SETI HEKTOR: Distant asteroid, a complex mini geological world

Artistic representation of the Trojan system showing the large 250 km dual shape Hektor and its 12 km moon. 

Credit: H. Marchis & F. Marchis

After 8 years of observations, scientists from the SETI Institute have found an exotic orbit for the largest Trojan asteroid, (624) Hektor, the only one known to possess a moon.

The formation of this system made of a dual primary and a small moon is still a mystery, but they found the asteroid could be a captured Kuiper body product of the reshuffling of giant planets in our solar system.

The results are being published today in Astrophysical Letters.

This study, based on W. M. Keck Observatory data and photometric observations from telescopes throughout the world, suggests that the asteroid and its moon are products of the collision of two icy asteroids.

This work sheds light on the complex youth of our solar system, when the building blocks that formed the core of Giant planets and their satellites were tossed around or captured during the giant planet migrations.

Franck Marchis

In 2006, a small team of astronomers led by Franck Marchis, astronomer at the Carl Sagan center of the SETI Institute, detected the presence of a small 12 km diameter moon around the large Trojan asteroid (624) Hektor using the 10 m Keck II telescope atop Mauna Kea, fitted with the NIRC-2 (Near-Infrared Camera 2) instrument behind the adaptive optics and laser guide star system (LGS-AO).

Since then, they collaborated with several researchers from University of California at Berkeley to determine the orbit of this moon and understand the origin of the system.

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. They are difficult to study since they are small and faint.

While the asteroid has been studied for 8 years, there were a couple of significant challenges before a paper could be published, according to Marchis.

"The major one was technical: the satellite can be seen only with a telescope like Keck Observatory's fitted with LSG-AO, but time on the mighty Keck's is highly prized and in limited availability," he said.

"Secondly, the 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."

The research, conducted with expert assistance from colleagues at the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) of the Observatoire de Paris, revealed that the 12 km moon orbits the large 250 km asteroid every 3 days at a distance of 600 km in an ellipse inclined almost 45 degrees with respect to the asteroid's equator.

More Information: "The puzzling mutual orbit of the binary Trojan asteroid (624) Hektor" published today by ApJL is co-authored by F. Marchis (SETI Institute), J. Durech (Charles University), J. Castillo-Rogez (Jet Propulsion Laboratory), F. Vachier (IMCCE-Obs. De Paris), M. Cuk (SETI Institute), J. Berthier (IMCCE-Obs. De Paris), M.H. Wong (UC Berkeley), P. Kalas (UC Berkeley), G. Duchene (UC Berkeley), M. A. van Dam (Flat Wavefronts), H. Hamanowa (Hamanowa observatory)and M. Viikinkoski (Tampere University) arxiv.org/ftp/arxiv/papers/1402/1402.7336.pdf

Medium sized Kuiper belt object less dense than water

Observations of the 2002 UX25 system with HST/HRC and Keck LGS-AO/NIRC2. 

The northward orientation arrow is 0.25 arcseconds long, for scale. 

In the first column, we show the image of both 2002 UX25 and its satellite. 

Credit: arXiv:1311.0553 

Michael Brown, a planetary scientist with California Institute of Technology, has found a medium sized object in the Kuiper belt (dubbed 2002 UX25) that doesn't appear to conform to theories of how such objects came to exist.

Michael Brown

In his paper to be published in Astrophysical Journal Letters, Brown notes that the mid-sized object appears to be less dense than it should be if it followed conventional thinking that suggests the larger the objects are in the belt, the more dense they should get.

The Kuiper belt, is of course, a group of rock-like objects (comets, dwarf planets, etc.) orbiting the sun that lie farther out than Neptune.

Such Kuiper belt objects (KBOs) are believed to have formed in ways similar to the way planets did, i.e. due to accretion of material over time.

Conventional theory suggests that small KBOs are less dense than water because of their porous nature—large KBOs grew more dense as they grew larger due to gravity causing them to compact.

If the theory is correct medium size KBOs should have medium density. But this new KBO that Brown has found doesn't conform to the theory at all, instead, its density is roughly the same as smaller KBOs, suggesting that it's not size that determines KBO density, but something else. And right now, Brown notes, nobody knows what that something else might be.

12 minute exposure of dwarf planet candidate (55637) 2002 UX25 with a 24" telescope.

2002 UX25 has a diameter of roughly 650 kilometers, putting it squarely in the mid-size KBO category, and it, like other KBOs, is believed to exist in very nearly the same state it's held since the formation of the solar system.

It's in studying such objects that scientists learn more about how everything in our solar system came to be the way it is.

Until now, most scientists agreed that KBOs of a size smaller than 350 kilometers across had a density less than that of water, whereas bigger ones had a greater density.

That theory might have to be changed however as 2002 UX25 is the first medium sized KBO to have its density measured and it clearly doesn't conform.

The discovery of 2002 UX25's density properties has already led to new theories, Brown notes, with some suggesting that scientists have been wrong to assume that KBOs and the planets formed at the same time.

Instead, they suggest, that it's possible that KBOs came first and afterwards as the planets were forming, eddies formed causing KBOs to knock into one another breaking them into different sized pieces.

More information: The density of mid-sized Kuiper belt object 2002 UX25 and the formation of the dwarf planets, arxiv.org/abs/1311.0553