Comet Or Asteroid? Hubble Discovers Unique Solar System Object

This photo, taken with the Hubble Space Telescope, reveals two asteroids with comet-like features orbiting each other. The asteroid pair, called 288P, was observed in September 2016 just before the asteroid made its closest approach to the Sun. 288P orbits in the main asteroid belt between Mars and Jupiter. Credit: NASA, ESA, and J. Agarwal

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 this week.

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.

This artist’s impression shows the binary asteroid 288P. The orbit of the asteroids is marked by a blue ellipse. Vaporization of water and dust creates a diffuse, comet-like head and tail. Credit: ESA/Hubble, L. Calcada.

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 (100 km). 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.

These images show how active 288P is. The apparent movement of the tail is a projection effect due to the relative alignment between the sun, Earth and 288P changing between observations. The tail direction is also affected by a change in the particle size. Initially, the tail was pointing towards the direction where larger dust particles (about 1 millimeter across) were popped off in late July. But from Sept. 20, 2016 onwards, the tail began to point in the opposite direction from the sun where much smaller particles are blown away from the nucleus by the pressure of sunlight. Credit: NASA, ESA, and J. Agarwal

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 known wide asteroid binaries in the Solar System. The observed activity of 288P also reveals information about its past:

This diagram shows the two areas where most of the asteroids in the solar system are found: the asteroid belt between Mars and Jupiter, and the trojans, two groups of asteroids moving ahead of and following Jupiter in its orbit around the Sun. There’s a further belt, not shown, here, of icy asteroids beyond the orbit of Neptune called the Kuiper Belt. Credit: ESA/Hubble, M. Kornmesser

“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 been a binary system for only about 5000 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.