ASTROBOB: Comet or asteroid? Hubble discovers unique solar system object

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DULUTH — The Hubble Space Telescope is always up to something. Now, a German-led group of astronomers have observed a one-of-a-kind object in the asteroid belt between Mars and Jupiter. Scratch that. Two-of-one-kind: a pair of asteroids orbiting each other that show comet-like features including a bright coma and a long tail. It's the first known binary asteroid also classified as a main-belt comet. The team's research was published in the journal Nature recently.

The object was discovered as an asteroid back in 2006 and temporarily given the name, 2006 VW139, a standard-style asteroid handle. But in 2011, the asteroid became active. Astronomers noticed it ejecting dust and growing a tail, something that comets do, so it was renamed 288P/300163 — a dual comet/asteroid designation. In September 2016, just before the 288P made its closest approach to the sun, it was close enough to Earth for a detailed view using the Hubble.

Hubble's photos proved we were tricked again, revealing that 288P wasn't a single object, but two asteroids of almost the same mass and size, orbiting each other at a distance of about 62 miles. Using Newton's Law, astronomers could measure the masses of the objects in the system. They also detected water ice vaporizing from the heat of the sun, similar to how the tail of a comet is created.

Most comets orbit well beyond the asteroid belt. They may drop into the inner solar system briefly and metamorphose from a grub-like, icy nucleus to a spectacular "butterfly" with colors and tail, but they soon return to Jupiter vicinity or far beyond. Instead, 288P is a far rarer object called a main belt comet, an asteroid orbiting between Mars and Jupiter that shows comet-like activity. Understanding the origin and evolution of these objects is a crucial element in our understanding of the formation and evolution of the whole solar system. It's believed that icy asteroids rather than icy comets delivered some of Earth's original water, so main belt comets may help us answer how that water came to Earth. Only about a dozen are known.

The various features of 288P — wide separation of the two components, near-equal component size, an elongated orbit and comet-like activity — also make it unique among the few wide-known asteroid binaries in the solar system. The observed activity of 288P also reveals information about its past:

"Surface ice cannot survive in the asteroid belt for the age of the solar system but can be protected for billions of years by a refractory (retains its strength in high temperatures) dust mantle only a few meters thick," said Jessica Agarwal, team leader and main author.

From this, the team concluded that 288P has been a binary system for only about 5,000 years. What most likely happened is that a larger object broke up because it was rotating so fast, and then the two fragments moved farther apart. More time working on the data coupled with further observation will provide a clearer view of how 288P came to be.