Comet, Asteroid, or what?

The recently discovered object P/2013 P5 (PANSTARRS), designated as a comet, is messing up the order of small objects in the solar system. It is circling the sun at the inner rim of the main asteroid belt, but images taken with the Hubble Space Telescope on September 10 and 23 reveal no less than six rapidly evolving dust tails, the typical attribute of a comet.
Credit: NASA, ESA, D. Jewitt (UCLA), J. Agarwal (MPI for Solar System Research), H. Weaver (Johns Hopkins University Applied Physics Laboratory), M. Mutchler (STScI), and S. Larson (University of Arizona)

The complexity of the tails is inconsistent with the hypothesis that a recent collision has sparked this unusual activity - six collisions within such a short time period are highly unlikely. Instead, the researchers suspect the tails are caused by a rapid rotation of the main body, fast enough the centrifugal force outmatches the weak gravitation holding the comet-asteroid together. The rotation might be accelerated by solar radiation pressure, applying a moment of force to the body. Confirmed by computer simulations, this hypothesis explaines the evolution of the tails well.

Asteroids are typically "dried-out" rocky objects, orbiting the sun mainly in the astroid belt located between Earth and Mars. At distances between 2 and 3.5 Astronomical Units, solar radiation has deprived them of all volatile substances. The tails of P/2013 P5 are therefore believed to consist almost entirely of dry dust. By contrast, comets originate from the outer regions of the solar system. They contain large amounts of frozen water and carbondioxide, which trigger the development of their magnificent dust- and ion tails once the comet approaches the sun. 

With P/2013 P5, astronomers have discovered something like a "comet-asteroid-hybrid" - but not for the first time. In 1996, images of the main-belt asteroid 1979 OW7 showed a distinct tail, causing the object to be re-labelled to "comet 133P/Elst-Pizarro". It was the first "main-belt comet" (MBC) or "active asteroid" discovered. Other MBCs known so far are 176P/LINEAR, 238P/Read, P/2008 R1 (Garradd), P/2010 R2 (La Sagra), P/2010 A2 (LINEAR), 596 Scheila, P/2012 F5 (Gibbs), and P/2013 R3. New P/2013 P5 just made the last entry of that list.

The IAU Minor Planet Center lists other "dual-status objects", that are not part of the main belt: (2060) Chiron (or 95P/Chiron), (4015) Wilson-Harrington (or 107P/Wilson-Harrington), (60558) Echeclus (or 174P/Echeclus).  3200 Phaethon, the famous parent body of the Geminid meteor shower, is also an active asteroid - so we can actually see the debris of such an object evaporate in the atmosphere every year in December!

Recently, The Spitzer Space telescope discovered residual cometary activity of asteroid (3552) Don Quixote, now believed to be an "extinct comet". Don Quixote is a different story however: it's orbit is stretched like that of a comet, but apart from Spitzer's obervations, earth-bound telecope were so far unable to detect any cometary behaviour. As an Amor-type Near Earth Object, Don Quixote comes fairly close to our planet from time to time (Phaethon is even an earth-crosser), but there is no immediate danger of collision.

With a discovery magnitude of  >20, P/2013 P5 is far out of reach for amateur telescopes. Ernesto Guido, Nick Howes and Martino Nicolini used a 2,0-meter remotely controlled telescope to obtain a follow-up ground based image on November 7. Although far from Hubble's capabilities, at least one of the tails is visible.

To find out more about the discovery of P/2013 P5, read the Nasa press release or the ApJL research paper.

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