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Artist’s conception of the New Horizons spacecraft flying past Pluto and Charon
More dramatic news from the far reaches of the solar system: NASA’s probe New Horizons has awakened from its nine year hibernation and is powering up to approach Pluto! Although it sounds like “New Horizons” is a boy band, NASA gave up on trying to launch every saccharine teenybopper act into the Kuiper belt (although that is a laudable goal): instead the probe is named after the fact that New Horizons is the first human spacecraft to explore the dwarf planet Pluto and its little moons Charon and Hydra. Launched in January of 2006, New Horizons set the record for the highest launch speed of a human-made object from Earth. The grand piano-sized spacecraft has spent the intervening years hurtling through the darkness of space–although it has periodically come to partial wakefulness to check in with mission control and to snap some dramatic flyby photos of famous locations along its trip (like this photo montage of Jupiter and Io). The craft also used Jupiter’s gravity well to increase its velocity.
Composite image of Jupiter and Io as photographed from New Horizons (NASA)
Since the time the probe was launched, astronomers have discovered two new miniature moons of Pluto: Kerberus and Styx. This means that New Horizons mission planners were forced to assess the possibility of a catastrophic collision with unseen debris or dust left over from these satellites. Computer models suggest that the likelihood of such an accident is remote, but, just in case, NASA has added two dramatic contingency plans for the mission. In one emergency plan, the probe’s satellite dish acts as a dust shield, in the other, the craft drops dangerously close to Pluto, where atmospheric drag has presumably cleared the surrounding space of particles. These worst case plans will almost certainly not be needed, although we will learn more as New Horizons gets closer to the dwarf planet.
After flying past Pluto next July, New Horizons will hurtle into the Kuiper belt where NASA hopes the probe will rendezvous with an icy Kuiper belt object so that we can learn more about these enigmatic leftovers from the creation of the solar system. The coming 7 months should be filled with excitement as we learn more about the Pluto system!
Neptune’s Moon Triton photographed by Voyager 2 (NASA)
It is absolutely freezing here in Brooklyn—a great vortex of bitter Arctic air has swirled south across huge swathes of the nation. The temperature here is 9° Fahrenheit (or -13° Celsius). Imagine how much worse things are in Minnesota, where it is -14° Fahrenheit (or -25° Celsius). Brrr! It hurts my fingers to write about it–even in my overheated study (well—bedroom, really). Now truly stretch your mind from the frozen heartland of America to the edge of the planetary solar system. The largest moon of the ice giant Neptune is the moon Triton, discovered in 1846 by English astronomer/brewer William Lassell, and named for the son of Poseidon. On the surface of Triton temperatures plunge to 36…which is to say 36 K (Kelvin). To translate that is -237° Celsius or a bone chilling -395° Fahrenheit.
Artist’s rendering of an ice volcano on Triton with Neptune in the background (NASA)
Triton is a strange moon. It is the seventh largest moon in the solar system and it is the only large moon to orbit its planet in a direction opposite from the planet’s rotation (which is called a retrograde orbit). Since there is no model for retrograde moons forming from accretion disks, Triton must be a captured object from the Oort cloud—and, indeed, the moon is extremely similar in composition to Pluto and other dwarf planets of the Solar system’s distant periphery. Despite the extreme cold of Triton’s surface, the moon is geologically active. Like Earth, the moon is probably differentiated into layers: a core, a mantle, and a crust. The crust is formed of ice: frozen water, methane, and nitrogen. A large polar cap covers the southern pole, but much of the rest of the moon is a“cantaloupe” surface of melted and refrozen ice. The surface is (geologically) young. Cryovolcanic activity and tidal forces have kept the ice active. Cryovolcanoes were first spotted on Triton during the Voyager 2 flyby in 1989 (the first time such phenomenon were ever observed). Because of tidal warming (caused by gravitational interaction with Neptune), Triton may have once had a liquid ocean beneath the crust, but this has likely solidified assuming that there is no radioactive decay from the rocky core.
A size comparison of Earth, Earth’s moon, and Triton
Triton is closer to Neptune than the Earth’s moon is to Earth…and Neptune is seventeen times more massive than Earth. This doesn’t bode well for the long term future of Triton. Within the next three and a half billion years, the moon will either be pulled into Neptune’s surface and swallowed or it will be ripped to pieces and form a spectacular ring structure like Saturn’s.
The Dwarf Planet Haumea
Haumea is a dwarf planet located in the Kuiper belt. The little planetoid was discovered in December 2004 by a team of Caltech astronomers. It is about a third the size of Pluto. The team initially called it “Santa” but, in keeping with the IAU’s naming convention for Kuiper belt objects they eventually named the worldlet after a matronly fertility goddess from Hawaiian mythology.
Artist’s conception of Haumea and its pink spot
Although Haumea is typical of other dwarf planets in the Oort cloud in that it is a hunk of rock covered with ice, there are a couple of very unique things about the body. Most notably Haumea is shaped like a lozenge (as opposed to being mostly spherical like other planets). Astronomers believe that Haumea has sufficient gravity to overcome the compressive strength of its material. In other words it chould be approximately spherical, however the planet is rotating with such velocity that it has become spindle shaped—like a water balloon thrown in a rifling spiral.
The extreme rapidity of Haumea’s rotation is its other defining characteristic. It rotates more rapidly than any planetlike object with a diameter greater than 100 kilometers. Haumea rotates completely every 3.9 hours so days there are incredibly short (although its huge orbit takes 283 years to complete—so years are long). It is believed that Haumea’s breakneck spin comes from a titanic collision with some other Oort belt object. Haumea’s two dinky moons were probably also created by the impact. Haumea has a large red spot on it–perhaps because of the presence of minerals–or the fractured perturbance left by an impact.
An Artist’s conception of Jupiter seen from Amalthea
If you wanted to build a vacation home with a truly spectacular view, one of the possibilities you might consider is Jupiter’s moon Amalthea. Discovered in 1892 by the American astronomer Edward Emerson Barnard (who also discovered Barnard’s star) Amalthea was the first Jovian moon discovered by someone other than Galileo Galilei. Amalthea is the largest inner satellite of Jupiter and from its surface Jupiter would appear to take up 46.5 degrees of the sky (from the horizon to directly overhead is 90 degrees). Amalthea is in synchronous rotation around Jupiter and so the planet would always appear in the same part of the sky (provided you were on the right part of the moon). From Amalthea the sun would disappear behind the planet’s bulk for an hour and a half each revolution.
A digital rendering of Amalthea
The 66 known Jovian moons are largely named after the lovers and children of Jupiter/Zeus, however Amalthea is an exception: it is named for Jupiter’s step-mother the goat/nymph Amalthea who fed and cared for the young god as he quickly grew to adulthood and whose impervious skin was fashioned into the aegis of the king of the god. The name Amalthea was used for the moon almost since it was discovered but was only formally adopted by the International Astronomical Union in 1975.
Amalthea Visited by the NASA spacecraft Galileo
Amalthea is a strange and mysterious moon which perplexes astronomers. Its irregularly shape is somewhat like a potato and it is covered with deep craters and tall mountains. The surface of the moon is deep red in color (in fact Amalthea is the reddest object in the solar system) however weird bright patches of green appearing on the mountain slopes–the nature of which is unknown. The moon appears to be formed of ice and rubble, but if had formed where it now is during the early days of Jupiter, it would have melted. The moon must have formed elsewhere and been captured by Jupiter—a recent paper speculated that it was originally a Trojan asteroid. Since Amalthea is made of ice and heterogeneous rubble scientists are perplexed at why gravity has not rearranged its into a more spherical shape. Since Amalthea is so close to Jupiter it’s orbit is decaying and it will one day fall into the gas giant (so you may want to get really good insurance on the vacation house I mentioned in the first paragraph).
Mimas (Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA)
Mimas is a moon of Saturn. Discovered late in the 18th century by the astonishing Sir William Herschel, Mimas is the smallest (known) astronomical body which is spherical from self-gravitation (here is an explanation of what that means). The moon’s most noteworthy feature is an enormous impact crater named Herschel which is 130 kilometres (81 mi) across–about the same as the distance between New York and Philadelphia. Wikipedia gives some additional dimensions of the crater:
Herschel’s diameter is almost a third of [Mimas’] diameter; its walls are approximately 5 kilometres (3.1 mi) high, parts of its floor measure 10 kilometres (6.2 mi) deep, and its central peak rises 6 kilometres (3.7 mi) above the crater floor.
If you were standing in the crater (which you should not do!) it would be a great broken plain surrounded by cliffs thirteen times taller than the Empire State Building. In the middle you could see a huge mountain slightly shorter than the tallest mountain in North America. Jagged craters and valleys as deep as Lake Baikal would lie around you.
Mimas orbits above Saturn. The dark lines are shadows cast by the rings (Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA)
A great series of impact cracks on the opposite side of the moon would seem to indicate that the collision which created Herschel nearly shattered Mimas (which is composed principally of ice).
The moon’s name might be of passing interest to followers of my Deities of the Underworld category. In Greek mythology, Mimas was one of the monstrous sons of Gaia. He was born with snakes for legs and he was clad in full armor. In the Aeneid, Virgil tells the story of how Hephaestus imprisoned Mimas under Vesuvius during Gaia’s great rebellion against the Olympian gods. As the imprisoned giant shakes so to does the area around the Bay of Naples.
Mimas--Yikes! (Also, what happened to his armor?)
As I was researching this article, I was struck by how many moons Saturn has! As a special bonus feature, here is an alphabetic list of Saturn’s named moons (several more remain anonymous): Aegaeon, Aegir, Albiorix, Anthe, Atlas, Bebhionn, Bergelmir, Bestla, Calypso, Daphnis, Dione, Enceladus, Epimetheus, Erriapus, Farbauti, Fenrir, Fornjot, Greip, Hati, Helene, Hyperion, Hyrrokkin, Iapetus, Ijiraq, Janus, Jarnsaxa, Kari, Kiviuq, Loge, Methone, Mimas, Mundilfari, Narvi, Paaliaq, Pallene, Pan, Pandora, Phoebe, Polydeuces, Prometheus, Rhea, Siarnaq, Skadi, Skoll, Surtur, Suttung, Tarqeq, Tarvos, Telesto, Tethys, Thrym, Titan and Ymir.
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