A new study by researchers at
Southwest Research Institute (SwRI) and Charles University (Prague) has
found that sunlight can have surprisingly important effects on the spins of
small asteroids. The study indicates that sunlight may play a more important
role in determining asteroid spin rates than collisions, which were
previously thought to control asteroid spin rates. Results will be published
in the Sept. 11 issue of Nature.
David Vokrouhlicky (Charles University), David Nesvorny and William Bottke
(both of the SwRI Space Studies Department) conducted the study, which
showed that sunlight absorbed and reemitted over millions to billions of
years can spin some asteroids so fast they could potentially break apart. In
other cases, it can nearly stop them from spinning altogether. The team even
noted that the effects of sunlight, combined with the gravitational tugs of
the planets, can slowly force asteroid rotation poles to point in the same
direction.
Until recently, researchers thought asteroid impacts controlled the rotation
speed and direction of small asteroids floating in space. The unusual spin
states of 10 asteroids observed by Stephen Slivan, a researcher at the
Massachusetts Institute of Technology, however, have cast doubt on this
idea. Slivan’s asteroids, the first in the 15- to 25-mile-diameter range to
have their spins extensively studied, are in the so-called Koronis asteroid
family, a cluster of asteroid fragments produced by a highly energetic
collision billions of years ago. Slivan found that not only do four of these
asteroids rotate at nearly the same speed, but they also have spin axes that
point in the same direction.
“The data clearly show that the spin vector alignment is real, but how they
got that way has been a big puzzle,” says Slivan. “I’m delighted that others
find this to be an interesting problem.”
“To picture just how weird these asteroids really are, imagine you were
handed a box of spinning tops just as you were about to launch aboard the
space shuttle. Given all the shaking produced by the launch, you would
expect the tops to have different spin speeds and orientations by the time
you reached orbit,” says Bottke. “Instead, imagine your surprise upon
opening the box if the tops were all spinning at the same speed and had
their handles pointing toward the constellation Cassiopeia. Now increase the
size of the tops by a factor of a million and pretend that the bouncing
during launch is equivalent to billions of years of asteroid collisions.
This is the strange situation we find ourselves with.”
The remaining six asteroids studied by Slivan either have extremely slow
spin rates, such that they rotate slower than the hour hand of a clock, or
very fast spin rates, such that they are near the limit beyond which loose
material on the surface of an asteroid would fly off.
“One would expect that collisions would have randomized these rotation
rates. It was a big surprise to find a cluster of asteroids with such odd
spin states,” says Nesvorny.
To explain the spin states of Koronis family asteroids, Vokrouhlicky,
Nesvorny and Bottke investigated how asteroids reflect and absorb light from
the sun and reradiate this energy away as heat. They found that while the
recoil force produced by the reradiation of sunlight is tiny, it can still
substantially alter an asteroid’s rotation rate and pole direction if it has
enough time to act.
“Like the story about the tortoise and the hare, slow and steady sunlight
wins the race over the fast-acting, but less effective, jolt of collisions
between asteroids. Sunlight in space never stops,” says Bottke, “and most
asteroids have been exposed to a lot of it because of their age.”
Using computer simulations, the team showed that sunlight has been slowly
increasing and decreasing the rotation rates of Koronis family asteroids
since they were formed 2 to 3 billion years ago. More remarkably, they found
that some simulated asteroids were captured into a special spin state that
forced the wobble of the asteroid’s spin axis (produced by gravitational
perturbations from the sun) to “beat” at the same frequency as the wobble of
the asteroid’s orbit (produced by gravitational perturbations from the
planets). This state, called a spin-orbit resonance, can drive an asteroid’s
rotation rate and spin axis to particular values.
“These results give us a new way to look at the asteroids,” says
Vokroulicky. “It is our hope that this work will stimulate observational
studies into many different regions of the main asteroid belt. We have only
scratched the surface of this interesting problem.”
NASA, the National Research Council and the Grant Agency of the Czech
Republic funded the study.
EDITORS: An image to support this story is available from
www.swri.org/press/koronis.htm.
