Study Puts Solar Spin
On Asteroids, Their Moons & Earth Impacts
(9 July 2008) Asteroids with moons,
which scientists call binary asteroids, are common in the solar
system.
A longstanding question has been how the majority of
such moons are formed. In this week's issue of the journal Nature, a trio of
astronomers from Maryland and France say the surprising answer is sunlight,
which can increase or decrease the spin rate of an asteroid.
Animation of
the spin-up and binary formation of an asteroid.
Derek Richardson, of
the University of Maryland, his former student Kevin Walsh, now Poincaré
Fellow in the Planetology Group in the Cassiopée Laboratory of CNRS at
the Côte d'Azur Observatory, France, and that group's leader, co-author
Patrick Michel outline a model showing that when solar energy "spins up" a
"rubble pile" asteroid to a sufficiently fast rate, material is slung off from
around the asteroid's equator. This process also exposes fresh material at the
poles of the asteroid.
If the spun off bits of asteroid rubble shed
sufficient excess motion through collisions with each other, then the material
coalesces into a satellite that continues to orbit its parent. Because the
team's model closely matches observations from binary asteroids, it neatly
fills in missing pieces to a solar system puzzle. And, it could have much more
down-to-earth implications as well. The model gives information on the shapes
and structure of near-Earth binary asteroids that could be vital should such a
pair need to be deflected away from a collision course with
Earth.
Finally, the authors say, these findings suggest that a sample
return mission to such a binary asteroid could bring back exposed pristine
material from the poles of the parent asteroid, providing a chance to probe the
internal composition of an asteroid without having to dig into
it.
Solar Spin Power
It's estimated that about 15 per cent
of near-Earth and main-belt asteroids with diameters less than 10 kilometres
have satellite Scientists have determined that these small binary asteroid
pairs were not formed at the beginning of the solar system, indicating that
some process still at work must have created them. "It was at first thought the
moons in these asteroid pairs probably formed through collisions and/or close
encounters with planets," said Richardson, an associate professor of astronomy
at the University of Maryland. "However, it was found that these mechanisms
could not account for the large number of binary asteroids present among
near-Earth and inner main belt asteroids."
Recent studies have outlined
a thermal process - known as the YORP effect after the scientists (Yarkovsky,
O'Keefe, Radzievskii, Paddack) who identified it - by which sunlight can speed
up or slow down an asteroid's spin. Widespread evidence of this mechanism can
be seen in the notable abundance of both fast and slow rotators among
[near-Earth asteroids] and small main belt asteroids, Walsh, Richardson and
Michel write in the Nature paper.
Animation of the spin-up and binary
formation of an asteroid.
The trio modelled different types of 'rubble
pile' asteroids -- chunks of rock held together by gravity. This work,
supported by the National Science Foundation and NASA, as well as the European
Space Agency and the French National Planetology Program, is the first to show
how the slow spinup of such asteroids leads over millions of years to mass loss
that can form binaries. "Our model almost exactly matches the observations of
our test case, binary asteroid KW4, which was imaged incredibly well by the
NSF-supported Arecibo radio telescope in Puerto Rico," Walsh
said.
Asteroid Deep Impacts
"Based on our findings, the
YORP effect appears to be the key to the origin of a large fraction of observed
binaries," said Michel. "The implications are that binary asteroids are
preferentially formed from aggregate objects [rubble piles], which agrees with
the idea that such asteroids are quite porous. The porous nature of these
asteroids has strong implications for defensive strategies if faced with an
impact risk to Earth from such objects, because the energy required to deflect
an asteroid depends sensitively on its internal structure," he
said.
Doublet craters formed by the nearly simultaneous impact of
objects of comparable size can be found in a number of places on Earth,
suggesting that binary asteroids have hit our planet in the past. Similar
doublet craters also can be found on other planets. The authors say that their
current findings also suggest that a space mission to a binary asteroid could
bring back material that might shed new light on the solar system's early
history.
The oldest material in an asteroid should lie underneath its
surface, explained Richardson, and the process of spinning off this surface
material from the primary asteroid body to form its moon, or secondary body,
should uncover the deeper older material. "Thus a mission to collect and return
a sample from the primary body of such a binary asteroid could give us
information about the older, more pristine material inside an asteroid, just as
the University of Maryland-led Deep Impact mission gave us information about
the more pristine material inside a comet," Richardson said. Michel added,
"Bringing back pristine material is the goal of our proposed Marco Polo
mission, which is currently under study by the European Space Agency, in
partnership with JAXA in Japan."
(source: University of
Maryland)
