In September 2013, astronomers working with the Hubble Space Telescope (HST) discovered that a newly found asteroid had a comet-like tail. Closer study revealed that the tail was composed of six distinct streams of dust. This was surprising enough, but when they took a closer look two weeks later they were astonished to find that the structure of the system had changed completely.
The asteroid drew attention from the start: asteroids are small and distant enough that they only ever appear as star-like points of light, but when P/2013 P5 was first discovered by the Pan-STARRS survey, it was distinctly fuzzy. A team led by David Jewitt of the University of California at Los Angeles applied for time on the HST so they could get a closer look, which is when the bizarre multiple tails were discovered.
“We were literally dumbfounded when we saw it,” said Dr Jewitt. “Even more amazing, 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’s going on? It quickly became obvious that the tails are streams of dust being ejected from the surface of the asteroid. This happens often enough when meteoroids collide with asteroids, raising a cloud of dust which is then blown into a tail by the Solar wind, but such tails are very short-lived. The multiple tails streaming from P/2013 P5 had persisted for at least a month, and the only mechanism astronomers know of that can cause material to vent from the surface of such small objects for so long is subsurface ices melting in the Sun’s heat, as with comets. But there is no trace of water or other volatiles in the tails, so that’s not the cause.
Dr Jewitt’s team have come up with one interesting idea, however: the Sun’s constant radiation pressure can cause objects to gradually begin spinning. What they suspect is that this asteroid’s rotational speed has climbed high enough that the loose material on the surface is starting to be affected by centrifugal action, causing it to “fall” towards the asteroid’s equator. At the equator, it’s speed is high enough to exceed the escape velocity of the asteroid, and dust and debris get flung out into space.
So far only a few hundred tonnes of material has been observed to leave the asteroid, a tiny fraction of the total mass, suggesting that this show could continue for a very long time. But is the hypothesis correct? Follow-up observations are planned, to confirm whether the dust is being ejected along the equatorial plane which would provide evidence for this idea. And of course, if this were happening then we would assume it to be a common event, so we would expect to find other asteroids doing the same thing. “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 November 7 issue of The Astrophysical Journal Letters.