[meteorite-list] As Dawn Approaches Mars, PSI Scientist Gear Up For GRaND Tests

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 12 Feb 2009 16:13:57 -0800 (PST)
Message-ID: <200902130013.QAA27900_at_zagami.jpl.nasa.gov>

Feb. 12, 2009

FROM:
Ed Stiles
Public Information Office
Planetary Science Institute
520-248-7119
psinews at psi.edu

AS DAWN APPROACHES MARS, PSI SCIENTISTS GEAR UP FOR GRaND TESTS

The Dawn spacecraft, which began its journey to the asteroid
belt in 2007, is now nearing Mars, and scientists at the
Tucson-based Planetary Science Institute (PSI) are preparing
to use the encounter to tune up Dawn's GRaND instrument.

Dawn's instrument payload includes a visible-light camera, a
visible-light and infrared mapping spectrometer, and GRaND
(the Gamma-Ray and Neutron Detector).

The GRaND team will be involved in several activities during
MCA (the Mars encounter). These include:

-- Pre-encounter testing and optimization of the instrument.
-- Measurement of background noise levels caused by comic rays
   and, possibly, by energetic particles.
-- Acquisition of neutron and gamma-ray data from Mars.
-- Calibration of the instrument by comparing GRaND's
   measurements of the Martian neutron and gamma-ray output to
   data acquired by the Odyssey spacecraft, which has been
   orbiting Mars for the past seven years.

"The main task between now and MCA is to make final adjustments
to instrument parameters," said Tom Prettyman, a senior scientist
at PSI and the lead investigator for GRaND. "Only minor adjustments --
if any -- are needed. The instrument has performed consistently
since it was first turned on following launch."

Dawn won't loiter at Mars. This encounter is only a slingshot
flyby in which the spacecraft will speed past Mars and use the
red planet's gravitational pull to increase its velocity by
about 2,500 mph relative to the sun.

Dawn is headed for Vesta and Ceres, the two most massive bodies
in the asteroid belt, which is located between Mars and Jupiter.
Scientists hope the encounters with Vesta and Ceres will help
them journey back in time to answer many questions relating
to the solar system's early formation.

GRaND will measure the energy spectra of neutrons and gamma rays
coming from the surfaces of Vesta and Ceres to determine the
geochemistry of these protoplanets.

The quantities of various elements on the protoplanets' surfaces
will provide scientists with clues to the formation and thermal
evolution of Vesta and Ceres, including the role of water in
planetary development, Prettyman said.

The neutrons and gamma rays that GRaND measures are emitted by
the decay of long-lived radioactive elements and during the
subatomic collisions that result from cosmic rays slamming
into the protoplanets' surfaces.

These same cosmic rays hit the spacecraft and its detectors,
creating background signals that must be subtracted from the
instrument's readings to arrive at accurate numbers for the
neutrons and gamma rays coming from Vesta, Ceres or Mars,
said PSI Senior Scientist William Feldman

"Galactic cosmic rays are mostly energetic protons that interact
with the spacecraft structural materials to make neutrons and
gamma rays, just as they would on a planetary surface," Prettyman
added. "The neutrons and gamma rays propagate through the
spacecraft and wind up in our instrument. In addition,
cosmic-ray interactions produce radioactive isotopes in the
spacecraft structures that contribute to the background
radiation measured by GRaND."

Since Odyssey has been orbiting Mars for seven years, scientists
know the amount of neutron and gamma-ray radiation coming from
the planet's surface, and the spectroscopy data from Odyssey
will be used to calibrate GRaND, Prettyman said, noting that
both he and Feldman also are on the Odyssey science team.

While the data acquired during the flyby will be used to
calibrate the instrument, it also has value for Mars research,
Prettyman said.

"Any time you compare data sets, there is the potential to find
something new," he said. "For Instance, Odyssey points right at
the center of Mars. We don't have a lot of data where Odyssey is
tipped relative to Mars like GRaND will be during the flyby. So
one of the things we might learn is that we need to refine our
background radiation models for Odyssey, which could ultimately
have an impact on Odyssey's discoveries and our understanding of
the distribution of water on Mars and the abundance of carbon
dioxide and other compounds."

Dawn will make its closest approach to Mars, flying within about
300 miles of the Martian surface on February 18 (Pacific Coast
Time), before speeding off toward Vesta, which it will begin to
orbit in 2011. After mapping Vesta, Dawn will fly on, making a
three-fourths orbit around the sun before reaching Ceres in 2015.

GRaND was developed by Los Alamos National Laboratory, and key
sensor components were manufactured by eV Products, Eljen
Technology, and Proteus, Inc. PSI is responsible for operating
the instrument and analyzing data acquired during the post-launch,
science phase of the mission.

Dawn isn???t an acronym. Instead, it reflects the mission's
purpose -- gathering data related to the dawn of our solar system.

The Jet Propulsion Laboratory manages the Dawn mission for NASA's
Science Mission Directorate. Christopher T. Russell, professor of
geophysics and space physics at UCLA, leads the overall mission.

--------------------------------------------------------------------------
CONTACT:
Tom Prettyman
Senior Scientist
520-622-6300
prettyman at psi.edu

William Feldman
Senior Scientist
505-667-7372
feldman at psi.edu

PSI INFORMATION:
Mark V. Sykes
Director
520-622-6300
sykes at psi.edu

PSI Homepage
http://www.psi.edu

PSI Press Releases
http://www.psi.edu/press/
--------------------------------------------------------------------------
Received on Thu 12 Feb 2009 07:13:57 PM PST