[meteorite-list] Dawn Falling For Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 12 Feb 2009 16:03:14 -0800 (PST)
Message-ID: <200902130003.QAA24925_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/features.cfm?feature=2031

NASA Spacecraft Falling For Mars
Jet Propulsion Laboratory
February 12, 2009

Launched in September of 2007, and propelled by any one of a trio of
hyper-efficient ion engines, NASA's Dawn spacecraft passed the orbit of
Mars last summer. At that time, the asteroid belt (where Dawn's two
targets, asteroid Vesta and the dwarf planet Ceres reside), had never
been closer. In early July the spacecraft began to lose altitude,
falling back towards the inner solar system. Then on October 31, 2008,
after 270 days of almost continuous thrusting, the ion drive turned off.

"Not only are our thrusters off and we are dropping in altitude, we are
plunging toward Mars," said Marc Rayman, the Dawn project's chief
engineer from NASA's Jet Propulsion Laboratory in Pasadena, Calif. "And
everybody here on Dawn could not be happier."

The team's joy at plummeting towards a planet named for the Roman god of
war is not unfounded. Mars, the final stop for many a NASA spacecraft,
was always an important, and weighty, waypoint for the Dawn mission. It
all has to do with one of the heavy subjects of rocket science, gravity
assists.

A gravity assist is the use of the relative movement and gravity of a
planet or other celestial body to alter the path and speed of a
spacecraft, typically in order to save fuel, time and expense. A
spacecraft traveling to an outer planet (or in this case asteroid) will
decelerate because the incessant tug of the sun's gravity slows it down.
By flying a spacecraft close by a large planet and its large gravity
field, some of the planet's speed as it orbits the sun is transferred to
the spacecraft. In Dawn's case, it is using the Red Planet's tremendous
angular momentum (the speed at which Mars orbits the sun) to give it a
little extra oomph.

"A big oomph actually," said Rayman. "The gravity of Mars will change
Dawn's path about the sun, enlarging its elliptical orbit and sending
the probe farther from the sun. It will also change Dawn's orbital plane
by more than 5 degrees. This is important because Dawn has to maneuver
into the same plane in which Vesta orbits the sun."

If Dawn had to perform these orbital adjustments on its own with no Mars
gravitational deflection, it would have required the spacecraft to fire
up its engines and change velocity by more than 5,800 miles per hour
(9,330 kilometers per second). Such velocity changes would have required
Dawn to carry an extra 230 pounds (104 kilograms) of xenon fuel.

"Without the gravity assist, our mission would not have been affordable,
even with the extraordinary capability of the ion propulsion system,"
said Rayman. "That's why we are happy Dawn is now plunging toward Mars."

Also happy for the opportunity to fly past the fourth rock from the sun
is Dawn's science team. With asteroid Vesta still more than
two-and-a-half years away, Mars provides the perfect opportunity to give
their highly-tuned instruments a workout.

"It is fortuitous that we need Mars to get out to Vesta and Ceres," said
Carol Raymond, Dawn's deputy principal investigator, from JPL. "Since
there are other spacecraft currently operating at Mars with similar
instrumentation, we will be able to check our measurements against their
knowledge of Mars, and carry that information farther out into the solar
system."

But the Mars gravity assist is not the final hurdle on Dawn's road to
the asteroid belt. The subsequent 30 months include more than 27 months
of blue-green tinged ion thrusting to successfully rendezvous with
Dawn's first target - Vesta.

While an accurately flown encounter with the planet Mars makes a big
difference in the life of NASA's asteroid pioneer, the planet itself
does not come out unscathed. Weighing in at all of 2,500 pounds (1,134
kilograms), Dawn has its own mass and thereby its own gravitational
field. In contrast, the somewhat more massive planet is almost 600
million-million-million times more substantial than that of the spacecraft.

"The laws of physics tell us that Mars will pay a price for helping
Dawn," said Rayman. "The flyby will cause Mars to slow in its orbit
enough that after one year, its position will be off by about the width
of an atom. If you add that up, it will take about 180 million years for
Mars to be out of position by one inch (2.5 centimeters). We appreciate
Mars making that sacrifice so Dawn can conduct its exciting mission of
discovery in the asteroid belt."

Dawn's 4.8-billion-kilometer (3-billion-mile) odyssey includes orbiting
asteroid Vesta in 2011 and the dwarf planet Ceres in 2015. These two
giants of the asteroid belt have been witness to much of our solar
system's history. By using Dawn's instruments to study both objects for
several months, scientists can more accurately compare and contrast the
two. Dawn's science instrument suite will measure geology, elemental and
mineral composition, shape, surface topography, geomorphology and
tectonic history, and will also seek water-bearing minerals. In
addition, the Dawn spacecraft's orbit characteristics around Vesta and
Ceres will be used to measure the celestial bodies' masses and gravity
fields.

The Dawn mission to Vesta and Ceres is managed by JPL, a division of the
California Institute of Technology in Pasadena, for NASA's Science
Mission Directorate, Washington. The University of California, Los
Angeles, is responsible for overall Dawn mission science. Other
scientific partners include Planetary Science Institute, Tucson, Ariz.;
Max Planck Institute for Solar System Research, Katlenburg-Lindau,
Germany; DLR Institute for Planetary Research, Berlin; Italian National
Institute for Astrophysics, Rome; and the Italian Space Agency. Orbital
Sciences Corporation of Dulles, Va., designed and built the Dawn
spacecraft.

To learn more about Dawn and its mission to the asteroid belt, visit:

http://www.nasa.gov/dawn

Media contact: DC Agle/JPL
818-393-9011
Received on Thu 12 Feb 2009 07:03:14 PM PST