[meteorite-list] Fresh Spin on Solar Powered Asteroids

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:53 2004
Message-ID: <200309101945.MAA25987_at_zagami.jpl.nasa.gov>

http://space.com/scienceastronomy/asteroid_spin_030910.html

Fresh Spin on Solar Powered Asteroids
By Robert Roy Britt
space.com
10 September 2003

Collisions can send asteroids whirling and twirling. But what can really put
space rocks into a spin is the subtle yet constant effect of sunlight.

Strange as it might seem, solar heat can have a greater impact on whether
and how a small asteroid rotates upon its own axis than do the explosive
impacts that cleave them into existence in the first place. A new study on
the effect seems to solve a mystery that began last fall.

In September, MIT researcher Stephen Slivan announced odd orientations of 10
small asteroids in the Koronis family. All the rocks are chips off two
larger blocks that collided at least two billion years ago. They range in
size from 12 to 26 miles (20-40 kilometers).

Slivan had expected each asteroid's spin axis to be randomly oriented, a
result of the group's chaotic origin and myriad random smaller crashes that
should have occurred since. Instead he found six of the rocks pointing one
distinct direction and four in another.

Reaction over time

Another group of researchers looked into how this could have happened.

They began with the knowledge that sunlight causes uneven heating on a space
rock -- just as Earth's day side is warmed while the night side is cooler.
The day side, especially the afternoon portion, re-radiates significantly
more solar energy back into space, creating a recoil force.

An asteroid's orbit around the Sun shifts over time due to this process,
which can slow the rock down and cause it to drift off course. It's called
the Yarkovsky effect, after the Russian engineer who came up with it around
1900.

Some scientists have suggested the Yarkovsky effect could be employed to
save Earth from a deadly impact. An incoming asteroid's path could be
altered by simply painting the rock white and letting the Sun do the heavy
lifting, providing there is a lengthy warning period.

The Yarkovsky effect is most pronounced on smaller objects. Other research
has shown that uneven heating shuttle's meteoroids -- baseball and
boulder-sized bits created in asteroid crashes -- into certain solar system
zones that serve as portals, guaranteeing the objects a shot at hitting
Earth.

YORP effect

A similar presumed effect, named YORP (for Yarkovsky and some colleagues),
long ago suggested that an asteroid's spin rate and orientation would also
be altered by sunlight. But scientists always assumed the effect would be
minor and, importantly, random.

In the new research, the effect of sunlight on asteroids was simulated by in
new computer model. The recoil effect, it turns out, can over the eons
substantially alter an asteroid's rotation rate and axis orientation in
semi-predictable ways, rather than just randomly.

"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," said William Bottke, a study team member from the
Southwest Research Institute.

The effect can cause asteroids to be captured into special situations in
which the wobble of the spin axis beats at the same frequency as the wobble
of the rock's orbit around the Sun, a path that can also affected
significantly by the gravity of a large planet.

This spin-orbit resonance, as astronomers call it, would determine the
length of a day on an asteroid by forcing its rotation rate to a certain
value. It also controls the direction of the polar axis.

More work needed

"These results give us a new way to look at the asteroids," said study
leader David Vokrouhlicky of Charles University in Prague. "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."

The study will be detailed in the Sept. 11 issue of the journal Nature.

Richard Binzel, an MIT astronomer who was not involved in the research, said
Slivan's findings last fall "almost defied belief." The new computer model
shows promise for explaining the observations, but other asteroid families
will need to be studied to prove the model out, Binzel said.

"A light touch might be the best way to influence asteroid spins," Binzel
writes in an accompanying Nature analysis.
Received on Wed 10 Sep 2003 03:45:55 PM PDT