Astronomers¹ using the NASA/ESA Hubble Space Telescope have observed a unique and baffling（令人困惑的） object in the asteroid²（小行星） belt that looks like a rotating lawn sprinkler（洒水车） or badminton shuttlecock. While this object is on an asteroid-like orbit, it looks like a comet, and is sending out tails of dust into space. Normal asteroids³ 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 spokes⁴（辐条） on a wheel. It was first spotted⁵ in August of this year as an unusually fuzzy-looking object by astronomers using the Pan-STARRS 1 telescope in Hawaii.

 

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 literally⁶ dumbfounded（目瞪口呆的） when we saw it," said lead investigator⁷ 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 belches⁸ 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 rotation⁹ rate increased to the point where its surface started flying apart, ejecting dust in episodic eruptions¹⁰ 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 intermittently¹¹ over a period of at least five months.

 

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 impulsive¹² dust-ejection events. Radiation pressure from the Sun smears¹³ 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 spun¹⁴ up to a high speed as pressure from the Sun's light exerted a torque（转矩，扭矩） 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 avalanche¹⁵ 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 radius¹⁶ 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 rotational¹⁷ breakup. Astronomers will also try to measure the asteroid's true spin rate.

 

Jewitt's interpretation¹⁸ 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.

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