When is a comet not a comet?

Normal asteroids appear as tiny points of light. But this asteroid, designated P/2013 P5, has six comet-like tails of dust radiating from it like the spokes on a wheel. It was first spotted in August of this year as an unusually fuzzy-looking object by astronomers using the Pan-STARRS 1 telescope in Hawaii [1].

Because nothing like this has ever been seen before, astronomers are scratching their heads to find an adequate explanation for its mysterious appearance.

The multiple tails were discovered in Hubble images taken on 10 September 2013. When Hubble returned to the asteroid on 23 September, its appearance had totally changed. It looked as if the entire structure had swung around.

"We were literally dumbfounded when we saw it," said lead investigator David Jewitt of the University of California at Los Angeles, USA. "Even more amazingly, its tail structures change dramatically in just 13 days as it belches out dust. That also caught us by surprise. It's hard to believe we're looking at an asteroid."

One explanation for the odd appearance is that the asteroid's rotation rate increased to the point where its surface started flying apart, ejecting dust in episodic eruptions that started last spring. The team rules out an asteroid impact because a lot of dust would have been blasted into space all at once, whereas P5 has ejected dust intermittently over a period of at least five months [2].

Careful modelling by team member Jessica Agarwal of the Max Planck Institute for Solar System Research in Lindau, Germany, showed that the tails could have been formed by a series of impulsive dust-ejection events [3]. Radiation pressure from the Sun smears out the dust into streamers. "Given our observations and modelling, we infer that P/2013 P5 might be losing dust as it rotates at high speed," says Agarwal. "The Sun then drags this dust into the distinct tails we're seeing."

The asteroid could possibly have been spun up to a high speed as pressure from the Sun's light exerted a torque on the body. If the asteroid's spin rate became fast enough, Jewitt said, the asteroid's weak gravity would no longer be able to hold it together. Dust might avalanche down towards the equator, and maybe shatter and fall off, eventually drifting into space to make a tail. So far, only a small fraction of the main mass, perhaps 100 to 1000 tonnes of dust, has been lost. The asteroid is thousands of times more massive, with a radius of up to 240 metres.

Follow-up observations may show whether the dust leaves the asteroid in the equatorial plane, which would be quite strong evidence for a rotational breakup. Astronomers will also try to measure the asteroid's true spin rate.

Jewitt's interpretation implies that rotational breakup may be a common phenomenon in the asteroid belt; it may even be the main way in which small asteroids "die" [4]. "In astronomy, where you find one, you eventually find a whole bunch more," Jewitt said. "This is just an amazing object to us, and almost certainly the first of many more to come."

The paper from Jewitt's team appears online in the 7 November issue of The Astrophysical Journal Letters.

Notes

[1] The comet was discovered by Micheli et al. on 27 August 2013. It was spotted in observations from 18 August 2013. The discovery was announced in a Minor Planet Electronic Circular.

[2] Agarwal calculated that the first ejection event occurred on 15 April, and the last one on 4 September 2013. Other eruptions occurred on 18 July, 24 July, 8 August, and 26 August 2013.

[3] A less likely option is that this emission is a result of water ice sublimating. Water ice can survive within the asteroid belt, although only at the outskirts or if buried deep enough within a large enough asteroid to be shielded. However, P5 is likely made of metamorphic rocks, making it incapable of holding ice in the same way that comets do. This, coupled with P5's orbit and its very small size, makes it very unlikely that its mass loss would be due to ice sublimation.

[4] This is not the first time that Hubble has observed a strange asteroid. In 2010, Hubble spotted a strange X-shaped asteroid (heic1016). However, unlike P/2013 P5, this was thought to have been formed by a collision. Later that year astronomers observed asteroid (596) Scheila, an object with a tail that was surrounded by a C-shaped cloud of dust (opo1113a). Again, this asteroid was thought to be the result of a collision between Scheila and a much smaller body -- only the second time that such an event has been spotted.