Meteorites
that fall to Earth usually come directly from the asteroid belt between Mars
and Jupiter, rather than from the population of larger space rocks that drifted
in from the asteroid belt's innermost edge to hang around our planet's
neighborhood.
The
finding is detailed in the journal Nature and explains why the makeup of
most meteorites doesn't match the composition of most near-Earth asteroids
(NEAs).
"Why
do we see a difference between the objects hitting the ground and the big
objects whizzing by?" said Richard Binzel, a planetary scientist at the
Massachusetts Institute of Technology. "It's been a headscratcher."
Binzel
and other researchers from MIT and Hawaii's Institute of Astronomy compared the
composition of NEAs with samples from thousands of meteorites found on
Earth.
Two
thirds of NEAs matched up with LL chondrite rocks, which are low in metals, but
just 8 percent of meteorites had a similar match. The meteorites lined up more
with the mixed rock population of the asteroid belt that lies between Mars and
Jupiter.
Small,
boulder-like rocks from the asteroid belt end up as meteorites striking Earth at
least in part because of uneven heating from the sun. Solar energy heats the
day side of a rotating rock, which then radiates the heat away and creates a
propelling force that can change the rock's path.
That
phenomenon, known as the Yarkovsky
effect, can turn many smaller asteroids into sun-guided missiles aimed at
Earth. A similar effect called the YORP
effect can also set asteroids spinning.
However,
the Yarkovsky effect acts more weakly on larger asteroids, so that it only
gradually nudges the bigger brutes toward Earth's vicinity.
The
new study confirmed that the largest NEAs come from the asteroid belt's
innermost edge, forming a family of rocks made up of remnants from a larger
asteroid.
NEAs
may seem less likely to threaten Earth, but an asteroid two-thirds of a mile
wide (1 km) could have devastating consequences for whatever area it happens to
strike. At minimum, widespread regional destruction would occur.
Binzel
hopes that knowing the makeup of such asteroids will allow humans to better
understand deal with such threats. One approach
to deal with a loosely-clumped asteroid might not work for a solid, stony rock.
"Odds
are, an object we might have to deal with would be like an LL chondrite, and
thanks to our samples in the laboratory, we can measure its properties in
detail," Binzel said. "It's the first step toward 'know thy
enemy'."
Such
hints and some
additional funding for surveillance might eventually allow humans to put
down the asteroid threat for good.
advertisement
