A Curiouser and Curiouser KBO

The shape of 2003 EL61 is at least 1,960 x 1,500 x 1,000 kilometers — making it a fast rotating squashed football.
NASA, ESA, and A. Feild (STScI)

With the controversy over the classification of Eris, Pluto, and "dwarf planets," a lesser known Kuiper Belt object (KBO) is vying to be the most interesting solar system object beyond Neptune. Soon after they found it, astronomers learned that 2003 EL61 is shockingly strange. Its spin period, just 3.9 hours, makes it the fastest rotating known body in our solar system larger than 100 kilometers (60 miles) across. Its shape is even weirder — it looks like a squashed American football, and at its widest point the highly elongated body likely exceeds even Eris's diameter. Astronomers also found two moons of it: an inner one with a 35-day non-circular orbit, and a brighter one in a 49-day circular orbit. Moreover, the object's density and rapid rotation imply that it is solid rock, with only the thinnest of icy veneers coating its surface.

Given those characteristics, it seems clear that something plowed into 2003 EL61, sped up its rotation, and left debris in the form of moons. However, in research presented at last week's American Astronomical Society Division for Planetary Sciences meeting in Pasadena, California, Kristina Barkume (Caltech) suggests that there are many more fragments from the collision than just the two satellites.

Barkume has been spectroscopically surveying KBOs between 500 and 1,000 kilometers in diameter to learn about their compositions and physical characteristics. She found something peculiar: at least 5 KBOs share similar orbits and a common, dominant spectral signature of water ice. What's most intriguing is that 2003 EL61's largest moon nicely fits into this grouping. "They have much stronger absorptions of water ice than all the KBOs for which we have data," says Barkume. "In particular, a majority of KBOs have no detectable water ice."

2003 EL61
Click on the image for an animation showing the rotation of 2003 EL61.
Michael E. Brown (Caltech)

The conclusion is that when the parent, 2003 EL61, got hit, it spewed fragments — a KBO family. The parent body is a remnant rocky core, and the children look alike. "It's as if Mercury had a family," says dynamicist Alessandro Morbidelli (Observatoire de la Cote d'Azur, France).

Another fascinating aspect of this family is its age. The collection presents some tantalizing, albeit circumstantial, evidence that the collision happened very recently, geologically speaking. Barkume is quick to note, "We can't say its age at the moment." But the fact that the ice signature is so strong, she adds, could mean that it is fresh.

Furthermore, the close similarity of the objects' orbits could say something about the collision that formed them. According to team leader, Michael E. Brown (Caltech), conventional thinking is that when an impact occurs in the Kuiper Belt, the event either scatters objects to far-flung distances or creates satellites — not both. To see moons and a group of free objects so closely related is odd. Perhaps the impact happened with just enough energy to send objects beyond the gravitational influence of 2003 EL61, but not much more.

For now, the EL61 family tree includes: 1999 OY3, 1995 SM55, 1996 TO66, 2002 TX300, 2003 OP32, and the moons of 2003 EL61. But Barkume hopes to examine many more family candidates in the coming months.

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