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

[Ron Baalke]

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.

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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/
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