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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.

Cause

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 fire.

The electronics box used to control the instrument cover pyrotechnics was not peer reviewed.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 designed.

Status

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.

Hardware

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 moving parts.

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.

Summary

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, see http://sunland.gsfc.nasa.gov/smex/wire/mission/technology.html. Some 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.

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