NASA's Far
Ultraviolet Spectroscopic Explorer astronomy satellite
is back in full operation, its aging onboard software
control system rejuvenated and its mission extended by
enterprising scientists and engineers after a near-death
experience in December 2004.
Observations with the orbiting telescope resumed Nov.
1, 2005, about 10 months after the third of four onboard
reaction wheels, used to precisely point the spacecraft and
hold it steady, stopped spinning. After two months of
experience tweaking the new control system in November and
December, FUSE operations returned in January to a level of
efficiency comparable to that achieved earlier, mission
managers said.
"It's really a level of performance that we never
thought we would see again," said William Blair, a research
professor in Physics
and Astronomy at Johns Hopkins and FUSE's chief of
observatory operations. "The old satellite still has some
spunk."
FUSE was launched in June 1999. Late in 2001, two of
the reaction wheels failed in quick succession, leaving the
satellite temporarily unusable. That time, science
operations were successfully resumed within about two
months through a modification of flight control software
and development of a creative new technique to establish
fine pointing control.
"The project aggressively pursued a similar track this
time, but it was even harder with just one operational
reaction wheel," said George Sonneborn, FUSE project
scientist at NASA's Goddard Space Flight Center in
Greenbelt, Md. "Some people would say what we're doing is
nearly impossible."
Initially, at least three reaction wheels were
required for the spacecraft to conduct its scientific
mission. The revised control mode developed in 2001
utilized the two remaining reaction wheels and drafted the
satellite's magnetic torquer bars into the effort to
provide control in all three axes. The MTBs (essentially,
controllable electromagnets) apply forces on the satellite
by interacting with Earth's magnetic field. Now, the FUSE
control system has been modified again to use magnetic
control on two axes, which provides a tenuous but
acceptable level of control in place of the missing
reaction wheels.
"It's like we had three strong muscles originally and
could point FUSE wherever we wanted to," Blair said. "Now
we have to control the pointing with one strong muscle and
two weak muscles. The revised control software is like a
good physical therapist, teaching the satellite to
compensate."
Since its launch, FUSE has obtained more than 52
million seconds of science data on everything from planets
and comets in our solar system to distant quasars and
active galaxies, and every major class of object in
between. This information, compiled in the form of
spectrographs rather than visual images, provides
astronomers with details about the physical properties and
characteristics of objects, from temperatures and densities
to chemical makeup.
Observations from the satellite have been used to
discover an extended tenuous halo of very hot gas
surrounding our Milky Way galaxy and have found evidence of
similar hot gas haloes around other galaxies. FUSE also has
detected molecular hydrogen in the atmosphere of the planet
Mars for the first time. This has implications for the
water history of our frozen neighbor. In addition, FUSE
observations first detected molecular nitrogen in dense
interstellar gas and dust clouds but at levels well below
what astronomers had expected, requiring a return to the
drawing board for theories of interstellar chemistry.
NASA has twice extended what originally was planned as
FUSE's three-year mission to carry out a broad range of
science programs for hundreds of astronomers from around
the world. To date, more than 350 publications based on
FUSE observations have been published in the professional
astronomy literature and many more are on the way. A new
set of planned observations for the coming year was
accepted in December by NASA, and the first of these has
already been obtained.
"The recovery of FUSE operations is a tremendous
testament to the dedication and ingenuity of the scientists
and engineers at Johns Hopkins and at the Orbital Sciences
Corp.," said Warren Moos, professor of physics and
astronomy and principal investigator for FUSE. "There are a
large number of astronomers in line waiting for FUSE
observations that are now being undertaken once again."
Johns Hopkins has primary responsibility for all
aspects of FUSE, including both the development and
operational phases of the mission. The FUSE science and
satellite control center is on the Homewood campus. FUSE
partners include Honeywell Technical Services, Johns
Hopkins' Applied Physics Laboratory, the Canadian Space
Agency, the French Space Agency, the University of Colorado
at Boulder, and the University of California, Berkeley, in
addition to Orbital Sciences Corp.
FUSE is a NASA Explorer mission. Goddard Space Flight
Center manages the Explorers Program for NASA Headquarters
in Washington, D.C.
For more on the FUSE mission and future status
updates, go to
fuse.pha.jhu.edu.