The WIRE spacecraft's scientific functionality was destroyed
shortly after launch on March 4th, 1999, when the satellite's instrument
cover was prematurely jettisoned. This allowed the interior of the
craft to heat and subsequently sublimate the solid hydrogen that was
supposed to keep the instruments cool. Outgassing resulting from the
sublimation of the cryogenic materials put the spacecraft into a spin
that was too great for the reaction control wheels to recover. Although
mission controllers started to work on ways to slow the spacecraft,
they could not get it under control in time to prevent complete loss
of the frozen hydrogen.
Sequence of Events
About twenty minutes after WIRE separated from its Pegasus XL
launch vehicle, it passed over McMurdo, Antarctica. During the ten
minutes of contact McMurdo had with WIRE, commands were sent to
initial cryogenic venting to release any gases that had accumulated
during launch. Forty minutes later, NORAD tracking showed three
separate objects; one about the size of the telescope
cover. By the time the spacecraft passed over Poker Flat, Alaska,
tumble rates were increasing, rather than decreasing as expected.
An analysis of the attitude control system was performed, but it
was found to be operating normally; the spin caused by the hydrogen
outgassing was just to great for it to recover from. Controllers were
not able to get the spacecraft under control, and within thirty six
hours, a four month supply of cryogenic hydrogen had been lost.
The fault was found to lie in the design of the electronics used to
control when the pyrotechnics used to jettison the telescope
cover. When the electronics were first powered on, unexpected
transients resulted due to the clock and a field programmable gate
array (FPGA) being in non-deterministic conditions for a short
period. The conditions during this time signalled the pyrotechnics to
The electronics box used to control the instrument cover
pyrotechnics was not peer reviewed. Other components were peer
reviewed, but at that time, the pyro control box had not yet been
WIRE was recovered and all its systems were found to be working
normally. No component failure was found to contribute to the loss of
cryogenics, it was simply a badly designed component.
Information presented here is derived from the WIRE National Space
Science Data Center (NSSDC) entry at
http://nssdc.gsfc.nasa.gov/nmc/tmp/1999-011A.html, and from the WIRE Mishap Investigation
Board Report, available at ftp://ftp.hq.nasa.gov/pub/pao/reports/1999/wire_summary.pdf.
WIRE, part of NASA's Small Explorers (SMEX) program, is a
spacecraft that was intended to conduct a narrow infrared survey to
examine starburst galaxies and look for protogalaxies. WIRE was
launched from the Western Range, Vandenberg Air Force Base, aboard a
Pegasus XL launch vehicle on March 4th, 1998. WIRE's scientific value
was lost when the cryogenic materials needed to cool its instruments
vented into space shortly after launch.
The instrument that was to make the infrared observations was a two
color (12 and 25 micrometers) infrared imaging telescope. This
telescope was cooled by solid hydrogen, had a 30 cm aperture opening,
and a 32 by 32 arc-minute field of view. The detectors used in the
instrument were two 128 by 128 arrays of blocked impurity band
SiAs. The telescope itself was of a Cassegrain design, using two
diamond turned mirrors, one dichroic, and a filter. It contained no
The cryostat that was to be used to cool the optics and detectors was the
first small, lightweight cryostat ever to fly in space. It was of a
dual stage design that allowed a guard-tank to keep the telescope at
-438 degrees Fahrenheit and prevent heat from reaching the inner
tank. The inner tank cooled the detectors, which needed to be below
-444 degrees Fahrenheit to operate. Such temperatures were thought
impossible to achieve in a space based instrument without the use of
helium. Because hydrogen is forty times more efficient than helium as
a cryogen, it becomes possible to make the cryostat small enough to
fit within the Pegasus fairing along with the telescope itself.
The spacecraft itself is made of M55J/954-3 and K1100/954-3
Cyanate Ester fiber reinforced composites. This structure made WIRE
the first spacecraft to use a fully bonded all composite structure, as
opposed to the common mechanically fastened aluminum
structure. The use of composites gives WIRE a 50 percent weight
reduction in the frame, leaving more weight for instruments. WIRE uses
gallium arsenide solar cells on deployed panels, with energy storage
in a 9 Amps per hour "Super" nickel cadmium battery. An S-band
transponder capable of 2 Kbps uplink and 2.25 Mbps downlink was is
for communications. The craft is managed by a 80386/80387 processor,
with data storage on a 30 Mb solid state recorder.
Attitude control is handled by a 8085 processor fed with sensor data
from a three-axis gyroscope package, a star tracker, a digital Sun
sensor, six coarse Sun sensors, a three-axis magnetometer, and a wide
angle Earth sensor. Orientation is controlled with four reaction
control wheels, and three magnetic torque rods.
As of February 9, 2000, WIRE was in a 540 km by 590 km orbit with a
97.56 degree inclination. WIRE is currently being used as a test bed
for other science opportunities, technology infusion, risk management,
educational outreach, and training. It is also being used for
astroseismology investigations, which is the study of star
oscillations. The star tracker onboard WIRE has enough accuracy to
detect the small variations in the brightness of a star.
For really good, in depth information about the technology on WIRE,
of the information here came from the WIRE National Space Science Data
Center (NSSDC) catalog entry, located at http://nssdc.gsfc.nasa.gov/nmc/tmp/1999-011A.html.