The Mars Climate Orbiter was a 338 kilogram robotic space probe launched by NASA on December
11, 1998 to study the Martian climate, atmosphere, and surface changes and to act as the communications
relay in the Mars Surveyor '98 program for Mars Polar Lander. However, on September 23,
1999, communication with the spacecraft was lost as the spacecraft went into orbital insertion,
due to ground-based computer software which produced output in non-SI units of pound-seconds
instead of the metric units of newton-seconds specified in the contract between NASA and
Lockheed. The spacecraft encountered Mars on a trajectory that brought it too close
to the planet, causing it to pass through the upper atmosphere and disintegrate.
Mission background History
After the loss of Mars Observer and the onset of the rising costs associated with the future
International Space Station, NASA began seeking cheaper, smaller probes for scientific interplanetary
missions. In 1994, the Panel on Small Spacecraft Technology was established to set guidelines
for future miniature spacecraft. The panel determined that the new line of miniature
spacecraft should be under 1000 kilograms with highly focused instrumentation. In 1995,
a new Mars Surveyor program began as a set of missions designed with limited objectives,
low costs, and frequent launches. The first mission in the new program was Mars Global
Surveyor, launched in 1996 to map Mars and provide geologic data using instruments intended
for Mars Observer. Following Mars Global Surveyor, Mars Climate Orbiter carried two instruments,
one originally intended for Mars Observer, to study the climate and weather of Mars.
The primary science objectives of the mission included:
determine the distribution of water on Mars monitor the daily weather and atmospheric
conditions record changes on the martian surface due
to wind and other atmospheric effects determine temperature profiles of the atmosphere
monitor the water vapor and dust content of the atmosphere
look for evidence of past climate change. Spacecraft design
The Mars Climate Orbiter bus measured 2.1 meters tall, 1.6 meters wide and 2 meters
deep. The internal structure was largely constructed with graphite composite/aluminum honeycomb
supports, a design found in many commercial airplanes. With the exception of the scientific
instruments, battery and main engine, the spacecraft included dual redundancy on the
most important systems. The spacecraft was 3-axis stabilized and included
eight, hydrazine monopropellant thrusters. Orientation of the spacecraft was determined
by a star tracker, two Sun sensors and two inertial measurement units. Orientation was
controlled by firing the thrusters or using three reaction wheels. To perform the Mars
orbital insertion maneuver, the spacecraft also included a LEROS 1B main engine rocket,
providing 640N of thrust by burning hydrazine fuel with nitrogen tetroxide oxidizer.
The spacecraft included a 1.3-meter high-gain antenna to transceive data with the Deep Space
Network over the x-band. The radio transponder designed for the Cassini–Huygens mission
was used as a cost-saving measure. It also included a two-way UHF radio frequency system
to relay communications with Mars Polar Lander upon an expected landing on December 3, 1999.
The space probe was powered with a 3-panel solar array, providing an average of 500 W
at Mars. Deployed, the solar array measured 5.5 meters in length. Power was stored in
12-cell, 16-amp-hour Nickel hydrogen batteries. The batteries were intended to be recharged
when the solar array received sunlight and power the spacecraft as it passed into the
shadow of Mars. When entering into orbit around Mars, the solar array was to be utilized in
the aerobraking maneuver, to slow the spacecraft until a circular orbit was achieved. The design
was largely adapted from guidelines from the Small Spacecraft Technology Initiative outlined
in the book, Technology for Small Spacecraft. In an effort to simplify previous implementations
of computers on spacecraft, Mars Climate Orbiter featured a single computer using an IBM RAD6000
processor capable of 5MHz, 10MHz and 20MHz operations. Data storage was to be maintained
on 128MB of random-access memory and 18MB of flash memory. The flash memory was intended
to be used for highly important data, including triplicate copies of the flight system software.
The cost of the mission was $327.6 million total for the orbiter and lander, $193.1 million
for spacecraft development, $91.7 million for launching it, and $42.8 million for mission
operations. Scientific instruments
Mission profile Launch and trajectory
The Mars Climate Orbiter probe was launched on December 11, 1998 at 18:45:51 UTC by the
National Aeronautics and Space Administration from Space Launch Complex 17A at the Cape
Canaveral Air Force Station in Florida, aboard a Delta II 7425 launch vehicle. The complete
burn sequence lasted 42 minutes bringing the spacecraft into a Hohmann transfer orbit,
with a final velocity of 5.5 km/s relative to Mars, and sending the probe into a 669
million kilometer trajectory. At launch, Mars Climate Orbiter weighed 638 kilograms including
propellant.
Encounter with Mars
Mars Climate Orbiter began the planned orbital insertion maneuver on September 23, 1999 at
09:00:46 UTC. Mars Climate Orbiter went out of radio contact when the spacecraft passed
behind Mars at 09:04:52 UTC, 49 seconds earlier than expected, and communication was never
reestablished. Due to complications arisen from human error, the spacecraft encountered
Mars at a lower than anticipated altitude and disintegrated due to atmospheric stresses.
Mars Reconnaissance Orbiter has since completed most of the intended objectives for this mission.
Cause of failure On November 10, 1999, the Mars Climate Orbiter
Mishap Investigation Board released a Phase I report, detailing the suspected issues encountered
with the loss of the spacecraft. Previously, on September 8, 1999, Trajectory Correction
Maneuver-4 was computed and then executed on September 15, 1999. It was intended to
place the spacecraft at an optimal position for an orbital insertion maneuver that would
bring the spacecraft around Mars at an altitude of 226 kilometers on September 23, 1999. However,
during the week between TCM-4 and the orbital insertion maneuver, the navigation team indicated
the altitude may be much lower than intended at 150 to 170 kilometers. Twenty-four hours
prior to orbital insertion, calculations placed the orbiter at an altitude of 110 kilometers;
80 kilometers is the minimum altitude that Mars Climate Orbiter was thought to be capable
of surviving during this maneuver. Post-failure calculations showed that the spacecraft was
on a trajectory that would have taken the orbiter within 57 kilometers of the surface,
where the spacecraft likely disintegrated because of atmospheric stresses.
The primary cause of this discrepancy was that one piece of ground software produced
results in a United States customary unit, while a second system that used those results
expected them to be in metric units. Software that calculated the total impulse produced
by thruster firings calculated results in pound-seconds. The trajectory calculation
used these results to correct the predicted position of the spacecraft for the effects
of thruster firings. This software expected its inputs to be in newton-seconds.
The discrepancy between calculated and measured position, resulting in the discrepancy between
desired and actual orbit insertion altitude, had been noticed earlier by at least two navigators,
whose concerns were dismissed. A meeting of trajectory software engineers, trajectory
software operators, propulsion engineers, and managers, was convened to consider the
possibility of executing Trajectory Correction Maneuver-5, which was in the schedule. Attendees
of the meeting recall an agreement to conduct TCM-5, but it was ultimately not done.
See also Gimli Glider, an aircraft involved in an incident
after poor metrication caused it to run out of fuel
Metrication Space exploration
Unmanned space mission Notes
^ a b Planned but unaccounted for event. References
External links Mars Surveyor '98 launch press kit
Mars Climate Orbiter arrival at Mars press kit
Mars Climate Orbiter Mission Profile by NASA's Solar System Exploration
http:nssdc.gsfc.nasa.govspacecraftDisplay.do?id=1998-073A