The semimajor axes of asteroid fragments up
to about 20 km in diameter drift due to the Yarkovsky effect, a subtle
non--gravitational mechanism related to radiation pressure recoil on
spinning and orbiting objects. Over the collisional lifetimes of
these objects (typically, $10$~Myr to $1$~Gyr), orbital semimajor
axes can be moved by a few times $10^{-2}$~AU for bodies between 1 and
10~km in mean radius. This has profound implications on the delivery
of multi--km near--Earth asteroids, because the Yarkovsky drift drives
many small main--belt asteroids into the resonances which transport
them to the Mars--crossing state and eventually to near--Earth space.
Recent work has shown that, without such a drift, the Mars--crossing
population would be depleted over times $\approx 100$~Myr, much less
than the age of the Solar System. Moreover, the Yarkovsky semimajor
axis mobility may spread in an observable way the tight semimajor axis
clustering of small asteroids produced as a consequence of disruptive
collisions.
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