Title: The Little Spacecraft That Could.
Subject(s): MOON -- Exploration; LUNAR Prospector (Artificial
satellite)
Source: Scientific American, Jun99, Vol. 280 Issue 6, p20, 2p, 1
diagram
Author(s): Alpert, Mark
Abstract: Discusses the scientific mission of the Lunar Prospector
spacecraft to gauge the size of the moon's core. Details on the
process of measurement; Theories on the moon's origin; Plans for its
use in the study of the moon's polar regions.
AN: 1893291
ISSN: 0036-8733
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Database: MasterFILE Elite

Section: SCIENCE AND THE CITIZEN

ASTRONOMY

THE LITTLE SPACECRAFT THAT COULD

After a string of remarkable discoveries, Lunar Prospector prepares
for a spectacular finale

Lunar Prospector is not an impressive-looking spacecraft. Shaped like
a soup can with its ends cut off, the 295-kilogram (650-pound) orbiter
is not much larger than a washing machine. But as it nears the end of
its 18-month mission, the plucky vessel continues to provide revealing
glimpses of the composition and structure of the moon. The spacecraft
first grabbed the public's attention in early 1998, when its
instruments detected evidence of ice in the perpetually shadowed areas
near the moon's poles. Now mission investigators have announced
another important finding: measurements indicating that the moon has a
relatively tiny core. The new data reinforce the theory that the moon
was created by a cataclysmic collision between Earth and another body
more than four billion years ago.

The investigators gauged the size of the moon's core in two ways.
Because of the Doppler effect, which shifts the frequency of Lunar
Prospector's radio signal as the spacecraft moves toward or away from
Earth, researchers were able to identify slight variations in the
craft's velocity as it orbited the moon. By carefully recording these
variations, the scientists mapped the lunar gravitational field and
calculated the moon's moment of inertia, which revealed the
distribution of the body's mass. Assuming that the moon's core, like
Earth's, is composed mostly of iron, researchers estimated that its
radius must be between 220 and 450 kilometers (140 to 280 miles). The
radius of the moon as a whole is 1,738 kilometers. "It's an indirect
measurement, with a lot of uncertainties," says Alan B. Binder, the
principal investigator for Lunar Prospector.

But Binder and his colleagues had an ingenious plan for refining the
estimate. Although the moon's core does not generate a planetary
magnetic field, as Earth's does, Lunar Prospector's magnetometer was
able to measure the weak field induced in the moon's core when the
body passed through the tail of Earth's magnetosphere. From these
data, the scientists calculated a core radius of between 300 and 425
kilometers.

At this size, the moon's core would contain only about 2 percent of
the body's mass. In contrast, Earth's core--which has a radius of
about 3,400 kilometers--comprises about 30 percent of the planet's
mass. The relative puniness of the moon's core suggests that the moon
was born with a severe iron deficiency. Astronomers have theorized
that about 4.5 billion years ago a rogue protoplanet, probably two or
three times as massive as Mars, slammed into Earth and blasted a huge
amount of debris into space. According to this theory, some of the
debris clumped together to form the moon. Binder believes the impact
occurred after most of Earth's iron had sunk to the planet's core. In
that case, the debris expelled into space would have been mostly
iron-poor rock from Earth's mantle and from the mantle of the
protoplanet. Lunar Prospector may find additional clues to the moon's
origins in the last months of its mission. Earlier this year the
spacecraft dropped from its 100-kilometer-high mapping orbit to a low
elliptical orbit that brings it as close as seven kilometers to the
moon's surface. The lower altitude allows the craft to take more
accurate readings, although it must fire its thruster every four weeks
to avoid hitting the moon.

A crash landing, however, is the craft's ultimate fate. On July 31 the
mission runs out of funding, and eventually the orbiter will run out
of fuel. But even the smashup may yield a scientific return:
investigators hope to maneuver Lunar Prospector for an impact in one
of the moon's permanently shadowed polar regions. Some scientists are
still skeptical about the presence of ice in these areas; to
strengthen the evidence, Binder and his colleagues would like to
analyze the plume of material that would be ejected by the
spacecraft's swan dive. Observatories in Earth orbit, including the
Hubble Space Telescope, might be able to see traces of water vapor in
the plume. "That would be an absolute confirmation of water," Binder
says. Officials at the National Aeronautics and Space Administration
have not yet approved the impact experiment.

MOON'S CORE

DIAGRAMS: Holds only 2 percent of the body's mass (left), whereas
Earth's core contains nearly one third of the planet's mass (below).

~~~~~~~~

By Mark Alpert
_________________

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Source: Scientific American, Jun99, Vol. 280 Issue 6, p20, 2p, 1
diagram.
Item Number: 1893291