A significant fraction of the large, mile-wide asteroids that hit the Earth
have satellites, according to a study done by Dr. William Bottke, a Texaco
Prize research fellow at Caltech, and Professor Jay Melosh at the University
of Arizona.
Bottke and Melosh's results are based on a computer simulation which shows
that some asteroids, after experiencing a close approach with the Earth, are
split into multiple fragments by Earth's gravitational forces, in the same way
that Jupiter was able to pull apart comet Shoemaker Levy-9 in 1992. In many
cases, these newly-formed fragments begin to orbit one another. Bottke and
Melosh suggest that these co-orbiting bodies can re-encounter and impact the
Earth during a later pass, creating two distinct craters.
These co-orbiting asteroids may be able to explain the curious impact
phenomena known as "doublet craters" found on Earth. Doublet craters are
formed by the nearly simultaneous impact of two asteroids of comparable size.
At least 3 of the 28 largest craters on Earth have a companion crater nearby
which shares its formation age. (An example would be the craters East and West
Clearwater Lake in Quebec, Canada.) Surveys of Venus show a similar fraction
of doublet impact structures, though Venus' impact record is complicated by
the planet's thick atmosphere, which prevents small asteroids from impacting
the surface.
These craters, which are frequently larger than 10 miles across, are too large
and too far separated to have been formed by a large asteroid breaking up
within Earth's atmosphere. Also, the odds of getting two separate asteroids to
impact the Earth at the same place and time are astronomical unless both
asteroids were orbiting each other before impact.
The asteroids most susceptible to disruption events by Earth's gravity are
"rubble-pile "asteroids, asteroids not composed of solid chunks of rock but
collections of blocky components and fine-grained material held together by
self-gravity. The idea that many asteroids are "rubble-piles" is supported by
(a) images of the asteroids Gaspra and Ida taken by the Galileo spacecraft,
which show them to be pockmarked with large craters capable of shattering a
solid object into many fragments, (b) observations by Alan Harris of the Jet
Propulsion Laboratory in Pasadena, California that show that no small asteroid
has a rotation period faster than 2-3 hours; rubble-pile asteroids rotating
faster than this limit would fly apart from centrifugal force, while solid
asteroids would not.
Asteroid satellites are already known to exist, though not all are produced by
planetary close encounters. Images from the Galileo spacecraft in 1994 showed
that Ida, an elongated 35 mile long asteroid residing in the main-asteroid
belt between the orbits of Mars and Jupiter, has a mile-long satellite now
called Dactyl. Since this particular pair could never approach the Earth, it
was probably formed as a by-product of two large asteroids colliding with each
other, an event which is common in the region where Ida resides but less
common near the Earth.
These results suggest that deflecting a comet or asteroid away from an impact
encounter with Earth may be more difficult than we expect. If we were to
attempt to deflect a rubble-pile asteroid using atomic weapons, we may instead
just disperse chunks of material into a cloud of high velocity debris which
could produce just as much damage at impact as a solid object. Also, if the
asteroid in question has a satellite, we will have to deflect more than one
target.