Strange paired asteroids behave like a comet


When is a comet not a comet? When it’s a couple of asteroids forever circling each other. Stuart Gary reports.


An international team of astronomers have found that this unusual object in the asteroid belt is, in fact, two asteroids orbiting each other that have comet-like features. These include a bright halo of material, called a coma, and a long tail of dust.
NASA, ESA, and J. DePasquale and Z. Levay (STScI)

A strange comet-like asteroid first discovered more than a decade ago has turned out to be even stranger than first thought. Astronomers have found that it is actually consists of two asteroids orbiting each other.

In findings reported in the journal Nature, a team led by Jessica Agarwal of the Max Planck Institute for Solar System Research, in Germany, say the pair represents the first known binary asteroid that can also classified as a comet.

The binary – a kilometre-wide object named 2006 VW139/288P -- was initially discovered by the University of Arizona's Spacewatch program in 2006, in the main asteroid belt between Mars and Jupiter.

This odd object, called 2006 VW139/288P, is the first known binary asteroid that is also classified as a main-belt comet. Roughly 5,000 years ago, 2006 VW139/288P probably broke into two pieces due to a fast rotation.
NASA, ESA, and J. Agarwal (Max Planck Institute for Solar System Research)
Its comet-like activity was first noticed through the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) at Hawaii’s Haleakala Observatory in 2011 as it approached perihelion – its closest orbital position to the Sun.

Then in September 2016, just before the asteroid made its closest approach to the Sun, astronomers took a more detailed look, using the Hubble Space Telescope.

The new images revealed that 288P is actually comprises two asteroids of almost identical mass and size, orbiting each other at a distance of about 100 kilometres.

The Hubble images confirmed ongoing activity in the binary system – with a bright comet-like coma of material surrounding the asteroids and streaming into a long tail.

Initially, the tail was producing relatively large one-millimetre diameter dust particles.

However, by September 20, the tail had begun to point away from the Sun – just like a comet’s would – and particle size had dropped to just 10 microns.

Like any object circling the Sun, 288P travels along an elliptical path, bringing it closer to and farther away during the course of its orbital year.

About 20 so-called active asteroids similar to 288P have been discovered – but so far, this is the only binary system.

Its comet-like properties are thought to arise as a result of an initial collision that exposed volatile elements previously buried deep inside. These begin to “outgas”, producing a coma and a tail.

The authors detected strong indications of the sublimation of water ice due to the increased solar heating — a similar similar to how the tail of comets are created.

This makes 288P the first known binary asteroid that also is classified as a main-belt comet.

Understanding the origin and evolution of main-belt comets — including asteroids orbiting that behave like them — will help scientists trying to understand the formation of the solar system as a whole.

The various features of 288P — wide separation of the two components, near-equal component size, high eccentricity and comet-like activity — make it unique among the few known wide asteroid binaries in the solar system.

The observations also reveals information about its past. Surface ice cannot survive in the asteroid belt for the age of the solar system -- but it can be protected for billions of years by a refractory dust mantle just a few metres thick.

As it’s still so active, the authors think that 288P has existed as a binary for only about 5,000 years. Its two components are members of a loose group of 11 asteroids that are thought to have been created as a result of a massive object shattering around 7.5 millions years ago.

  1. http://www.nature.com/nature/journal/v549/n7672/full/nature23892.html?foxtrotcallback=true#affil-auth
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