The Mars Climate Orbiter (MCO) fired its main engine at 09:01
UTC, September 23rd 1999, to begin orbital insertion into Mars
orbit. Telemetry with the craft was lost five minutes later as it
was occulted by Mars (went behind Mars as viewed from Earth), but
communication was expected to be recovered twenty minutes later when
it was supposed to reappear from behind Mars. No signal from Mars
Climate Orbiter would ever be heard on Earth after 09:04:52 UTC,
Thursday, September 23, 1999.
Technically, the MCO was fairly successful. The only unexpected
problem arose because of the craft's asymmetrical solar array
configuration, unlike that of Mars Global Surveyor. This meant that
solar wind exerted forces off the center of mass of the
spacecraft, which caused the craft to gain angular momentum. This
momentum was imparted to the craft's reaction control wheels, which
became saturated with momentum more often than had been predicted
from data obtained by the earlier voyage of Mars Global
Surveyor. Angular momentum desaturation (AMD) events had to take
place, which caused small errors in the craft's trajectory. The actual
trajectory of the craft and the trajectory the operations team thought
it was on began to diverge by a factor of 4.45, the conversion factor
from pounds to Newtons.
Anomalies in navigation and Doppler signatures of angular
momentum desaturation (AMD) events were observed, but they were only
informally reported. As such, nobody realized the serious threat these
anomalies were suggesting. Doppler solutions that consistently
indicated a shallow flight path as MCO approached the planet and data
collected after trajectory correction maneuver four (TCM-4) which
showed a lower than expected periapse weren't addressed. Tracking
data collected during the twenty four hours preceding orbital
insertion also showed a declining periapse, which was by this time
beginning to approach 80 km, the lowest considered survivable by
MCO. A contingency trajectory correction maneuver, TMC-5, had been
roughed out, but never fully planned. The use of TCM-5 was discussed
but never executed because of concerns about its
planning. Nevertheless, TCM-5 may have given MCO the periapse boost it
needed to stay alive.
Mars Climate Orbiter was occulted by Mars at 09:04:59 UTC, forty nine
seconds earlier than predicted. By that time, it was too late to save
the craft from burning up in the atmosphere or skipping off into
space.
The root cause was found to be the use of English units for
thruster performance data generated by a ground based computer
program. This data was used in the construction of trajectory
models for the spacecraft, and the modellers assumed the data was in
metric units as per the specifications.
The following is from the MCO Phase 1 Mishap Report, available at
ftp://ftp.hq.nasa.gov/pub/pao/reports/1999/MCO_report.pdf:
Root cause:
- Failure to use metric units in the coding of a ground
software file, "Small Forces," used in trajectory models
Contributing Causes:
- Undetected mismodeling of spacecraft velocity changes
- Navigation Team unfamiliar with spacecraft
- Trajectory correction maneuver number 5 not performed
- Systems engineering process did not adequately address transition
from development to operations
- Inadequate communications between project elements
- Inadequate operations Navigation Team staffing
- Inadequate training
- Verification and validation process did not adequately address
ground software
MPL Recommendations:
- Verify the consistent use of units throughout the MPL spacecraft
design and operations
- Conduct software audit for specification compliance on all data
transferred between JPL and Lockheed Martin Astronautics
- Compare prime MPL navigation projections with projections by
alternate navigation methods
- Train Navigation Team in spacecraft design and operations
- Prepare for possibility of executing trajectory correction
maneuver number 5
- Establish MPL systems organization to concentrate on trajectory
correction maneuver number 5 and entry, descent and landing
operations
- Take steps to improve communications
- Augment Operations Team staff with experienced people to support
entry, descent and landing
- Train entire MPL Team and encourage use of Incident, Surprise,
Anomaly process
- Develop and execute systems verification matrix for all
requirements
- Conduct independent review on all mission critical events
- Construct a fault tree analysis for remainder of MPL mission
- Assign overall Mission Manager
- Perform thermal analysis of thrusters feedline heaters and
consider use of pre-conditioning pulses
- Reexamine propulsion subsystem operations during entry, descent,
and landing
It was not an unforeseen technical or mechanical failure on the
orbiter that lead to its loss. Instead, it was a disorganized and
informal ground crew that led to the demise of the Mars Climate
Orbiter. The lessons learned from this mishap could not save the Mars
Polar Lander mission, partly because the investigation of the MCO
disaster left the operations team starved for expertise, and partly
because of the proximity of the two missions in time.
Much of this information was found on the National Space Science
Data Catalog (NSSDC) site, http://nssdc.gsfc.nasa.gov/, and in the
Mars Climate Orbiter Mishap Investigation Report, Phase 1,
ftp://ftp.hq.nasa.gov/pub/pao/reports/1999/MCO_report.pdf.
The Mars Climate Orbiter (MCO) spacecraft began as one half of a two part
mission, the Mars Surveyor '98 program. It was originally called Mars
Surveyor 98 Orbiter, or alternatively, Mars '98 Orbiter. The other half
of the Mars Surveyor '98 program was the equally ill-fated Mars Polar
Lander.
Mission Profile
MCO was lifted from Earth on December 11th 1998 at 18:45:51 UTC
aboard a Delta II Lite launch vehicle equipped with four strap-on solid
rocket boosters and a Star 48 (PAM-D) third stage. During the cruise,
four trajectory correction maneuvers and a number of angular momentum desaturation events took place.
At 09:01 UTC, on September 23rd 1999, MCO began it's orbital
insertion burn. This burn was to last 16 minutes and 23 seconds,
during which time the craft would pass behind Mars. Telemetry was lost
with MCO at 09:04:52 UTC as Mars occulted the craft, and MCO was never heard from again.
If the mission had gone as planned, then MCO would have found
itself in a 14 hour elliptical capture orbit of approximately 150 by
21,000 km. Aerobraking by using the solar panel to provide drag would
then have begun, and would have continued until the orbit had changed
to about 90 by 405 km. This orbit would have ideally been obtained on
the 22nd of November, 1999. On the 1st of December, thrusters would
have been used to move the craft into a nearly circular orbit of 421
km.
The Mars Polar Lander (MPL) was expected to land on Mars on December 3rd
1999, and the first phase of the MCO was to serve as a communications
relay between the lander and Earth until the end of the lander mission
on the 29th of February 2000. MCO was to fly over the MPL landing site
ten times a day for 5 or 6 minutes each time. Information between MPL
and MCO would have been transmitted over a 2 way UHF link of 128
kbit/s.
Science and mapping operations were to begin on March 3rd 2000, and
were scheduled to last a full Martian year, which is 687 days. After
that, MCO was to be placed in a stable orbit and was destined to serve
as a UHF relay station for the Mars 2001 mission.
Objectives
The first objective of the Mars Climate Orbiter mission was to act
as a relay station for the Mars Polar Lander mission, which was
expected to arrive on the heels of MCO. After completing it's science
objectives, MCO was to again serve as a relay station, this time for
the Mars 2001 mission.
The science objectives of the MCO were, as the name implies, to
study the Martian climate. Specifically, MCO was to examine daily
weather and atmospheric conditions, find the dust and water content of
the atmosphere, determine temperature profiles, and look for evidence
of past climate change.
Hardware
The Mars Climate Orbiter craft was basically a box, 2.1m high, 1.6m
wide, and 2m deep. The interior consisted of stacked propulsion and
equipment modules. It carried a 1.3 meter high gain dish antenna atop
a mast above the propulsion module. This antenna was mounted on a 2
axis gimbal and was used to communicate with Earth on the X-band via
the Cassini Deep Space Transponders.
Power was provided by a 5.5m long solar array which was attached to
the side of the craft with a 2 axis gimbal. The solar array had a
total surface area of 11 square meters, and consisted of three panels
of GaAs/Ge solar cells, which could provide 1000 W at 1 AU. Power was
stored in nickel hydride (NiH2) common pressure vessel batteries.
The propulsion system consisted of a bipropellent 640 N main
engine, which used hydrazine and nitrogen trioxide as fuel. A total of
eight smaller thrusters using a single propellant, hydrazine, were
used for attitude control and maneuvering. Four of these thrusters were
7 N pitch/yaw thrusters, and the other four were 0.3 N roll
thrusters.
Attitude was controlled by a combination of the mentioned
thrusters and reaction control wheels. The spacecraft used an
inertial measurement unit, and star tracker, and analog Sun
sensors to find it's orientation in space.
Instruments on board the craft were the Mars Color Imager (MARCI),
and the Pressure Modulator Infared Radiometer (PMIRR). MARCI was used
to acquire approach images of Mars, and was supposed to be used to take
pictures of the Martian atmosphere and surface. MARCI combined nadir
pointed pushframe wide angle and medium angle cameras. The wide angle
camera had a field of view of 140 degrees on seven spectral bands, two
ultraviolet and seven visible. The medium angle camera had a 6 degrees
field of view on ten spectral channels. PMIRR was a multispectral limb
and nadir scanning atmospheric sounder. It had nine channels, and was
supposed to provide vertical profiles of atmospheric temperature,
dust, water vapor, and condensate clouds.
The instruments, UHF antenna, and battery enclosure were located at
the bottom of the craft.
MCO had a launch mass of 625 kg, 291 kg of which was propellant.
Mars Climate Orbiter was part of NASA's Mars Surveyor
Program. Development of the spacecraft cost about 192.1 million
dollars, launch costs and mission operation costs were about 91.7
million and 42.8 million dollars respectively.