[meteorite-list] STEREO Hunts for Remains of an Ancient Planet near Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 9 Apr 2009 15:24:43 -0700 (PDT)
Message-ID: <200904092224.PAA27767_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2009/09apr_theia.htm

STEREO Hunts for Remains of an Ancient Planet near Earth
NASA Science News
04.09.2009

April 9, 2009: NASA's twin STEREO probes are entering a mysterious
region of space to look for remains of an ancient planet which once
orbited the Sun not far from Earth. If they find anything, it could
solve a major puzzle--the origin of the Moon.

"The name of the planet is
Theia," says Mike Kaiser, STEREO project scientist at the Goddard Space
Flight Center. "It's a hypothetical world. We've never actually seen it,
but some researchers believe it existed 4.5 billion years ago - and that
it collided with Earth to form the Moon."

The "Theia hypothesis" is a brainchild of Princeton theorists Edward
Belbruno and Richard Gott. It starts with the popular Great Impact
theory of the Moon's origin. Many astronomers hold that in the formative
years of the solar system, a Mars-sized protoplanet crashed into Earth.
Debris from the collision, a mixture of material from both bodies, spun
out into Earth orbit and coalesced into the Moon. This scenario explains
many aspects of lunar geology including the size of the Moon's core and
the density and isotopic composition of moon rocks.

It's a good theory, but it leaves one awkward question unanswered: Where
did the enormous protoplanet come from?

Belbruno and Gott believe it came from a Sun-Earth Lagrange point.

Sun-Earth Lagrange points are regions of space where the pull of the Sun
and Earth combine to form a "gravitational well." The flotsam of space
tends to gather there much as water gathers at the bottom of a well on
Earth. 18th-century mathematician Josef Lagrange proved that there are
five such wells in the Sun-Earth system: L1, L2, L3, L4 and L5 located
as shown in the diagram below.

When the solar system was young, Lagrange points were populated mainly
by planetesimals, the asteroid-sized building blocks of planets.
Belbruno and Gott suggest that in one of the Lagrange points, L4 or L5,
the planetesimals assembled themselves into Theia, nicknamed after the
mythological Greek Titan who gave birth to the Moon goddess Selene.

"Their computer models show that Theia could have grown large enough to
produce the Moon if it formed in the L4 or L5 regions, where the balance
of forces allowed enough material to accumulate," says Kaiser. "Later,
Theia would have been nudged out of L4 or L5 by the increasing gravity
of other developing planets like Venus and sent on a collision course
with Earth."

If this idea is correct, Theia itself is long gone, but some of the
ancient planetesimals that failed to join Theia may still be lingering
at L4 or L5.

"The STEREO probes are entering these regions of space now," says
Kaiser. "This puts us in a good position to search for Theia's
asteroid-sized leftovers."

Just call them "Theiasteroids."

Astronomers have looked for Theiasteroids before using telescopes on
Earth, and found nothing, but their results only rule out
kilometer-sized objects. By actually entering L4 and L5, STEREO will be
able to hunt for much smaller bodies at relatively close range.

"The search actually began last month when both spacecraft rolled 180
degrees so that they could take a series of 2-hour exposures of the
general L4/L5 areas. In the first sets of images, amateur astronomers
found some known asteroids and new comet Itagaki was imaged just a
couple of days after the announcement of its discovery. No Theiasteroids
however."

Hunting for Theiasteroids is not STEREO's primary mission, he points
out. "STEREO is a solar observatory. The two probes are flanking the sun
on opposite sides to gain a 3D view of solar activity. We just happen to
be passing through the L4 and L5 Lagrange points en route. This is
purely bonus science."

"We might not see anything," he continues, "but if we discover lots of
asteroids around L4 or L5, it could lead to a mission to analyze the
composition of these asteroids in detail. If that mission discovers the
asteroids have the same composition as the Earth and Moon, it will
support Belbruno and Gott's version of the giant impact theory."

The search will continue for many months to come. Lagrange points are
not infinitesimal points in space; they are broad regions 50 million
kilometers wide. The STEREO probes are only in the outskirts now.
Closest approach to the bottoms of the gravitational wells comes in
Sept-Oct. 2009. "We have a lot of observing ahead of us," notes Kaiser.

Readers, you may be able to help. The STEREO team is inviting the public
to participate in the search by scrutinizing photos as they come in from
the spacecraft. If you see a dot of light moving with respect to the
stars, you may have found a Theiasteroid. Links to the data and further
instructions may be found at sungrazer.nrl.navy.mil
<http://sungrazer.nrl.navy.mil>.

Let the hunt begin!
Received on Thu 09 Apr 2009 06:24:43 PM PDT