Bizarre Orbit of Weird-Looking Asteroid's Moon Revealed
H. Marchis & F. Marchis
Artist impression of the Trojan system showing Hektor and its moon.
NASA's Dawn spacecraft orbited the massive asteroid Vesta in 2011 and 2012, giving us an unprecedented look at the protoplanet's landscape, craters and mineral composition. The probe, which is now on its way to dwarf planet Ceres, not only revealed the evolution of Vesta, it also provided vital clues as to the evolution of our solar system. Now,in new images published by NASA
, an unusually colorful Vesta landscape is on display. Using data from the mission, scientists at Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany have produced a rather psychedelic view of this otherwise bland landscape. Dawn's camera system is equipped with seven filters, each filter sensitive to a specific wavelength of light. Normally, Vesta would look gray to the naked eye, but when analyzing the ratios of light through Vesta's filters, the landscape pops with color. Shown here, the flow of material inside and outside a crater called Aelia is demonstrated. As different minerals reflect and absorb different wavelengths of light, this composite image is alive with color, each shade representing different kinds of minerals littering Vesta's landscape.
This is Antonia, a crater located inside the huge Rheasilvia basin in the southern hemisphere of Vesta. From this image, planetary scientists have been ableto deduce that
"the light blue material is fine-grain material excavated from the lower crust. The southern edge of the crater was buried by coarser material shortly after the crater formed. The dark blue of the southern crater rim is due to shadowing of the blocky material."
The impact crater Sextilia can be seen in the lower right of this image. The mottled dark patches are likely impact ejecta from a massive impact and the redish regions are thought to be rock that melted during the impact. The diversity of the mineralogy is obvious here. "No artist could paint something like that. Only nature can do this," said Martin Hoffman, a member of the framing camera team at Max Planck Institute.
Earlier images of Vesta have shown an unusual "pitted terrain" on the floors of the craters named Marcia (left) and Cornelia (right). Once again, the varied colors demonstrate the different minerals and processes that cover Vesta's surface.
of Vesta shows the abundance of hydrogen on Vesta's surface. Note that the hydrogen signal is enhanced near the asteroid's equator. The hydrogen is likely from hydroxyl or water bound to minerals in Vesta's surface.
Another, earlier view of Antonia crater, demonstrating the mineral diversity of the region.
If you had to imagine the weirdest-looking space rock, you might imagine the form of asteroid 624 Hektor, the largest known Trojan asteroid in the solar system. Its bi-lobed shape makes it look like a really big cartoon dog bone, or even a huge peanut. What’s more, despite its knobbly appearance (and just as knobbly gravitational field), it even has its own moon. Now astronomers have taken a long, hard look at this exotic rocky beast and pinned down the bizarre orbit 624 Hektor’s moon, finding that the orbit will remain remarkably stable for billions of years.
The 250 kilometer (155 mile) wide 624 Hektor was discovered in 1907 by German astronomer August Kopff, but the existence of its 12 kilometer (7.5 mile) moon wasn’t revealed until 2006 by a team led by SETI Institute astronomer Franck Marchis. Now, eight years of observations of the motion of 624 Hektor’s natural satellite have finally hit paydirt.
Trojans are a special sub-class of asteroid. Trapped in an orbital resonance with Jupiter, they orbit the sun with the same period occupying two regions 60 degrees ahead and 60 degrees behind the gas giant. 624 Hektor is the only Trojan known to possess its own moon.
The technical hurdles are many when trying to observe Trojan asteroids — they are distant and very faint, requiring time on the world’s most powerful observatories, a hurdle that contributed to the long period of time it took to pin down the moon’s orbit.
“The satellite can be seen only with a telescope like Keck Observatory’s fitted with LSG-AO (laser guide-star adaptive optics), but time on the mighty Keck’s is highly prized and in limited availability,” said Marchis in a SETI Institute news release.
The Keck Observatory, located atop Mauna Kea, Hawaii, consists of two telescopes and in this study, Marchis’ team used the Keck II telescope that is outfitted with a precision adaptive optics system that fires a powerful laser high into the atmosphere, creating an artificial star as seen by the telescope’s optics. The motion of the guide star reveals turbulence in the upper atmosphere (the same effect that causes stars to “twinkle”), which can be actively compensated for, revealing deep space objects that would have otherwise been impossible to resolve from ground-based telescopes.
Two adaptive optics observations made in July 2006 and October 2008 with the Keck II telescope. The center of each image shows the elongated shape of Hektor. The small, faint moon is shown in the cyan circle. F. Marchis/WMKO
But it was 624 Hektor’s moon’s orbit that also contributed to the complexities of the observation. “(T)he orbit of the satellite is so bizarre that we had to develop a complex new algorithm to be able to pin it down and understand its stability over time,” added Marchis.
With assistance from the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) of the Observatoire de Paris, the moon was found to orbit 624 Hektor every 3 days at a distance of 600 kilometers (372 miles) in an ellipse inclined to 45 degrees from the asteroid’s equator
“The orbit of the moon is elliptical and tilted relative to the spin of Hektor, which is very different from other asteroids with satellites seen in the main-belt,” said SETI Institute scientist and co-author Matija Cuk. “However, we did computer simulations, which include Hektor being a spinning football shape asteroid and orbiting the sun, and we found that the moon’s orbit is stable over billions of years.”
With these observations, the researchers have been able to theorize how 624 Hektor came to form in the strange bi-lobe shape and why it has its own moon. Using Keck II and photometric observations of the asteroid since 1957, the researchers managed to refine the asteroid’s shape and they believe the system was created through the slow collision of two asteroids that fused together. The moon was formed from ejecta from this collision.
“We built several models of equal quality from the photometric data, but we favored a model made of two lobes since some of the best adaptive optics observations suggest that the Trojan asteroid has a dual structure,” said Josef Durech, co-author and researcher at the Charles University in Prague.
Also, it appears Hektor didn’t originate at Jupiter’s orbit; it migrated from the Kuiper Belt, a region of icy rocky bodies around the orbit of Pluto, during the early history of the solar system.
“We also show that Hektor could be made of a mixture of rock and ices, similar to the composition of Kuiper belt objects, Triton and Pluto,” said Julie Castillo-Rogez, researcher at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “How Hektor became a Trojan asteroid, located at only 5 times the Earth–sun distance, is probably related to the large scale reshuffling that occurred when the giant planets were still migrating.”
These fascinating results have been published in the Astrophysical Journal Letters.
Eight years after its discovery, Hektor’s moon remains nameless and Marchis and his team will consider any nomination from the public. If I had to make a suggestion, I’d vote for the name Astyanax, the son of Hector, Crown Prince of Troy and Princess Andromache of Cilician Thebe. Unfortunately, Astyanax has a rather gnarly and untimely death according to Greek mythology, so the discoverers may want to seek out another, more palatable name for their bizarre little asteroid satellite. (That said, I’m still voting for Astyanax.)