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Asteroid explodes in death spin

Jenny Winder, News Writer
Mar 24, 2015, 18:59 UTC

Sen—A team of astronomers have managed for the first time to measure the rotational speed of a rare "active asteroid" and their observations suggest the space rock spun so fast that it exploded.

The large majority of known asteroids move cleanly along their orbits in the asteroid belt between the orbits of Mars and Jupiter. Active asteroids, which mimic comets with tails formed by long-lasting ice sublimation, have been confounding scientists since they were discovered several years ago.  

Sublimation is the process where ice transforms into vapour directly without a liquid water stage.

A new type of active asteroid was discovered in 2010 which explosively ejected dust. Scientists have debated whether such an explosion is a result of a high-speed collision with another minor object, or a consequence of “rotational disruption”. This is a process of launching dust and fragments by spinning so fast that the large centrifugal forces produced exceed the object’s own gravity, causing it to break apart.

Astronomers have so far identified four objects suspected of such activity. All four are very small making them faint and challenging to observe.

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Active asteroid P/2012 F5. A wide-angle view of the main nucleus and smaller fragments embedded in a long dust trail (top). A close-up view with the trail removed to enhance the visibility of the fragments (bottom). Image credit: M. Drahus, W. Waniak (OAUJ)/W. M. Keck Observatory

Then a team led by Michal Drahus, of the Jagiellonian University, in Krakow, Poland—where Nicolaus Copernicus studied in the 15th Century—was awarded time at the W. M. Keck Observatory in Hawaii to observe one of the active asteroids using the Keck II telescope.

“When we pointed Keck II at P/2012 F5 last August, we hoped to measure how fast it rotated and check whether it had sizable fragments. And the data showed us all that,” Drahus said.

They discovered at least four fragments of the object and calculated the object’s rotation period by measuring small periodic fluctuations in brightness, as the irregular nucleus rotates about its spin axis and reflects different amounts of sunlight. They found a very short rotation period of 3.24 hours—fast enough to cause the object impulsively to explode. 

“This is really cool because fast rotation has been suspected of catapulting dust and triggering fragmentation of some active asteroids and comets. But up until now we couldn’t fully test this hypothesis as we didn’t know how fast fragmented objects rotate,” Drahus said.

Active asteroid P/2012 F5 was discovered in 2012 by Alex R. Gibbs, using the Mount Lemmon 1.5 metre reflector. It was initially classified as a comet, based solely on its “dusty” look. Then two independent teams suggested that all the dust had been emitted in a single pulse that would have occurred about a year before the discovery. 

By 2013 the dust had cleared, allowing another team using the University of Hawaii’s 2.2-metre telescope on Mauna Kea to detect the star-like nucleus with a maximum size of two kilometres.

“We suspected that this upper limit was close to the actual size of the object. Consequently, we chose to observe P/2012 F5 because—despite its small size—it appeared to be the largest and easiest to observe active asteroid suspected of rotational disruption,” said Jessica Agarwal of the Max Planck Institute for Solar System Research, one of the co-authors of a paper published in Astrophysical Journal Letters.

P/2012 F5 is now the first freshly fragmented object in the Solar System with a well-determined spin rate that turns out to be the fastest among the active asteroids. These two features of the object are consistent with the “rotational disruption” scenario. But alternative explanations, such as fragmentation due to an impact, cannot be completely ruled out.