4 Vesta - NASA Dawn Mission Status Update 7JUL2011

http://www.jpl.nasa.gov/news/news.cfm?release=2011-206  

Dawn Team Members Check out Spacecraft
Jet Propulsion Laboratory
July 07, 2011

Dawn Mission Status Update

Mission managers for NASA's Dawn spacecraft are studying the
spacecraft's ion propulsion system after Dawn experienced a loss of
thrust on June 27. Dawn team members were able to trace the episode to
an electronic circuit in the spacecraft's digital control and interface
unit, a subsystem that houses the circuit and a computer that provides
the "brains" to Dawn's ion propulsion system. That circuit appeared to
lose an electronic signal. As a result, the valves controlling the flow
of xenon fuel did not open properly. Dawn automatically put itself into
a more basic configuration known as "safe-communications" mode, where
the spacecraft stopped some activities and turned its high-gain antenna
to Earth.

Engineers were able to return the spacecraft to a normal configuration
and restart the spacecraft's thrusting on June 30 by switching to a
second digital control and interface unit with equivalent capabilities.
One set of images for navigation purposes was not obtained on June 28
because the spacecraft was in safe-communications mode, and one other
set, on July 6, was not obtained to allow the spacecraft to spend the
time thrusting. Other sets of navigation images have been and will be
acquired as expected. The ion propulsion system is now functioning normally.

"Dawn is still on track to get into orbit around Vesta, and thanks to
the flexibility provided by our use of ion propulsion, the time of orbit
capture actually will move earlier by a little less than a day," said
Marc Rayman, Dawn's chief engineer and mission manager. "More
importantly, the rest of Dawn's schedule is unaffected, and science
collection is expected to begin as scheduled in early August."

In an unrelated event, the visible and infrared mapping spectrometer on
Dawn reset itself on June 29. At the time of the reset, the instrument
was gathering calibration data during the spacecraft's approach to the
giant asteroid Vesta. Some of its planned observations were completed
successfully before automatic sensors turned the instrument off.

On June 30, Dawn team members were able to trace the reset to an
internal error in the instrument's central processing unit, though they
don't yet know why the internal error occurred. By temporarily turning
the instrument back on, the Dawn team confirmed that the instrument is
otherwise in a normal configuration. They powered the instrument back
off, as originally planned for this time. Team members are working to
determine when they will turn it back on again.

After arriving at Vesta, Dawn will spend about one year orbiting the
asteroid, which is also known as a protoplanet because it is a large
body that almost became a planet. Data collected at Vesta will help
scientists understand the earliest chapter of our solar system's history.

The Dawn mission to Vesta and Ceres is managed by the Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, for NASA's Science Mission Directorate, Washington. Dawn is a
project of the directorate's Discovery Program, managed by NASA's
Marshall Space Flight Center in Huntsville, Ala. 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, Germany; Italian National Institute for Astrophysics, Rome; and
the Italian Space Agency, Rome. 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 and http://dawn.jpl.nasa.gov .



Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.