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.