The intrepid never-say-die space telescope known as
FUSE has finally
reached its mission's end
and will be turned off after more than eight years of
discoveries on everything from planets and
nearby stars to galaxies and quasars billions of
light-years away.
The satellite's control room on the Johns Hopkins
Homewood campus will go dark on Oct. 18,
leaving the satellite itself — its pointing system so
often pulled from the brink of inoperability by
enterprising engineers and scientists in that control room
— to continue circling the Earth. Eventually,
decades from now, its orbit will decay and the satellite
will burn up in the atmosphere.
"It has been a great run. Who would have believed that
a mission designed for three years
would have gone on producing great science for eight," said
Warren Moos, Gerhard H. Dieke Professor
of Physics and
Astronomy at Johns Hopkins and FUSE principal
investigator. "What a testament to the
ingenuity and hard work of the FUSE team."
FUSE, short for Far Ultraviolet Spectroscopic
Explorer, was a Johns Hopkins-managed NASA
mission that complemented the Hubble Space Telescope
with observations at short ultraviolet light
wavelengths below the range in which Hubble operates. Its
original, three-year science mission was
extended by NASA three times.
Astronomers from around the world have published more
than 1,200 papers based on data from
the satellite, which was launched atop a Delta-II rocket in
June 1999 from Cape Canaveral. FUSE
produced not pictures of distant objects but spectrographs,
which are digital "charts" that break
down the light emitted by those objects. By analyzing FUSE
data, astronomers were able to measure
temperatures, densities and chemical compositions of
objects near and far, helping to place them in
context in the history of the universe.
Groundbreaking science done during FUSE's eight years
includes a rare glimpse into molecular
hydrogen in Mars' atmosphere, confirmation of a hot gas
halo surrounding the Milky Way galaxy and a
first-ever observation of molecular nitrogen outside our
solar system, among others.
George Sonneborn, FUSE project scientist at NASA
Goddard Space Flight Center in Greenbelt,
Md., said, "JHU has done a fantastic job delivering a
top-quality science mission for NASA."
FUSE is the largest astrophysics mission that NASA has
ever handed off to a university to
develop and then operate. The 18-foot-tall, 3,000-pound
satellite has been run by a group of about 25
scientists, engineers and support staff from a
basement-level control room in the Bloomberg Center
for Physics and Astronomy.
Though FUSE's mission has been astoundingly successful
from a scientific point of view, it faced
a number of "death-defying" adventures dating back to late
2001. That's when, during a two-week
period, two of the four "momentum wheels" that helped
researchers aim the satellite at its targets
stopped working. At least three wheels were needed to point
FUSE with the precision needed for
accurate astronomical observations.
Left with no other choices, the operations team
conceived of and created a modified control
system that used other devices onboard the satellite
— electromagnets called magnetic torquer bars —
to stabilize the satellite's pointing by periodically
pushing or pulling the satellite against Earth's
magnetic field. In just eight weeks, FUSE was back in
business.
"It was a tenuous control at first, but it was
certainly better than nothing," recounted Bill Blair,
a research professor in the Henry A. Rowland Department of
Physics and Astronomy and FUSE's chief
of operations. "But with time, tweaks and experience, we
got back into a very respectable science
operations mode."
The satellite remained stable until Dec. 27, 2004,
when yet another momentum wheel stopped
operating, leaving the satellite with just one.
"True to form, we figured out how to use the magnetic
trick to work with the single remaining
wheel. But it was a lot harder than it sounds," Blair
said.
It took most of 2005 to coax the satellite back into
an effective operational mode, but from
November 2005 into the spring of this year, FUSE was again
gathering data. On May 8, however, the
last momentum wheel malfunctioned. Researchers and
engineers believe that some source of excess
friction slowed the wheel. After studying the problem, the
operations team was able to restart the
wheel and put the telescope back into service.
"From June 12 to July 12, the wheel performed again
almost flawlessly," Blair said. "But then
the 'wheels came off,' so to speak. The wheel operated
perfectly right up to the end, and then it just
stopped dead, probably indicating a catastrophic failure of
some kind."
After a month of creative troubleshooting, the FUSE
team had to face the sad fact that the
satellite's science mission was, at last, at its end.
"The magnetic system just doesn't have enough muscle
by itself to point and hold the satellite
for astronomical observations," Blair said. "We contacted
NASA and told them the science mission was
over."
Now the team has less than a year to close out the
mission, including reprocessing and archiving
some 131 million seconds of science data, writing final
reports and providing final documentation of the
mission. FUSE's scientific data will remain available to
astronomers for years, however, through a data
archive at the Space
Telescope Science Institute, which is located on the
Homewood campus.
One part of the closeout process is concluding
on-orbit operations and turning off the satellite.
Until now, FUSE has continued to take end-of-mission
calibration data and perform engineering tests
of various kinds. But Oct. 18, FUSE gets "put to sleep" for
good. After 30 years or so, its orbit will
decay and FUSE will burn up in the atmosphere.
"After that Thursday, FUSE will be just another piece
of space junk, orbiting the earth every
100 minutes or so. It is a sad and ignominious end to such
an outstandingly successful mission," Blair
said. "But a tremendous scientific legacy is left behind. I
commend the team of scientists and
engineers who have worked so hard over the years to wring
every last bit of science we could out of
this amazing satellite."
For more on FUSE, go to fuse.pha.jhu.edu.