Hector, champion of the Trojans

It’s a complex geological world in miniature asteroid “Trojan” 624 Hektor, the largest and the only one equipped with a moon in the category of bodies that follow the same orbit of Jupiter. New research suggests that it is born from the encounter between two icy asteroids, similar to those in the Kuiper belt, pulled by Jupiter on their journey towards the inner solar system.

Hektor and its moon

Artistic representation of Hektor and its moon. The interior of the asteroid, composed of a mixture of rock and ice, is shown in section.


The asteroid 624 Hektor is very interesting. Discovered in 1907, with its 250 km wide, is the largest of the “Trojans” asteroids that move in the same orbit of Jupiter. Has an extremely elongated shape and orbit on itself in less than seven hours. Unique among the Trojans, it also has a moon, discovered in 2006 by a team led by Franck Marchis of the SETI Institute. Now, after eight years of observations, an international team of astronomers is led by the same Marchis, and a new study was published in the Astrophysical Journal Letters that sheds new light on the origin and bizarre characteristics of 624 Hektor and its moon.

The study, based mainly on observations with the powerful telescopes at the WM Keck Observatory adaptive optics in Hawaii, suggests that the asteroid and its moon may be the result of a collision between two frozen asteroids, caught in the Kuiper belt by gravitational attraction of Jupiter as the giant gas replace its orbit in the solar system, at the dawn of its formation.

The moon, which has a diameter of about 12 km, orbits around the asteroid every three days, at a distance of 600 km and with an inclination of at least 45 degrees to the equator of the main body. The calculation of its orbit was particularly laborious, both to the difficulties of observation and that for its bizarre path. “The moon’s orbit is elliptical and tilted relative to the rotation of Hektor, which is quite different from what we observe in other asteroids with satellites in the main belt,” said Matija Cuk, scientist at the Carl Sagan Center at the SETI Institute that is one of the authors of the research. “However, our computer simulations show that the moon’s orbit is stable over billions of years.”

Do not know the exact features of 624 Hektor, but the research team has refined a model that the asteroid has a split-hydrofoil shape. Scientists believe that this dual structure has been determined by a low-speed collision between two asteroids, while the moon could have originated from material ejected during the collision.

two obs

Two observations made in 2006 and 2008 with the Keck II telescope. At the center of each image is easily distinguishable the oblong shape of Hektor, while the small and weak moon is indicated by the blue circle.

Credit: NASA

Even the peculiar composition of the asteroid provides important information about its origin. “In our study we have shown that Hektor may be composed of a mix of rock and ice, a composition similar to the objects of the Kuiper belt, Pluto and Triton,” said Julie Castillo – Rogez NASA’s Jet Propulsion Laboratory, one of the study participants. “The reason why Hektor has become a Trojan asteroid, located just 5 times the Earth-Sun distance, is probably related to the large-scale rearrangement occurred at the time when giant planets were still migrating in the solar system.”

The complex shape of the asteroid and the bizarre orbit of its moon will be a matter of debate in the scientific community. In the meantime, the authors of the research are looking for a name for the moon, which has not own one yet.

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