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Scientists reveal puzzling orbit of Trojan asteroid's moon

Charles Black, Founder and CEO of Sen
Feb 28, 2014, 8:00 UTC

Sen—Astronomers have been studying the largest Trojan asteroid named (624) Hektor—the only Trojan asteroid known to have a moon—to understand its origins and its moon's orbit. 

The scientists believe the asteroid, which at 250 km is the largest of the Trojan asteroids, is likely to have formed from the collision of two icy asteroids. The study shows Hektor is made from rock and ice and probably formed far out from the Sun with other Kuiper belt objects before being captured during the migration phase of the Solar System when the order of objects was reshuffled as the giant planets migrated. 

The researchers, based at the SETI Institute and assisted by astronomers at the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) of the Observatoire de Paris, studied data captured over the last 8 years from the W. M. Keck Observatory and other telescopes around the world.

Trojan asteroids are those that are temporarily trapped in regions 60 degrees in front or 60 degrees behind the planet Jupiter in its orbit around the Sun.

Hektor spins rapidly on its axis, completing a rotation in less than 7 hours. The researchers worked on modelling the shape of the asteroid based on photometric data. “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 of the study, in a statement. 

Artist illustration of Hektor and its moon

Artist illustration showing the large 250 km dual shape Hektor and its 12 km moon. The interior of the asteroid made of a mixture of rock and ices is shown through a cutaway. This work suggests the asteroid is porous with a denser core of compacted ices. Image credit: H. Marchis & F. Marchis

“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. 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,” said Julie Castillo-Rogez, researcher at the Jet Propulsion Laboratory, in a statement.

The moon, which has a diameter of about 12 km, orbits Hektor every 3 days at a distance of 600 km in an ellipse inclined almost 45 degrees with respect to 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,” explained Matija Cuk, co-author and scientist at the Carl Sagan Center of the SETI Institute, in a statement. “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.”

The asteroid (624) Hektor was discovered in 1907 by August Kopff. Its moon was only discovered in 2006 by a team led by Franck Marchis, an astronomer at the Carl Sagan center of the SETI Institute.

The research is published in Astrophysical Letters.