a retrograde orbit, astronomers believe that the moon originated in the Kuiper Belt and later became trapped by Neptune’s gravity.

Another theory has it that Triton was once a dwarf planet with a companion. In this scenario, Neptune captured Triton and flung its companion away when the giant gas moved further out into the solar system, billions of years ago.

Also like Pluto, 55% of Triton’s surface is covered with frozen nitrogen, with water ice comprising 15–35% and dry ice (aka. frozen carbon dioxide) forming the remaining 10–20%. Trace amounts of methane and carbon monoxide ice are believed to exist there as well, as are small amounts of ammonia (in the form of ammonia dihydrate in the lithosphere).

Triton’s density suggests that its interior is differentiated between a solid core made of rocky material and metals, a mantle composed of ice, and a crust. There is enough rock in Triton’s interior for radioactive decay to power convection in the mantle, which may even be sufficient to maintain a subterranean ocean. As with Jupiter’s moon of Europa, the proposed existence of this warm-water ocean could mean the presence of life beneath the icy crusts.

Atmosphere and Surface Features:

Triton has a considerably high albedo, reflecting 60–95% of the sunlight that reaches it. The surface is also quite young, which is an indication of the possible existence of an interior ocean and geological activity. The moon has a reddish tint, which is probably the result of the methane ice turning to carbon due to exposure to ultraviolet radiation.

Triton is considered to be one of the coldest places in the Solar System. The moon’s surface temperature is approx. -235°C while Pluto averages about -229°C. Scientists say that Pluto may drop as low as -240°C at the furthest point from the Sun in its orbit, but it also gets much warmer closer to the Sun, giving it a higher overall temperature average.

It is also one of the few moons in the Solar System that is geologically active, which means that its surface is relatively young due to resurfacing. This activity also results in cryovolcanism, where water ammonia and nitrogen gas burst forth from the surface instead of liquid rock. These nitrogen geysers can send plumes of liquid nitrogen 8 km above the surface of the moon.

Triton (lower left) compared to the Moon (upper left) and Earth (right), to scale. Credit: NASA/JPL/USGS
Triton (lower left) compared to the Moon (upper left) and Earth (right), to scale. Credit: NASA/JPL/USGS

Because of the geological activity constantly renewing the moon’s surface, there are very few impact craters on Triton. Like Pluto, Triton has an atmosphere that is thought to have resulted from the evaporation of ices from its surface. Like its surface ices, Triton’s tenuous atmosphere is made up of nitrogen with trace amounts of carbon monoxide and small amounts of methane near the surface.

This atmosphere consists of a troposphere rising to an altitude of 8km, where it then gives way to a thermosphere that reaches out to 950 km from the surface. The temperature of Triton’s upper atmosphere, at 95-100 K (ca.-175 °C/-283 °F) is higher than that at the surface, due to the influence of solar radiation and Neptune’s magnetosphere.

A haze permeates most of Triton’s troposphere, thought to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton’s atmosphere also has clouds of condensed nitrogen that lie between 1 and 3 km from the surface.

Observations taken from Earth and by the Voyager 2 spacecraft have shown that Triton experiences a warm summer season every few hundred years. This could be the result of a periodic change in the planet’s albedo (i.e. its gets darker and redder) which could be caused by either frost patterns or geological activity.

Using the CRIRES instrument on ESO’s Very Large Telescope, a team of astronomers has been able to see that the summer is in full swing in Triton’s southern hemisphere. Credit: ESO
Using the CRIRES instrument on ESO’s Very Large Telescope, a team of astronomers has been able to see that the summer is in full swing in Triton’s southern hemisphere. Credit: ESO

This change would allow more heat to be absorbed, followed by an increase in sublimation and atmospheric pressure. Data collected between 1987 and 1999 indicated that Triton was approaching one of these warm summers.

Exploration:

When NASA’s Voyager 2 made a flyby of Neptune in August of 1989, the mission controllers also decided to conduct a flyby of Triton – similar to Voyager 1‘s encounter with Saturn and Titan. When it made its flyby, most of the northern hemisphere was in darkness and unseen by Voyager.

Because of the speed of Voyager’s visit and the slow rotation of Triton, only one hemisphere was seen clearly at close distance. The rest of the surface was either in darkness or seen as blurry markings. Nevertheless, the Voyager 2 spacecraft managed to capture several images of the moon and spotted geysers of liquid nitrogen blasting out of two distinct features on the surface.

In August of 2014, in anticipation of New Horizons impending encounter with Pluto, NASA restored these photos and used them to create the first global color map of Triton. Produced by Paul Schenk, a scientist at the Lunar and Planetary Institute in Houston, the map was also used to make a movie (shown below) that recreated the historic Voyager 2 encounter in time for the 25th anniversary of the event.

Yes, Triton is indeed an unusual moon. Aside from its rather unique characteristics (retrograde motion, geological activity) the moon’s landscape is likely to be an amazing sight. For anyone standing on the surface, surrounded by colorful ices, plumes of nitrogen and ammonia, a nitrogen haze and Neptune’s big blue disc hanging on the sky, the experience would seem like something akin to a hallucination.

In the end, it is too bad that the Solar System will one day be saying good-bye to this moon. Because of the nature of its orbit, the moon will eventually fall into Neptune’s gravity well and break up. At which point, Neptune will have a huge ring like Saturn, until those particles crash into the planet as well.

That too would be something to behold. One can only hope that humanity will still be around in 3.6 billion years to witness it!

We have many interesting articles on Triton, Neptune, and the outer planets of the Solar System here at Universe Today.

Here’s one about the New Map of Triton, and one about the Underground Ocean it might be hiding, and 40 Years of Summer on Triton. And here’s Why You Shouldn’t Buy Real Estate on Triton.

In the Observatory also has an interview with Emily Lakdawalla, the senior editor and planetary evangelist for the Planetary Society, titled “Where Should We Look for Life in the Solar System?

Sources:

New Horizons Mission to Pluto

Humans have been sending spacecraft to other planets, as well as asteroid and comets, for decades. But rarely have any of these ventured into the outer reaches of our Solar System. In fact, the last time a probe reached beyond the orbit of Saturn to explore the worlds of Neptune, Uranus, Pluto and beyond was with the Voyager 2 mission, which concluded back in 1989.

But with the New Horizons mission, humanity is once again peering into the outer Solar System and learning much about its planets, dwarf planets, planetoids, moons and assorted objects. And as of July 14th, 2015, it made its historic rendezvous with Pluto, a world that has continued to surprise and mystify astronomers since it was first discovered.

Background:

In 1980, after Voyager 1‘s flyby of Saturn, NASA scientists began to consider the possibility of using Saturn to slingshot the probe towards Pluto to conduct a flyby by 1986. This would not be the case, as NASA decided instead to conduct a flyby of Saturn’s moon of Titan – which they considered to be a more scientific objective – thus making a slingshot towards Pluto impossible.

Because no mission to Pluto was planned by any space agency at the time, it would be years before any missions to Pluto could be contemplated. However, after Voyager 2′s flyby of Neptune and Triton in 1989, scientists once again began contemplating a mission that would take a spacecraft to Pluto for the sake of studying the Kuiper Belt and Kuiper Belt Objects (KBOs).

Voyager 2. Credit: NASA
Artist’s impression of the Voyager spacecraft in flight. Credit: NASA/JPL

In May 1989, a group of scientists, including Alan Stern and Fran Bagenal, formed an alliance called the “Pluto Underground”. Committed to the idea of mounting an exploratory mission to Pluto and the Kuiper Belt, this group began lobbying NASA and the US government to make it this plan a reality. Combined with pressure from the scientific community at large, NASA began looking into mission concepts by 1990.

During the course of the late 1990s, a number of Trans-Neptunian Objects (TNOs) were discovered, confirming the existence of the Kuiper Belt and spurring interest in a mission to the region. This led NASA to instruct the JPL to re-purpose the mission as a Pluto and KBO flyby. However, the mission was scrapped by 2000, owing to budget constraints.

Backlash over the cancellation led NASA’s Science Mission Directorate to create the New Frontiers program which began accepting mission proposals. Stamatios “Tom” Krimigis, head of the Applied Physics Laboratory’s (APL) space division, came together with Alan Stern to form the New Horizons team. Their proposal was selected from a number of submissions, and officially selected for funding by the New Frontiers program in Nov. 2001.

Despite additional squabbles over funding with the Bush administration, renewed pressure from the scientific community allowed the New Horizons team managed to secure their funding by the summer of 2002. With a commitment of $650 million for the next fourteen years, Stern’s team was finally able to start building the spacecraft and its instruments.

Engineers working on the New Horizons spacecraft's Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument. Credit: NASA
Engineers working on the New Horizons spacecraft’s Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument. Credit: NASA

Mission Profile:

New Horizons was planned as a voyage to the only unexplored planet in the Solar System, and was originally slated for launch in January 2006 and arrival at Pluto in 2015. Alan Stern was selected as the mission’s principal investigator, and construction of the spacecraft was handled primarily by the Southwest Research Institute (SwRI) and the Johns Hopkins Applied Physics Laboratory, with various contractor facilities involved in the navigation of the spacecraft.

Meanwhile, the US Naval Observatory (USNO) Flagstaff Station – in conjunction with NASA and JPL – was responsible for performing navigational position data and related celestial frames. Coincidentally, the UNSO Flagstaff station was where the photographic plates that led to the discovery of Pluto’s moon Charon came from.

In addition to its compliment of scientific instruments (listed below), there are several cultural artifacts traveling aboard the spacecraft. These include a collection of 434,738 names stored on a compact disc, a piece of Scaled Composites’s SpaceShipOne, and a flag of the USA, along with other mementos. In addition, about 30 g (1 oz) of Clyde Tombaugh’s ashes are aboard the spacecraft, to commemorate his discovery of Pluto in 1930.

The New Horizons spacecraft takes off on Jan. 19, 2006 from the Kennedy Space Center for its planned close encounter with Pluto. Credit: NIKON/Scott Andrews/NASA
The New Horizons spacecraft takes off on Jan. 19, 2006 from the Kennedy Space Center for its planned close encounter with Pluto. Credit: NIKON/Scott Andrews/NASA

Instrumentation:

The New Horizons science payload consists of seven instruments. They are (in alphabetically order):

  • Alice: An ultraviolet imaging spectrometer responsible for analyzing composition and structure of Pluto’s atmosphere and looks for atmospheres around Charon and Kuiper Belt Objects (KBOs).
  • LORRI: (Long Range Reconnaissance Imager) a telescopic camera that obtains encounter data at long distances, maps Pluto’s farside and provides high resolution geologic data.
  • PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation) an energetic particle spectrometer which measures the composition and density of plasma (ions) escaping from Pluto’s atmosphere.
  • Ralph: A visible and infrared imager/spectrometer that provides color, composition and thermal maps.
  • REX: (Radio Science EXperiment) a device that measures atmospheric composition and temperature; passive radiometer.
  • SDC: (Student Dust Counter) built and operated by students, this instrument measures the space dust peppering New Horizons during its voyage across the solar system.
  • SWAP: (Solar Wind Around Pluto) a solar wind and plasma spectrometer that measures atmospheric “escape rate” and observes Pluto’s interaction with solar wind.
Instruments New Horizons will use to characterize Pluto are REX (atmospheric composition and temperature; PEPSSI (composition of plasma escaping Pluto's atmosphere); SWAP (solar wind); LORRI (close up camera for mapping, geological data); Star Dust Counter (student experiment measuring space dust during the voyage); Ralph (visible and IR imager/spectrometer for surface composition and thermal maps and Alice (composition of atmosphere and search for atmosphere around Charon). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
The instruments New Horizons will use to characterize Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Launch:

Due to a series of weather-related delays, the New Horizons mission launched on January 19th, 2006, two days later than originally scheduled. The spacecraft took off from Cape Canaveral Air Force Station, Florida, at 15:00 EST (19:00 UTC) atop an Atlas V 551 rocket. This was the first launch of this particular rocket configuration, which has a third stage added to increase the heliocentric (escape) speed.

The spacecraft left Earth faster than any spacecraft to date, achieving a launch velocity of 16.5 km/s. It took only nine hours to reach the Moon’s orbit, passing lunar orbit before midnight (EST) on the same day it was launched. It has not, however, broken Voyager 1‘s record – which is currently traveling at 17.145 km/s (61,720 km/h, 38,350 mph) relative to the Sun – for being the fastest spacecraft to leave the Solar System.

Inner Solar System:

Between January and March, 2006, mission controllers guided the probe through a series of trajectory-correction maneuvers (TCMs). During the week of February 20th, 2006, controllers conducted in-flight tests on three of the major on board science instruments. On April 7th, the spacecraft passed the orbit of Mars, moving at roughly 21 km/s (76,000 km/h; 47,000 mph) away from the Sun.

At this point in its journey, the spacecraft had reached a distance of 243 million kilometers from the Sun, and approximately 93.4 million km from Earth. On June 13th, 2006, the New Horizons spacecraft passed the tiny asteroid 132524 APL at a distance of 101,867 km (63,297 mi) when it was closest.

Using the Ralph instrument, New Horizons was able to capture images of the asteroid, estimating to be 2.5 km (1.6 mi) in diameter. The spacecraft also successfully tracked the asteroid from June 10th-12th, 2006, allowing the mission team to test the spacecraft’s ability to track rapidly moving objects.

First images of Pluto in September 2006. Credit: NASA
First images of Pluto taken by New Horizons in September 2006. Credit: NASA

From September 21st-24th, New Horizons managed to capture its first images of Pluto while testing the LORRI instruments. These images, which were taken from a distance of approximately 4,200,000,000 km (2.6×109 mi) or 28.07 AU and released on November 28th, confirmed the spacecraft’s ability to track distant targets.

Outer Solar System:

On September 4th, 2006, New Horizons took its first pictures of Jupiter at a distance of 291 million kilometers (181 million miles). The following January, it conducted more detailed surveys of the system, capturing an infrared image of the moon Callisto, and several black and white images of Jupiter itself.

By February 28th, 2007, at 23:17 EST (03:17, UTC) New Horizons made its closest approach to Europa, at a distance of 2,964,860 km (1,842,278 mi). At 01:53:40 EST (05:43:40 UTC), the spacecraft made its flyby of Jupiter, at a distance of 2.3 million km (1.4 million mi) and received a gravity assist.

The Jupiter flyby increased New Horizons‘ speed by 4 km/s (14,000 km/h; 9,000 mph), accelerating the probe to a velocity of 23 km/s (83,000 km/h; 51,000 mph) relative to the Sun and shortening its voyage to Pluto by three years.

The encounter with Jupiter not only provided NASA with the opportunity to photograph the planet using the latest equipment, it also served as a dress rehearsal for the spacecraft’s encounter with Pluto. As well as testing the imaging instruments, it also allowed the mission team to test the communications link and the spacecraft’s memory buffer.

Black and white image of Jupiter viewed by LORRI in January 2007
Black and white image of Jupiter viewed by LORRI in January 2007. Credit: NASA/John Hopkins University Applied Physics Laboratory/Southwest Research Institute

One of the main goals during the Jupiter encounter was observing its atmospheric conditions and analyzing the structure and composition of its clouds. Heat-induced lightning strikes in the polar regions and evidence of violent storm activity were both observed. In addition, the Little Red Spot,  was imaged from up close for the first time. The New Horizons spacecraft also took detailed images of Jupiter’s faint ring system. Traveling through Jupiter’s magnetosphere, the spacecraft also managed to collect valuable particle readings.

The flyby of the Jovian systems also gave scientists the opportunity to examine the structure and motion of Io’s famous lava plumes. New Horizons measured the plumes coming from the Tvashtar volcano, which reached an altitude of up to 330 km from the surface, while infrared signatures confirmed the presence of 36 more volcanoes on the moon.

Callisto’s surface was also analyzed with LEISA, revealing how lighting and viewing conditions affect infrared spectrum readings of its surface water ice. Data gathered on minor moons such as Amalthea also allowed NASA scientists to refine their orbit solutions.

After passing Jupiter, New Horizons spent most of its journey towards Pluto in hibernation mode. During this time, New Horizons crossed the orbit of Saturn (June 8, 2008) and Uranus on (March 18, 2011). In June 2014, the spacecraft emerged from hibernation and the team began conducting instrument calibrations and a course correction,. By August 24th, 2014, it crossed Neptune’s orbit on its way to Pluto.

Capturing Callisto
New Horizons Long Range Reconnaissance Imager (LORRI) captured these two images of Jupiter’s outermost large moon, Callisto, during its flyby in February 2007. Credit: NASA/JPL

Rendezvous with Pluto:

Distant-encounter operations at Pluto began on January 4th, 2015. Between January 25th to 31st, the approaching probe took several images of Pluto, which were released by NASA on February 12th. These photos, which were taken at a distance of more than 203,000,000 km (126,000,000 mi) showed Pluto and its largest moon, Charon.

Investigators compiled a series of images of the moons Nix and Hydra taken from January 27th through February 8th, 2015, beginning at a range of 201,000,000 km (125,000,000 mi), while Kerberos and Styx were captured by photos taken on April 25.

On July 4th, 2015, NASA lost contact with New Horizons after it experienced a software anomaly and went into safe mode. On the following day, NASA announced that they had determined it to be the result of a timing flaw in a command sequence. By July 6th, the glitch had been fixed and the probe had exited safe mode and began making its approach.

The New Horizons spacecraft made its closest approach to Pluto at 07:49:57 EDT (11:49:57 UTC) on July 14th, 2015, and then Charon at 08:03:50 EDT (12:03:50 UTC). Telemetries confirming a successful flyby and a healthy spacecraft reached Earth on 20:52:37 EDT (00:52:37 UTC).

During the flyby, the probe captured the clearest pictures of Pluto to date, and full analyses of the data obtained is expected to take years to process. The spacecraft is currently traveling at a speed of 14.52 km/s (9.02 mi/s) relative to the Sun and at 13.77 km/s (8.56 mi/s) relative to Pluto.

Full trajectory of New Horizons space probe (sideview). Credit: pluto.jhuapl.edu
Full trajectory of New Horizons space probe (sideview). Credit: pluto.jhuapl.edu

Future Objectives:

With its flyby of Pluto now complete, the New Horizons probe is now on its way towards the Kuiper Belt. The goal here is to study one or two other Kuiper Belt Objects, provided suitable KBOs are close to New Horizons‘ flight path.

Three objects have since been selected as potential targets, which were provisionally designated PT1 (“potential target 1”), PT2 and PT3 by the New Horizons team. These have since been re-designated as 2014 MU69 (PT1), 2014 OS393 (PT2), and 2014 PN70 (PT3).

All of these objects have an estimated diameter of 30–55 km, are too small to be seen by ground telescopes, and are 43–44 AU from the Sun, which would put the encounters in the 2018–2019 period. All are members of the “cold” (low-inclination, low-eccentricity) classical Kuiper Belt, and thus very different from Pluto.

Even though it was launched far faster than any outward probe before it, New Horizons will never overtake either Voyager 1 or Voyager 2 as the most distant human-made object from Earth. But then again, it doesn’t need to, given that what it was sent out to study all lies closer to home.

What’s more, the probe has provided astronomers with extensive and updated data on many of planets and moons in our Solar System – not the least of which are the Jovian and Plutonian systems. And last, but certainly not least, New Horizons is the first spacecraft to have it made it out to such a distance since the Voyager program.

And so we say so long and good luck to New Horizons, not to mention thanks for providing us with the best images of Pluto anyone has ever seen! We can only hope she fares well as she makes its way into the Kuiper Belt and advances our knowledge of the outer Solar System even farther.

We have many interesting articles about the New Horizons spacecraft and Pluto here on Universe Today. For example, here are some Interesting Facts About PlutoHow Long Does it Take to Get to Pluto, Why Pluto is No Longer Considered a Planet, and Is There Life on Pluto?

For more information on the Kuiper Belt, check out What is The Kuiper Belt? and NASA’s Solar System Exploration entry on the Kuiper Belt and Oort Cloud.

Astronomy Cast also has some fascinating episodes on Pluto, including On Pluto’s Doorstep – Live Hangout with New Horizons Team

And be sure to check out the New Horizons mission homepage at NASA.