Headquarters, Washington, DC May 12, 2000
Goddard Space Flight Center, Greenbelt, MD
BOARD FINDS CAUSE OF HESSI MISHAP
The HESSI Mishap Board released its final report today saying
that the High Energy Solar Spectroscopic Imager (HESSI) spacecraft
was damaged March 21 during pre-flight vibration tests because of
a malfunction in the vibration test system at NASA's Jet
Propulsion Laboratory in Pasadena, CA. (The vibration tests
simulate the stresses that the spacecraft experiences during the
rollout, release and flight to orbit aboard the Pegasus launch
The damage was caused when the test device, called a
"shaker," delivered approximately 20 G's, ten times the
appropriate level for the test, to the spacecraft. (A G is a unit
of force equal to the gravity exerted on a body at rest.) As a
result, the spacecraft's structure was damaged and three of the
four solar arrays were severely damaged. Satellites are routinely
subjected to vibration testing as part of preparing them for
Mishap Board Chairman Denny Kross, Manager, Engineering
Systems at NASA's Marshall Space Flight Center, Huntsville, AL,
said a misalignment between two pieces of the test stand led to an
abnormally high level of static friction (what engineers call
"stiction"). The computer used to control the test then tried to
compensate and induced too large a shock into the satellite.
"It's similar to what happens when you are trying to close a
sticky, wooden window that's just a little out of kilter in the
frame," Kross said. "As soon as the window starts to stick, your
brain says, 'push down harder.' And if you are not careful, you
can push so hard that, when the window does break free, its slams
down onto the bottom of the window sill."
To prepare for the test, the satellite is mounted on a device
called a slip table, which attaches atop a large slab of granite
mounted to the floor. A thin layer of oil is continuously pumped
between the slip table and the granite slab to allow the slip
table to move freely when stimulated by the "shaker." A computer
controls how hard the spacecraft is shaken, and accelerometers
measure the response of the spacecraft to the shaking.
Engineers found that the shaker mechanism had shifted on its
mounting base, due to a failed support bearing. The problem was
not discovered until after the accident. The broken bearing
shifted the position of the shaker mechanism causing the
misalignment between the slip table and the granite mass, and this
misalignment in turn created friction between the two pieces of
hardware. The computer, sensing this friction, calculated an
inappropriate drive signal and the resulting pulse was
significantly higher that expected, damaging the satellite.
Two primary factors contributed to the accident, Kross said.
One was the absence of a scheduled maintenance program requiring
periodic inspections of the shake table. The other was the lack
of a procedure requiring the test team to look for any shaker
performance problems in the pre-test data. "Had either of these
procedures been in place, this incident could have been avoided,"
The Board has made a number of recommendations in its report
for changing processes and procedures used by NASA for vibration
tests. "The JPL test team responded magnificently in the wake of
this incident," he said. "They are revamping their inspection
program so the test fixture is physically and visually inspected
at regular intervals. They are adding steps in the testing
procedure so that results of earlier tests are reviewed and
analyzed to look for early indications of stiction before the
final tests are conducted."
Kross said other recommendations, such as refurbishing the
shaker and implementing over-test protection methods, also are
underway at JPL. As a result of the Board's work, new procedures
were put in place at NASA's Goddard Space Flight Center,
Greenbelt, MD, where similar testing is done, and alerts have been
sent to other sites in the U.S. and overseas where satellites
undergo vibration checks.
The HESSI satellite will be repaired and re-assembled at the
University of California, Berkeley, which is serving as the prime
contractor on the project and is home to the principal
investigator. HESSI will be returned to JPL for continued
spacecraft testing after re-assembly. Launch plans will be
announced when available.
NASA's development cost for the HESSI spacecraft was budgeted
at $40 million. Development, launch vehicle and mission operations
costs bring the total mission value to $75 million.
HESSI is a Small Explorer mission and is managed by Goddard
under the Explorer Program. The science team includes co-
investigators from Switzerland, Scotland, Japan, France and The
Netherlands. More information on the mission can be found at: