JPL and NASA News

Bill Wheaton, IPAC - 1999 April

WIRE Lost

The NASA Small Explorer (SMEX) program is intended to provide frequent opportunities for the flight of modest missions at low cost. An important part of the SMEX concept is a cultural shift within NASA to accept increased risk of failure, by deliberately reducing the stringent and extremely expensive layers of quality control that had previously been the agency rule. Since the program began, four SMEXs (SAMPEX, FAST, TRACE, and SWAS) have been launched, and all have been successful. On 4 March the odds finally caught up with WIRE, the Wide-field InfraRed Explorer, an IPAC and JPL mission which we described just this last February.

After a successful launch in a Orbital Sciences Pegasus XL air-dropped rocket, we all went home from IPAC feeling great. Unknown to us (but apparent to mission scientists and controllers later that night) the 3-axis stabilized spacecraft failed to stop its slow spinning as expected. A complete understanding must await the findings of a review board assembled to analyze the failure. However, according to NASA, it appears that for reasons unknown the cover on the cryostat (an elaborate thermos bottle, designed to hold the telescope, detectors, and the solid hydrogen intended to keep everything cold for the four months required to complete the observations) was ejected shortly after launch, three days early.

As the spacecraft was not yet safely pointed at deep space, radiation from the Earth and probably sunlight were able to enter the cryostat, so that unexpectedly large amounts of energy reached the solid H2 cryogen. The hydrogen then vaporized and vented into space at a much higher rate than designed, and reaction from the venting caused the spacecraft to spin faster and faster. An heroic effort by controllers led to quick uploading of a software patch that cycled the magnetic torque coils used for slow control of spacecraft orientation, in such a way as to act against the spin. This was ultimately successful, and by 11 March, 3-axis lock was achieved. But too late: the hydrogen was exhausted several days earlier, effectively ending the possibility of accomplishing the mission science. The $79 million cost may be appreciated better perhaps if we think of it as a few score of life-work equivalents, although spread among hundreds of people. Now that control has been regained, some technological return is expected from a plan to use WIRE's advanced communications and data handling systems for engineering tests. According to Dr. Ed Weiler, NASA associate administrator for space science, future missions such as SIRTF, the Space InfraRed Telescope Facility (described here in October 1998), will eventually accomplish much of the science WIRE was expected to do. Inevitably this will be later (SIRTF is currently scheduled for launch in December 2001), and more slowly due to SIRTF's narrower field-of-view.

So, four successful missions in five launches, or 80% success: not a bad ratio considering the prohibitive cost of an old-style mission's draconian quality control measures. It seems one can hardly advocate abandoning the SMEX concept, but of course such abstract calculation does nothing to mitigate the pain those who have devoted many years of effort to WIRE and now find their baby still-born.

Deep Space 1

Deep Space 1, the technology development mission launched on an interplanetary trajectory last October 24, is now over 50 million km from Earth. Since launch DS1 has been testing its suite of 12 new technologys:

• Solar Electric Propulsion:
  This is the ion drive, which we discussed here in detail in July 1998. It has by now accumulated over 850 hours of successful operation in space. A comet landing mission using ion drives, DS4, is now being considered for 2004 launch as a follow-on application.
• Solar Concentrator Arrays:
  With its ion drive, DS1 requires plenty of power. The solar concentrator arrays supply about 2.6 kW, and were tested early in the mission. They have cylindrical Fresnel lenses to increase the effective solar flux by a factor of over 7, yielding a big decrease in the required area of the solar cells, and also a reduction in their cost.
• Ion and Electron Spectrometer:
  An important question addressed by this experiment is whether the ion drive so disturbs the local environment that valid space physics measurements of particles, plasmas, and electromagnetic fields cannot be made. The package also tests a new integrated, low-mass instrument design, and returns cruise and encounter science data.
• Miniature Integrated Camera and Imaging Spectrometer:
  The MICAS camera and spectrometer uses a common 10 cm telescope, and provides the imaging information for the AutoNav system, described below. It includes two 500-1000 nm visual channels and two imaging spectrometers, one in the UV (80-185 nm with 50 spectral channels), and one in the IR (1200-2400 nm, with 100 channels).
• Autonomous Navigation:
  In the past, spacecraft navigation has been performed by the Deep Space Network of large radio antennas which JPL operates in California, Spain, and Australia. Because operation of this system is expensive, and as it is taxed to the limit by present and planned missions, onboard navigation is very attractive. AutoNav is accomplished by optical observation of asteroids against the field of background stars, with computation on board the spacecraft. AutoNav had some trouble at first analyzing images from MICAS, but after modification of the on-board software, it has recently been able to determine its position to within 2000 km, sufficient for deep-space cruise requirements, completely without external aid.
• Ka-Band Solid State Amplifier:
  Operating at a frequency four times higher than the X-band system used previously, this amplifier will reduce both antenna size and the power required at a given data rate on future missions.
• Beacon Monitor Operations:
  This system allows the spacecraft to monitor its own status and evaluate its need for ground-based assistance, reducing demands on the DSN. Four discrete status signals can be transmitted: one meaning "all well, no assistance needed", one meaning "tracking needed when convenient", one requesting more extensive telemetry within a certain time, and one indicating a situation exists requiring immediate ground assistance and intervention.
• Small Deep Space Transponder:
  This is a compact new design intended to be widely applicable in future deep-space missions, which is also has the ability to use the Ka-band amplifier and to generate the signals needed by the beacon monitor system.
• Autonomous Remote Agent:
  Again to reduce the ground costs associated with missions, but also to permit quicker response to circumstances that may develop on a spacecraft very distant from Earth, the Autonomous Remote Agent is a virtual member of the ground team, on board the spacecraft. An on-board mission manager carries the mission plan, expressed as high-level goals. A planning and scheduling engine uses the goals and a comprehensive knowledge of the spacecraft state and mission constraints to develop a plan which an executive translates into commands to spacecraft systems. The system has the ability to respond to unexpected situations, and has access to much more complete information about the spacecraft and instrument systems than would be available to ground controllers, so that it can recover from all but extraordinary faults.
• Low Power Electronics:
  A special test set of 0.9 V, 0.25 micron feature-width electronics is included to validate new very-low power micro electronics, with particular attention to effects due to the space radiation environment.
• Power Activation and Switching Module:
  This includes 40 V, 3 A power switches and custom integrated circuits providing voltage and current sensing, current limiting, and switching control, in a package that quadruples the density of previous practice.
• Multifunctional Structure:
  A test of a new packaging technology combining load-bearing elements with electronic housings and thermal control is incorporated which will greatly reduce the mass of future spacecraft cabling and chassises.

DS1 recently started a 150 hr ion drive maneuver that will set it on course for a flyby on about 28 July 1999 of asteroid 1992 KD, at a closest approach distance expected to be less than 10 km. If all remains well, a possible mission extension may then take it to two comets.

Other News

Concern about gyro failures on the Hubble Space Telescope HST has caused NASA to split a planned summer 2000 revisit into two parts, and schedule the first next October or late September. HST has 6 gyros, and can operate with only 3; however, with the failure of a third gyro in January, loss of just one more would put the observatory into a "safe" mode, precluding all science observations until the next servicing mission.

According to NASA administrator Dan Goldin, in testimony about the NASA budget given before the House Subcommittee on Space and Aeronautics, science spacecraft flights are due to increase from an annual rate of two in the early 1990s to seven at the present time to an average of fourteen per year during FY 2000 to FY 2004.

Bill Wheaton
Waw@ipac.caltech.edu