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The Planetary Society Weblog

By Emily Lakdawalla

Patroclus and Menoetius: Asteroids, comets, or what?

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Last week while there was all that news coming out about 2003 UB313, another nifty story got a little lost in the shuffle. There was a paper published in Nature about an asteroid that seems to belong to just about every club for small bodies in the solar system. Asteroid 617 Patroclus is a Trojan asteroid, which means that it (along with a thousand or so others) wanders around a gravitationally stable point 60 degrees behind Jupiter in its orbit. For the last 10 years, Patroclus has also been the only known binary Trojan, meaning that it has a companion. The news in the Nature study is that the companion, named Menoetius, is nearly as big as Patroclus (which is very unusual in binary systems); but what's odder about the pair is that the study suggests that both bodies are less dense than water ice. That is basically unheard of for bodies that are classified as "asteroids." That density means that Patroclus and Menoetius just have to have formed in the outer solar system, as comets. It's long been suspected that some asteroids are actually the dead cores of old comets, but Patroclus is the first Trojan to be definitively identified to be one of these. I wrote up a page with a lot more details about Asteroid 617 Patroclus and Menoetius if you're interested in reading more.