Can Moons Have Moons?

Can Moons Have Moons?

The Earth has a single moon, while Saturn has more than 60, with new moons being discovered all the time. But here’s a question, can a moon have a moon? Can that moon’s moon have its own moon? Can it be moons all the way down?

First, consider that we have a completely subjective idea of what a moon is. The Moon orbits the Earth, and the Earth orbits the Sun, and the Sun orbits the center of the Milky Way, which orbits within the Local Group, which is a part of the Virgo Supercluster. The motions of objects in the cosmos act like a set of Russian nesting dolls, with things orbiting things, which orbit other things. So maybe a better question is: could any of the moons in the Solar System have moons of their own? Well actually, one does.

Right now, NASA’s Lunar Reconnaissance Orbiter is happily orbiting around the Moon, photographing the place in high resolution. But humans sent it to the Moon, and just like all the artificial satellites sent there in the past, it’s doomed. No satellite we’ve sent to the Moon has ever orbited for longer than a few years before crashing down into the lunar surface. In theory, you could probably get a satellite to last a few hundred years around the Moon.

But why? How come we can’t make moons for our moon to have a moon of it’s own for all time? It all comes down to gravity and tidal forces. Every object in the Universe is surrounded by an invisible sphere of gravity. Anything within this volume, which astronomers call the “Hill Sphere”, will tend to orbit the object.

So, if you had the Moon out in the middle of space, without any interactions, it could easily have multiple moons orbiting around it. But you get problems when you have these overlapping spheres of influence. The strength of gravity from the Earth tangles with the force of gravity from the Moon.

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

Although a spacecraft can orbit the Moon for a while, it’s just not stable. The tidal forces will cause the spacecraft’s orbit to decay until it crashes. But further out in the Solar System, there are tiny asteroids with even tinier moons. This is possible because they’re so far away from the Sun. Bring these asteroids closer to the Sun, and someone’s losing a moon.

The object with the largest Hill Sphere in the Solar System is Neptune. Because it’s so far away from the Sun, and it’s so massive, it can truly influence its environment. You could imagine a massive moon distantly orbiting Neptune, and around that moon, there could be a moon of its own. But this doesn’t appear to be the case.

NASA is considering a mission to capture an asteroid and put it into orbit around the Moon. This would be safer than having it orbit the Earth, but still keep it close enough to extract resources. But without any kind of orbital boost, those tidal forces will eventually crash it onto the Moon. So no, in our Solar System, we don’t know of any moons with moons of their own. In fact, we don’t even have a name for them. What would you suggest?

Weekly Space Hangout – July 19, 2013

Here’s our Weekly Space Hangout for July 19, 2013. Watch as a team of space and astronomy journalists discuss the big space stories of the week. We do this every Friday at 12:00 pm Pacific Time / 3:00 pm Eastern Time. You can join us live, or watch the archive here or on Google+.

Host: Fraser Cain

Participants: Sondy Springmann, Amy Shira Teitel, Jason Major, David Dickenson, Dr. Matthew Francis

And here are the stories that we covered.

Researcher Finds a New Moon Around Neptune in Hubble Data

This composite Hubble Space Telescope picture shows the location of a newly discovered moon, designated S/2004 N 1, orbiting the giant planet Neptune, nearly 4.8 billion km (3 billion miles) from Earth. Credit: NASA, ESA, and M. Showalter (SETI Institute).

It took sharp and patient eyes, but researcher Mark Showalter of the SETI Institute has found a tiny moon orbiting Neptune that’s never been seen before. Showalter used archival data from the Hubble Space Telescope to find the moon, designated S/2004 N 1, which is estimated to be no more than 19 km (12 miles) across, making it the smallest known moon in the Neptunian system. This is the 14th known moon of Neptune.

S/2004 N 1 is so small and dim that it is roughly 100 million times fainter than the faintest star that can be seen with the naked eye, NASA said. Even Voyager 2 –which flew past Neptune in 1989 to survey planet’s system of moons and rings – didn’t catch a view of this moon, even though data from Voyager 2 revealed several other moons.

Neptune photographed by Voyage. Image credit: NASA/JPL
Neptune photographed by Voyager 2. Image credit: NASA/JPL

Showalter was studying the faint arcs, or segments of rings, around Neptune earlier this month.

“The moons and arcs orbit very quickly, so we had to devise a way to follow their motion in order to bring out the details of the system,” he said. “It’s the same reason a sports photographer tracks a running athlete — the athlete stays in focus, but the background blurs.”

The method involved tracking the movement of a white dot that appears over and over again in more than 150 archival Neptune photographs taken by Hubble from 2004 to 2009.

Showalter noticed the white dot about 100,000 km (65,400 miles) from Neptune, located between the orbits of the Neptunian moons Larissa and Proteus. Showalter plotted a circular orbit for the moon, which completes one revolution around Neptune every 23 hours.

Showalter should get the “Eagle Eyes” award for 2013!

Source: HubbleSite

Saturn Storm’s ‘Suck Zone’ Shown In Spectacular Cassini Shots

A false-color image, taken by the Cassini spacecraft, of a huge hurricane at Saturn's north pole. Credit: NASA/JPL-Caltech/SSI

Checking out the above pictures of a Saturn hurricane, one can’t help but wonder: how close was the Cassini spacecraft to spiralling down into gassy nothingness?

These dizzying images of a hurricane on Saturn, of course, came as the spacecraft zoomed overhead at a safe distance. NASA’s goal in examining this huge hurricane is to figure out its mechanisms and to compare it to what happens on our home planet.

Hurricanes on Earth munch on water vapor to keep spinning. On Saturn, there’s no vast pool of water to draw from, but there’s still enough water vapor in the clouds to help scientists understand more about how hurricanes on Earth begin, and continue.

“We did a double take when we saw this vortex because it looks so much like a hurricane on Earth,” stated Andrew Ingersoll, a Cassini imaging team member at the California Institute of Technology in Pasadena. “But there it is at Saturn, on a much larger scale, and it is somehow getting by on the small amounts of water vapor in Saturn’s hydrogen atmosphere.”

A false-color view of Saturn's storm, as seen through Cassini's wide-angle camera. The blue bands at the edge are Saturn's rings. Credit: NASA/JPL-Caltech/SSI
A false-color view of Saturn’s storm, as seen through Cassini’s wide-angle camera. You can see the eye in dark red, the jet stream in yellowish-green, and low-lying clouds in orange. The blue bands at the edge are Saturn’s rings. Credit: NASA/JPL-Caltech/SSI

There’s one big change in hurricane activity you’d observe if suddenly shifted from Earth to Saturn: this behemoth — 1,250 miles (2,000 kilometers) wide, about 20 times its Earthly counterparts — spins a heckuva lot faster.

In the eye, winds in the wall speed more than four times faster than what you’d find on Earth. The hurricane also sticks around at the north pole. On Earth, hurricanes head north (and eventually dissipate) due to wind forces generated by the planet’s rotation.

“The polar hurricane has nowhere else to go, and that’s likely why it’s stuck at the pole,” stated Kunio Sayanagi, a Cassini imaging team associate at Hampton University in Hampton, Va.

Cassini initially spotted the storm in 2004 through its heat-seeking infrared camera, when the north pole was shrouded in darkness during winter.

The spacecraft first caught the storm in visible light in 2009, when NASA controllers altered Cassini’s orbit so that it could view the poles.

Saturn, of course, is not the only gas giant in the solar system with massive hurricanes. Jupiter’s Great Red Spot has been raging since before humans first spotted it in the 1600s. It appears to be shrinking, and could become circular by 2040.

Neptune also has hurricanes that can reach speeds of 1,300 miles (2,100 kilometers) an hour despite its cold nature; it even had a Great Dark Spot spotted during Voyager’s flypast in 1989 that later faded from view. Uranus, which scientists previously believed was quiet, is a pretty stormy place as well.

Check out this YouTube video for more details on how Saturn’s storm works.

Source: Jet Propulsion Laboratory

Is Triton Hiding an Underground Ocean?

Voyager 2 mosaic of Neptune’s largest moon, Triton (NASA)

At 1,680 miles (2,700 km) across, the frigid and wrinkled Triton is Neptune’s largest moon and the seventh largest in the Solar System. It orbits the planet backwards – that is, in the opposite direction that Neptune rotates – and is the only large moon to do so, leading astronomers to believe that Triton is actually a captured Kuiper Belt Object that fell into orbit around Neptune at some point in our solar system’s nearly 4.7-billion-year history.

Briefly visited by Voyager 2 in late August 1989, Triton was found to have a curiously mottled and rather reflective surface nearly half-covered with a bumpy “cantaloupe terrain” and a crust made up of mostly water ice, wrapped around a dense core of metallic rock. But researchers from the University of Maryland are suggesting that between the ice and rock may lie a hidden ocean of water, kept liquid despite estimated temperatures of  -97°C (-143°F), making Triton yet another moon that could have a subsurface sea.

How could such a chilly world maintain an ocean of liquid water for any length of time? For one thing, the presence of ammonia inside Triton would help to significantly lower the freezing point of water, making for a very cold — not to mention nasty-tasting — subsurface ocean that refrains from freezing solid.

In addition to this, Triton may have a source of internal heat — if not several. When Triton was first captured by Neptune’s gravity its orbit would have initially been highly elliptical, subjecting the new moon to intense tidal flexing that would have generated quite a bit of heat due to friction (not unlike what happens on Jupiter’s volcanic moon Io.) Although over time Triton’s orbit has become very nearly circular around Neptune due to the energy loss caused by such tidal forces, the heat could have been enough to melt a considerable amount of water ice trapped beneath Triton’s crust.

Related: Titan’s Tides Suggest a Subsurface Sea

Another possible source of heat is the decay of radioactive isotopes, an ongoing process which can heat a planet internally for billions of years. Although not alone enough to defrost an entire ocean, combine this radiogenic heating with tidal heating and Triton could very well have enough warmth to harbor a thin, ammonia-rich ocean beneath an insulating “blanket” of frozen crust for a very long time — although eventually it too will cool and freeze solid like the rest of the moon. Whether this has already happened or still has yet to happen remains to be seen, as several unknowns are still part of the equation.

“I think it is extremely likely that a subsurface ammonia-rich ocean exists in Triton,” said Saswata Hier-Majumder at the University of Maryland’s Department of Geology, whose team’s paper was recently published in the August edition of the journal Icarus. “[Yet] there are a number of uncertainties in our knowledge of Triton’s interior and past which makes it difficult to predict with absolute certainty.”

Still, any promise of liquid water existing elsewhere in large amounts should make us take notice, as it’s within such environments that scientists believe lie our best chances of locating any extraterrestrial life. Even in the farthest reaches of the Solar System, from the planets to their moons, into the Kuiper Belt and even beyond, if there’s heat, liquid water and the right elements — all of which seem to be popping up in the most surprising of places — the stage can be set for life to take hold.

Read more about this here on Astrobiology.net.

Inset image: Voyager 2 portrait of Neptune and Triton taken on August 28, 1989. (NASA)

Weekly SkyWatcher’s Forecast: April 30-May 6, 2012

Large Magellanic Cloud - Image Courtesy of NASA

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Greetings, fellow SkyWatchers! Are you ready for another week filled with bright planets, a meteor shower, challenging lunar features, interesting stars and astronomy history? Then you have come to the right place! Bring along your telescopes and binoculars and meet me in the backyard…

Monday, April 30 – Karl Frederich Gauss was born on this day in 1777. Known as the “Prince of Mathematics,” Gauss contributed to the field of astronomy in many ways – from computing asteroid orbits to inventing the heliotrope. Out of Gauss’ many endeavors, he is most recognized for his work in magnetism. We understand the term “gauss” as a magnetic unit – a refrigerator magnet carries about 100 gauss while an average sunspot might go up to 4000. On the most extreme ends of the magnetic scale, the Earth produces about 0.5 gauss at its poles, while a magnetar can produce as much as 10 to the 15th power in gauss units!

While we cannot directly observe a magnetar, those living in the Southern Hemisphere can view a region of the sky where magnetars are known to exist – the Large Magellanic Cloud – or you can use the projection method to view a sunspot! If you have a proper solar filter, magnetism distorts sunspots as they near the limb – called the “Wilson Effect”

Tuesday, May 1 – On this day in 1949 Gerard Kuiper discovered Nereid, a satellite of Neptune. If you’re game, you can find Neptune – usually hanging around in Capricornus – about an hour before dawn. While it can be seen in binoculars as a bluish “star,” it takes around a 6″ telescope and some magnification to resolve its disc. Today’s imaging technology can even reveal its moons!

While you’re out this morning, keep an eye on the sky for the peak of the Phi Bootid meteor shower, whose radiant is near the constellation of Hercules. While the best time to view a meteor shower is around 2:00 a.m. local time, you will have best success watching for these meteors when the Moon is as far west as possible. The average fall rate is about 6 per hour.

Our lunar mission for tonight is to move south, past the crater rings of Ptolemaeus, Alphonsus, Arzachel, and Purbach, until we end up at the spectacular crater Walter.

Named for Dutch astronomer Bernhard Walter, this 132- by 140-kilometer-wide lunar feature offers up amazing details at high power. It is worthwhile to take the time to study the differing levels, which drop to a maximum of 4,130 meters below the surface. Multiple interior strikes abound, but the most fascinating of all is the wall crater Nonius. Spanning 70 kilometers, Nonius would also appear to have a double strike of its own—one that’s 2,990 meters deep!

Wednesday, May 2 – On the lunar surface, we can enjoy a strange, thin feature. If you used last night’s map, you’re well acquainted with this area! Look toward the lunar south where you will note the prominent rings of craters Ptolemaeus, Alphonsus, Arzachel, Purbach, and Walter descending from north to south. Just west of them, you’ll see the emerging Mare Nubium. Between Purbach and Walter you will see the small, bright ring of Thebit with a crater caught on its edge. Look further west and you will see a long, thin, dark feature cutting across the mare. Its name? Rupes Recta – better known as The Straight Wall, or sometimes Rima Birt. It is one of the steepest known lunar slopes rising around 366 meters from the surface at a 41 degree angle.

Be sure to mark your lunar challenge notes and we’ll visit this feature again!

Another great target for a bright night is Delta Corvi. 125 light-years away, it displays a yellowish color primary and slightly blue secondary that’s an easily split star in any telescope, and a nice visual double with Eta in binoculars. Use low power and see if you can frame this bright grouping of stars in the same eyepiece field.

Before you put the telescope away for the evening, be sure to visit with Mars. If you’ve been keeping track, the red planet is slowly moving away from us and dimming even more. Tonight it should have reached an apparent -0.0 magnitude. Compare it to other nearby stars and gauge its brightness for yourself. How has its apparent position against the background stars changed over the weeks? Have you noted features like Syrtis Major or Amazonis Planitia? How have the polar caps changed?

Thursday, May 3 – Tonight we’ll use what we learned previously to locate another unusual feature – Montes Recti or the “Straight Range.” You’ll find this curiosity tucked between Plato and Sinus Iridum on the north shore of Mare Imbrium.

To binoculars or small scopes at low power, this isolated strip of mountains will appear as a white line drawn across the grey mare. It is believed this feature may be all that is left of a crater wall from the Imbrium impact. It runs for a distance of around 90 kilometers, and is approximately 15 kilometers wide. The Straight Range and some of its peaks reach up to 2072 meters! Although this doesn’t sound particularly impressive, that’s over twice as tall as the Vosges Mountains in central western Europe, and on the average very comparable to the Appalachian Mountains in the eastern United States.

Friday, May 4 – Tonight you are on your own without a map. Lunar features are easy when you become acquainted with them! Return to the Moon and explore with binoculars or telescopes the area to the south around another easy and delightful lunar feature you should recognize, the crater Gassendi. At around 110 kilometers in diameter and 2010 meters deep, this ancient crater contains a triple mountain peak in its center. As one of the most “perfect circles” on the Moon, the south wall of Gassendi has been eroded by lava flows over a 48 kilometer expanse and offers a great amount of detail to telescopic observers on its ridge- and rille-covered floor. For those observing with binoculars? Gassendi’s bright ring stands on the north shore of Mare Humorum…an area about the size of the state of Arkansas!

Northeast of Regulus by about a fistwidth is 2.61 magnitude Gamma Leonis – also known as Algieba. This is one of the finest double stars in the sky, but a little difficult at low power since the pair is both bright and close. Separated by about twice the diameter of our own solar system, this 90 light-year distant pair is slowly widening.

Another two fingerwidths north is 3.44 magnitude Zeta Leonis – also named Aldhafera. Located about 130 light-years away, this excellent star has an optical companion which is viewable in binoculars – 35 Leonis. Remember this pair, because it will lead you to galaxies later!

Saturday, May 5 – In 1961 Alan Shepard became the first American in “space” (as we now refer to that region above the sky), taking a 15 minute suborbital ride aboard the Mercury craft Freedom 7.

Return to the Moon tonight to have a look on the terminator near the southern cusp for two outstanding features. The easiest is crater Schickard – a class V mountain-walled plain that spans 227 kilometers. Named for German astronomer Wilhelm Schickard, this beautiful old crater with the subtle interior details has another crater caught on its northern wall named Lehmann.

Look further south for one of the Moon’s most incredible features – Wargentin. Among the many strange things on the lunar surface, Wargentin is unique. Once upon a time, it was a very normal crater and had been that way for hundreds of millions of years – then it happened. Either a fissure opened in its interior, or the meteoric impact that formed it caused molten lava to begin to rise. Oddly enough, Wargentin’s walls were without large enough breaks to allow the lava to escape and it continued to fill the crater to the rim. Often referred to as “the Cheese,” enjoy Wargentin tonight for its unusual appearance and be sure to note Nasmyth and Phocylides as well!

Before we leave, let’s have a look east at 3.34 magnitude Theta Leonis. Also known as Chort, mark this one in your memory, as well as 3.94 magnitude Iota to the south as markers for a galaxy hop. Last is easternmost 2.14 magnitude Beta. Denebola is the “Lion’s Tail” and has several faint optical companions.

Sunday, May 6 – Earlier we learned about awesome magnetic energy, but what happens when you find magnetism in a very unlikely place? Tonight might be Full Moon, but we can still have a look at the lunar surface just a little southeast of the grey oval of Grimaldi. The area we are looking for is called the Sirsalis Rille and on an orb devoid of magnetic fields – it’s magnetic! Like a dry river bed, this ancient “crack” on the surface runs 480 kilometers along the surface and branches in many areas.

For those who like curiosities, our target for tonight will be 1.4 degrees northwest of 59 Leonis, which is itself about a degree southwest of Xi. While this type of observation may not be for everyone, what we are looking for is a very special star – a red dwarf named Wolf 359 (RA 10 56 28.99 Dec +07 00 52.0).

Discovered photographically by Max Wolf in 1959, charts from that time period will no longer be accurate because of the star’s large proper motion. It is one of the least luminous stars known, and we probably wouldn’t even know it was there except for the fact that it is the third closest star to our solar system. Located only 7.5 light-years away, this miniature star is about 8% the size of our Sun – making it roughly the size of Jupiter. Oddly enough, it is also a “flare star” – capable of jumping another magnitude brighter at random intervals. It might be faint and difficult to spot in mid-sized scopes, but Wolf 359 is definitely one of the most unusual things you will ever observe!

Until next week? Ask for the Moon, but keep on reaching for the stars!

How Did Neptune Get Its Name?

Neptune Hurricanes
The "surface" of Neptune, its uppermost layer, is one of the most turbulent and active places in the Solar System. Credit: NASA/JPL

How did Neptune get its name? Shortly after its discovery, Neptune was only referred to as “the planet exterior to Uranus” or as “Le Verrier’s planet”. The first suggestion for a name came from Johann Galle, who proposed the name Janus. Another proposal was Oceanus. Urbain Le Verrier, who discovered the planet, claimed the right to name his discovery: Neptune. Soon Neptune became the internationally accepted name.

In roman mythology, Neptune was the god of the sea. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets, all of which, except for Earth, were named for Greek and Roman mythology. Most languages today use some variant of the name “Neptune” for the planet.

Now that you know how the planet was named, how about some facts about the planet itself. Size wise, the planet has an equatorial radius 24,764 km, a polar radius of 24,341 km, and a surface area of 7.6408×10,sup>9km2. It has a volume of 6.254×1013km3, a mass of 1.0243×1026kg, and a mean density of 1.638 g/cm3.

Its atmosphere is composed primarily of hydrogen and helium along with traces of hydrocarbons and nitrogen. It also contains a high proportion of ices like: water, ammonia, and methane. Astronomers occasionally categorize Neptune as an ice giant. The interior of Neptune is primarily composed of ices and rock. Traces of methane in the outermost regions account for the planet’s blue appearance. Neptune’s atmosphere is notable for its active and visible weather patterns. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h.Because of its great distance from the Sun, Neptune’s outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching ?218°C. Temperatures at the planet’s center are approximately 5,000°C. Neptune is one of the most interesting planets in our solar system. There are plenty of other articles about the planet here on Universe Today.

We have written many articles about Neptune for Universe Today. Here’s an article about the size of Neptune, and here’s an article about the atmosphere of Neptune.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We’ve also recorded an entire episode of Astronomy Cast all about Neptune. Listen here, Episode 63: Neptune.

Source: NASA

“Pluto-Killer” Sets Sights on Neptune

Infrared image of Neptune from Keck Observatory in Hawaii. Credit: Mike Brown/CalTech

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The confessed (and remorseless) “Pluto Killer” Mike Brown has turned his gaze – and the 10-meter telescope at the Keck Observatory in Hawaii – on Neptune, our solar system’s furthest “official” planet. But no worries for Neptune – Mike isn’t after its planetary status… he’s taken some beautiful infrared images instead!

Normally only visible as a featureless blue speck in telescopes, Brown’s image of Neptune — along with its largest moon Triton —  shows the icy gas giant in infrared light, glowing bright red and orange.

Neptune and Triton in infrared. Credit: Mike Brown/CalTech.

Brown’s initial intention was not just to get some pretty pictures of planets. The target of the imaging mission was Triton and to learn more about the placement of its methane, nitrogen and seasonal frosts, and this sort of research required infrared imaging. Of course, Neptune turned out to be quite photogenic itself.

“The big difference is doing the imaging in the infrared where methane absorbs most of the photons,” said Brown. “So the bright places are high clouds where the sunlight reflects off of them before it had a chance to pass through much of the atmosphere. Dark is clear atmosphere full of methane absorption.

“I just thought it was so spectacular that I should post it.”

No argument here, Mike!

Neptune, now officially the outermost planet in our solar system, is the fourth largest planet and boasts the highest wind speeds yet discovered — 1,250 mph winds scream around its frigid skies! Like the other gas giants Neptune has a system of rings, although nowhere near as extravagant as Saturn’s. It has 13 known moons, of which Triton is the largest.

With its retrograde orbit, Triton is believed to be a captured Kuiper Belt Object now in orbit around Neptune. Kuiper Belt Objects are Mike Brown’s specialty, as he is the astronomer most well-known for beginning the whole process that got Pluto demoted from the official planet list back in 2006.

Read more on Skymania.com here.

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Jason Major is a graphic designer, photo enthusiast and space blogger. Visit his website Lights in the Dark and follow him on Twitter @JPMajor or on Facebook for the most up-to-date astronomy awesomeness!

Hubble’s New Views of Neptune

Four images of Neptune taken a few hours apart by the Hubble Space Telescope on June 25-26, 2011. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

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To celebrate the first complete orbit of the planet Neptue since its discovery in 1846, the Hubble Space Telescope took a series of images with the Wide Field Camera 3, showing the different faces of the planet as it rotates on its axis. The images were take on June 25-26, 2011.

Even with a telescope as powerful as Hubble, the planet still appears fairly small, but some details are visible. While its blue color is the most distinctive feature, the turbulent conditions in the planet’s atmosphere also show up. Neptune’s thick atmosphere is largely made up of hydrogen and helium and is thought to host the Solar System’s most furious storms, with winds of up to 2000 km/h.

See more about Neptune and these images from ESA’s Hubble page (including access to wallpaper-sized images) and tead more about Neptune’s discovery and anniversary in our article by Tammy Plotner.