The Hubble space telescope sure has seen some strange things in deep space, but this most recent find is possibly one of the strangest. Introducing the six-tailed comet... or is it an asteroid? Or some kind of weird alien sprinkler system in the asteroid belt?
The object - called P/2013 P5 - was discovered in August by the Pan-STARRS 1 telescope in Hawaii, an observation that baffled astronomers. All they could see was a nondescript fuzzy blob. So, employing the help of Hubble, the space telescope slewed around and observed the mystery object twice in September. What it saw was something just as baffling as what Pan-STARRS astronomers originally described: it was a weird multi-tailed comet-like object. Or so it appeared.
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Apart from the fact that P5 appears to have jets of gas and dust projecting in all directions - like spokes on a bicycle wheel - between the Sept. 10 and Sept. 23, "the entire structure had swung around," writes a Hubble science news release. This is all very un-comet-like - and, indeed, un-asteroid-like - behavior.
"We were literally dumbfounded when we saw it," said David Jewitt, lead investigator from the University of California at Los Angeles. "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."
So what is it? Needless to say, there are theories.
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The asteroid belt, that is located between the orbits of Mars and Jupiter, is home to a huge population of asteroids. Asteroids are known to be affected by the sun's radiation - like their cometary cousins, they heat up. But given enough time, asteroids can undergo what is known as the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect.
The uneven heating on an asteroid's surface can cause it to "spin up." In a nutshell, the YORP effect heats up the sun-facing side of an asteroid, but as the asteroid rotates, it radiates heat into space from the dark side. This heat radiation is carried away by infrared photons that themselves exert a tiny amount of momentum, or recoil, as they depart the asteroid. Over long periods (thousands to millions of years), this radiation acts to speed up an asteroid's spin.
But what of P5? Could the YORP effect explain these strange spoke features?
Jessica Agarwal of the Max Planck Institute for Solar System Research in Lindau, Germany, modeled a spinning asteroid to show the tails may be caused by impulsive dust-ejection events. Basically, we could be witnessing an asteroid that has been "spun up" at such a high rate by the sun that it is starting to lose integrity - centrifugal forces are causing it to eject dusty jets into space. "Given our observations and modelling, we infer that P/2013 P5 might be losing dust as it rotates at high speed," said Agarwal. "The sun then drags this dust into the distinct tails we're seeing."
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The space rock, which measures up to 240 meters wide, has probably only lost about 100-1000 tons of dust - a tiny fraction of its total mass. Although the spinning jets of dust seems the most likely explanation, follow-up observations are needed to see if the jets are being emitted in a plane, from the asteroid's equator.
"In astronomy, where you find one, you eventually find a whole bunch more," said Jewitt. "This is just an amazing object to us, and almost certainly the first of many more to come."
Jewitt's team has published a paper describing this fascinating object in the Nov. 7 issue of The Astrophysical Journal Letters (doi:10.1088/2041-8205/778/1/L21).
Image: Left: A Hubble observation of P/2013 P5 exhibiting the spoke-like dusty jets. Right: Diagram showing the spinning asteroid generating the jets. Credits: NASA, ESA, D. Jewitt (University of California, Los Angeles), J. Agarwal (Max Planck Institute for Solar System Research), H. Weaver (Johns Hopkins University Applied Physics Laboratory), M. Mutchler (STScI), and S. Larson (University of Arizona)