[meteorite-list] PSI Researchers Explore Orbital Environment for Dawn Spacecraft at Vesta

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 4 Oct 2010 12:19:27 -0700 (PDT)
Message-ID: <201010041919.o94JJRoX007475_at_zagami.jpl.nasa.gov>

NEWS RELEASE FROM THE PLANETARY SCIENCE INSTITUTE

SENT:
Oct. 4, 2010

FROM:
Alan Fischer
Public Information Office
Planetary Science Institute
520-885-5648
520-622-6300
fischer at psi.edu

PSI RESEARCHERS EXPLORE ORBITAL ENVIRONMENT FOR DAWN SPACECRAFT AT VESTA

A detailed study of the dynamics of the Dawn spacecraft as it
orbits the large Arizona-sized, non-spherical asteroid Vesta
next summer in 2011 reveals one of the most complex operational
environments for a NASA mission to date. This work has
implications for future mission planning to other large asteroids.

A paper on the research titled "The Dynamical Environment of Dawn at
Vesta" by Pasquale Tricarico, Associate Research Scientist with the
Planetary Science Institute in Tucson, Ariz., and PSI Director Mark
Sykes was recently published in Planetary and Space Science.

The Dawn spacecraft is using an efficient ion-propulsion system that
enables it to make the first ever double rendezvous. After completing
its study of Vesta, Dawn will depart and orbit the dwarf planet Ceres
in 2015.

Ion propulsion generates very gentle acceleration over long periods
of time, in contrast with chemical propulsion that produces high
thrust over short periods of time, Tricarico said. Dawn currently
uses this system to provide propulsion for its voyage to Vesta. Ion
propulsion will also be used to slowly transit from high to low
mapping orbits. Close passes are expected to reveal the dramatic
surface of what was a planetary embryo, including the crater from a
giant impact event thought to result in about 6 percent of the
meteorites falling on the Earth, he said.

"One of our goals was to figure out how low of an orbit Dawn could
get to without endangering the mission," said Tricarico. "The closer
you get, the better the imaging resolution and the greater the ability
of the Gamma-Ray and Neutron Detector to determine the elemental
composition of Vesta's surface." The Dawn GRaND instrument is run by
PSI Senior Scientist Tom Prettyman.

The hypothesized gravity field from Vesta's irregular shape combined
with Dawn's slow changes in orbit results in certain altitudes at which
Dawn could experience significant orbital perturbations as it passes
through resonances. Simulations indicate that when Dawn passes through
the altitude at which its orbital period equals Vesta's 5.4 hour
rotational period, the spacecraft could hypothetically become "trapped"
in its orbit. Data from these simulations indicate that such trapping
could be escaped by thrusting at the appropriate orbital phase.

"The perturbations look dramatic when plotted, but in actuality what
the spacecraft experiences is more like an extremely gentle ocean swell.
These effects on Dawn's orbit present some interesting operational
challenges, but nothing that either threatens the spacecraft or risks
the success of the mission," Sykes said.

Once through these resonance regions, Tricarico determined that the
spacecraft would be safe down to an average orbital radius of 400 km -
a minimum of 110 km above the surface.

"This is well below the lowest orbital radius of 460 km, planned by
Dawn's navigation team, giving the spacecraft a good margin of safety,"
said Tricarico. "Other operational factors limit the altitude at which
the spacecraft can operate.'

The research is relevant not just for Dawn, but also or future missions
to non-spherical bodies.

"Missions using low-thrust ion propulsion are going to become very
popular because they give you very cost-effective access to targets and
create the ability to do multiple target missions," he said.

"The present work used in-house open source software, previously
developed, which we are happy to provide to colleagues in support of
their future mission planning activities," said Tricarico.

This work was supported by the National Aeronautics and Space
Administration under subcontract from UCLA to the Planetary Science
Institute in support of the Dawn Discovery Mission.

The Dawn mission to asteroid Vesta and dwarf planet Ceres is managed
by JPL, for NASA's Science Mission Directorate, Washington, D.C. The
University of California, Los Angeles is responsible for overall
Dawn mission science.

Additional information about Dawn is online at

http://dawn.jpl.nasa.gov .


Contact:
Pasquale Tricarico
Associate Research Scientist
520-547-3954
tricaric at psi.edu

PSI INFORMATION:
Mark V. Sykes
Director
520-622-6300
sykes at psi.edu
Received on Mon 04 Oct 2010 03:19:27 PM PDT