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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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