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By R. Cowen
When
it comes to luring asteroids into the inner solar system,
a little nudge goes a long way.
Most
asteroids inhabit an elliptical set of tracks, known as
the main asteroid belt, between the orbits of Mars and Jupiter.
Although unlikely to spell doomsday for Earth, rocks occasionally
get flung from the belt onto paths that intersect our planet's
orbit. A new study suggests that tiny motions induced by
the sun's energy can play a crucial role in sending asteroids
on such an inward journey.
Researchers
realized in the 1980s that asteroids occupying certain zones,
known as resonances, within the outer part of the main belt
are profoundly influenced by Jupiter's gravity. The giant
planet's pull can dramatically elongate the orbits of these
asteroids, causing their paths to cross those of the inner
planets. More recently, scientists have calculated that
another set of resonances in the main belt nearer Mars also
acts as an escape hatch, ejecting some rocks into the inner
solar system.
These
special zones are numerous but extremely narrow, making
it hard to explain how so many asteroids end up in the inner
solar system. In the March 5 SCIENCE, Paolo Farinella of
the University of Trieste in Italy and David Vokrouhlicky
of Charles University in Prague, Czech Republic, present
computer simulations showing a nongravitational effect so
tiny it has often been ignored could account for the migration.
Named
for the Russian engineer who discovered it a century ago,
the Yarkovsky effect results from the way a spinning asteroid
absorbs and reradiates solar energy. Because an asteroid's
surface gets hotter the longer sunlight falls on it, it
does not reradiate energy evenly throughout its day or year.
If
different parts of the surface don't reemit radiation equally,
the asteroid will receive a net kick in a particular direction,
just as a rocket spewing a jet of gas recoils in the opposite
direction.
Farinella
and Vokrouhlicky calculate that over a period of 10 million
to 1 billion years, the typical interval between collisions
among such small asteroids, the Yarkovsky effect can shift
an orbit by a few million kilometers. This effect is large
enough to push a significant number of asteroids with diameters
of less than 20 km into resonances that can deliver them
into the inner solar system.
The
smaller the asteroid, the greater the Yarkovsky effect.
This could explain why the tiniest members of one family
of asteroids, known as the Astrids, have the widest range
of orbits, Farinella and Vokrouhlicky note.
"We're
learning more and more that small effects [like this] can
have important consequences," says Joseph A. Burns
of Cornell University. For instance, the Yarkovsky effect
may help explain why two classes of asteroid fragments,
or meteorites, each with a distinct composition, take very
different amounts of time to reach Earth.
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