For the first time, Nasa’s Hubble Space Telescope has spotted an asteroid, dubbed P/2013 P5, with six comet-like tails of dust.
“We were literally dumbfounded when we saw it,” said lead investigator David Jewitt of the University of California at Los Angeles. “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.”
The findings have been published in the Astrophysical Journal Letters.
P/2013 P5 has been ejecting dust periodically for at least five months. Astronomers believe it is possible the asteroid’s rotation rate increased to the point where its surface started flying apart. They do not believe the tails are the result of an impact with another asteroid because they have not seen a large quantity of dust blasted into space all at once.
Scientists using the Pan-STARRS survey telescope in Hawaii announced their discovery of the asteroid on 27 August. P/2013 P5 appeared as an unusually fuzzy-looking object. The multiple tails were discovered when Hubble was used to take a more detailed image on 10 September.
When Hubble looked at the asteroid again on 23 September, its appearance had totally changed. It looked as if the entire structure had swung around.
Careful modelling by team member Jessica Agarwal of the Max Planck Institute for Solar System Research showed that the tails could have been formed by a series of impulsive dust-ejection events. She calculated that dust-ejection events occurred 15 April, 18 July, 24 July, 8 August, 26 August and 4 September. Radiation pressure from the Sun stretched the dust into streamers.
Radiation pressure could have spun P/2013 P5 up. Jewitt said the spin rate could have increased enough that the asteroid’s weak gravity no longer could hold it together. If that happened, dust could slide toward the asteroid’s equator, shatter and fall off, and drift into space to make a tail. So far, only about 100 to 1 000 tons of dust, a small fraction of the P/2013 P5’s main mass, has been lost.
Astronomers will continue observing P/2013 P5 to see whether the dust leaves the asteroid in the equatorial plane. If it does, this would be strong evidence for a rotational breakup. Jewitt’s interpretation implies that rotational breakup must be a common phenomenon in the asteroid belt. It might even be the main way small asteroids die.