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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 7 Nov 2013 08:39:08 -0800 (PST)
Message-ID: <201311071639.rA7Gd8kM020702_at_zagami.jpl.nasa.gov>

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 M?nchen, Germany
Tel: +49-89-3200-6855
Cell: +44 7512 318322
Email: nguttrid at partner.eso.org
Received on Thu 07 Nov 2013 11:39:08 AM PST