  The origin of the Martian moons Deimos and Phobos is controversial. One
hypothesis for their origin is that they are captured asteroids, but the
mechanism requires an extremely dense martian atmosphere, and the mechanism by
which an asteroid in solar orbit could shed sufficient orbital energy to be
captured into Mars orbit has not been well elucidated. Since the discovery by
the space probe Galileo that the asteroid Ida has a moon "Dactyl", a
significant number of asteroids have been discovered to have smaller asteroids
in orbit about them. The existence of asteroid moons provides a mechanism for
the capture of the Martian moons (and the small moons of the outer planets).
When a binary asteroid makes a close approach to a planet, tidal forces can
strip the moon from the asteroid. Depending on the phasing, the asteroid can
then be captured. Clearly, the same process can be used to explain the origin
of any of the small moons in the solar system.
