[meteorite-list] Hubble Astronomers Observe Bizarre Six-Tailed Asteroid (2013 P5)

[Ron Baalke]

http://spacetelescope.org/news/heic1320/

When is a comet not a comet?
Hubble astronomers observe bizarre six-tailed asteroid
7 November 2013


[Images]

Astronomers using the NASA/ESA Hubble Space Telescope have observed a 
unique and baffling object in the asteroid belt that looks like a rotating 
lawn sprinkler 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 asteroids 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 
spokes on a wheel. It was first spotted in August of this year as an unusually 
fuzzy-looking object by astronomers using the Pan-STARRS 1 telescope in 
Hawaii [1].

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 literally dumbfounded when we saw it," said lead investigator 
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 belches 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 rotation 
rate increased to the point where its surface started flying apart, ejecting 
dust in episodic eruptions 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 intermittently over a period 
of at least five months [2].

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 impulsive dust-ejection events [3]. Radiation 
pressure from the Sun smears 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 spun up to a high speed as pressure 
from the Sun's light exerted a torque 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 avalanche 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 radius 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 rotational 
breakup. Astronomers will also try to measure the asteroid's true spin 
rate.

Jewitt's interpretation 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.

Notes

[1] The comet was discovered by Micheli et al. on 27 August 2013. It was 
spotted in observations from 18 August 2013. The discovery was announced 
in a Minor Planet Electronic Circular.

[2] Agarwal calculated that the first ejection event occurred on 15 April, 
and the last one on 4 September 2013. Other eruptions occurred on 18 July, 
24 July, 8 August, and 26 August 2013.

[3] A less likely option is that this emission is a result of water ice 
sublimating. Water ice can survive within the asteroid belt, although 
only at the outskirts or if buried deep enough within a large enough asteroid 
to be shielded. However, P5 is likely made of metamorphic rocks, making 
it incapable of holding ice in the same way that comets do. This, coupled 
with P5's orbit and its very small size, makes it very unlikely that its 
mass loss would be due to ice sublimation.

[4] This is not the first time that Hubble has observed a strange asteroid. 
In 2010, Hubble spotted a strange X-shaped asteroid (heic1016). However, 
unlike P/2013 P5, this was thought to have been formed by a collision. 
Later that year astronomers observed asteroid (596) Scheila, an object 
with a tail that was surrounded by a C-shaped cloud of dust (opo1113a). 
Again, this asteroid was thought to be the result of a collision between 
Scheila and a much smaller body - only the second time that such an event 
has been spotted.

Notes for editors

The Hubble Space Telescope is a project of international cooperation between 
ESA and NASA.

The international team of astronomers in the Hubble study consists of 
D. Jewitt (UCLA, USA), J. Agarwal (Max Planck Institute for Solar System 
Research, Germany), H. Weaver (The Johns Hopkins University Applied Physics 
Laboratory, USA), M. Mutchler (STScI, USA), and S. Larson (University 
of Arizona, USA). The paper, entitled "The Extraordinary Multi-Tailed 
Main-Belt Comet P/2013 P5', is published in The Astrophysical Journal 
Letters.

More information

Image credit: 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)

Contacts

David Jewitt
University of California at Los Angeles
Los Angeles, USA
Tel: +1-310-825-2521
Email: jewitt at ucla.edu

Nicky Guttridge
ESA/Hubble, Public Information Officer
Garching bei München, Germany
Tel: +49-89-3200-6855
Cell: +44 7512 318322
Email: nguttrid at partner.eso.org