A lightweight NASA instrument from California has arrived
in the Netherlands, one step closer in its journey to examine
how gases escape from the nucleus of a comet.

The Microwave Instrument for the Rosetta Orbiter is one
of 17 instruments that will fly aboard the European Space
Agency’s major mission to a comet. Rosetta will be the first
spacecraft to orbit a comet, and the microwave instrument will
be the first of its type to be sent to any solar system object
other than Earth.

"We’ll look at the abundance of the gases, their
temperatures, the speed at which they’re coming off, and the
temperature of the comet’s nucleus," said Dr. Margaret
Frerking, the microwave instrument’s project manager at NASA’s
Jet Propulsion Laboratory, Pasadena, Calif.

The JPL-built device was incorporated into the main
spacecraft structure in Alenia, Italy, and arrived in
Noordwjik, Netherlands, to begin a series of tests by the
European Space Agency. The next step in its journey is its
path to Kourou, French Guinea, for its January 2003 launch
into space. Rosetta will swing near Earth and two large
asteroids before reaching its chosen dance partner, Comet
Wirtanen, on Nov. 28, 2011. At that point in Wirtanen’s 5.5-
year orbit, the comet will be at about as far from the Sun as
Jupiter and five times as far from the Sun as Earth.

Rosetta will drop a lander onto Wirtanen’s nucleus, and
the orbiter will circle the comet at distances as close as 2
kilometers (1.2 miles).

From the orbiter, the microwave instrument will monitor
how the release of vapors from the comet’s icy nucleus changes
as Wirtanen moves closer to the Sun. Gases and dust escaping
from the surface of a comet form a cloud-like "coma" around
the nucleus and a tail pointed away from the Sun.

"The spacecraft will remain in orbit around Wirtanen for
20 months as the comet moves in from Jupiter’s distance from
the Sun to about Earth’s distance," said JPL’s Dr. Samuel
Gulkis, principal investigator for the instrument. "During
that time, the nucleus will warm significantly, and we’ll be
able to watch the whole process as the comet evolves from an
inactive iceball to having a fully developed coma."

The instruments onboard the orbiter will include a camera
to study surface details, a microscope to analyze dust grains
coming off the nucleus, spectrometers to examine surface and
coma materials in various wavelengths, and an experiment to
probe the comet’s interior with radio waves.

The microwave instrument is a very high frequency radio
spectrometer, weighing about 20 kilograms (44 pounds). It is
designed for studying water, carbon dioxide, ammonia and
methanol gases, four of the most abundant gases from comets.
The device is sensitive to slight differences in emission
wavelengths from those gases, allowing it to measure the
quantities coming off the nucleus, along with their
temperatures and speeds.

"We want to get a good estimate of the amount of mass
being lost by the comet so we can play that backward to get at
what the comet was like shortly after it was formed," Gulkis
said. That will help pin down ideas about how comets and
planets were produced during the infancy of our solar system.

The microwave instrument will also be able to measure
both the surface temperature of the nucleus and the
temperature just below the surface. "That temperature
difference will tell us about the insulating properties of the
surface and help us understand the thermal physics of what’s
going on inside the nucleus," Gulkis said.

As Rosetta passes the stony asteroid Otawara and the
carbon-rich asteroid Siwa on its roundabout route to Wirtanen,
the microwave instrument will examine thermal properties of
those minor planets’ surfaces and check whether they have any
permafrost layer leaking small quantities of water vapor into
space.

Online information is available about Rosetta at
http://sci.esa.int/rosetta and about the microwave instrument
at http://mirowww.jpl.nasa.gov . JPL, a division of the
California Institute of Technology in Pasadena, manages the
instrument for NASA’s Office of Space Science, Washington,
D.C.