Mars Climate Orbiter

The Mars Climate Orbiter (formerly the Mars Surveyor '98 Orbiter) was one of two NASA spacecraft in the Mars Surveyor '98 program, the other being the Mars Polar Lander (formerly the Mars Surveyor '98 Lander). The two missions were to study the Martian weather, climate, water and carbon dioxide budget, in order to understand the reservoirs, behavior, and atmospheric role of volatiles and to search for evidence of long-term and episodic climate changes.

The Mars Climate Orbiter was intended to enter orbit at an altitude of 140.5–150 km (460,000-500,000 ft.) above Mars. However, a navigation error caused the spacecraft to reach as low as 57 km (190,000 ft.). The spacecraft was destroyed by atmospheric stresses and friction at this low altitude. The navigation error arose because the contractors for craft's thrusters did not use SI units to express their performance^[1].

Science objectives

The Orbiter had as its primary science objectives to:

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.

Specifically it was to observe and study dust storms, weather systems, clouds and dust hazes, ozone, distribution and transport of dust and water, the effects of topography on atmospheric circulation, atmospheric response to solar heating, and surface features, wind streaks, erosion, and color changes. The orbiter was to use two instruments to carry out these investigations. The Mars Climate Orbiter Color Imager (MARCI) was to acquire daily atmospheric weather images and high resolution surface images and the Pressure Modulated Infrared Radiometer (PMIRR) was to allow measurement of the atmospheric temperature, water vapor abundance, and dust concentration. The orbiter was also to serve as a data relay satellite for the Mars Polar Lander and other future NASA and international lander missions to Mars.

Spacecraft and subsystems

The Mars Climate Orbiter was a box-shaped spacecraft about 2.1 m high, 1.6 m wide, and 2 m deep, consisting of stacked propulsion and equipment modules. The total spacecraft launch mass of 629 kg included 291 kg of propellant. An 11-square-metre solar array wing, measuring 5.5 m tip-to-tip, was attached by 2-axis gimbal to one side and a 1.3 m diameter high-gain dish antenna was attached by 2-axis gimbal to a mast at the top of the propulsion module. The MARCI and PMIRR instruments, as well as a UHF antenna and battery enclosure, were mounted to the bottom of the equipment module.

Propulsion was achieved via a 640 N bipropellant (hydrazine/nitrogen tetroxide) main engine, mounted with the propellant tanks in the propulsion module, and hydrazine thrusters. The orbiter was 3-axis stabilized. Attitude control and maneuvering capability was provided by four 7 N thrusters (pitch/yaw) and four 0.3 N thrusters (roll) in combination with reaction wheels. Attitude was determined using an inertial measurement unit, a star tracker, and analog Sun sensors with knowledge of 25 mrad and stability of 1.5 mrad/(1 s) and 3 mrad/(3 s).

Spacecraft power was provided by 3 panels of GaAs/Ge solar cells on the 5.5-metre-long single-wing solar array which each provide 1000 W of power on Earth or 500 W on Mars. Power was stored in nickel metal hydride (NiMH) common pressure vessel batteries. Thermal control was achieved through a combination of louvers, MLI, Kapton, paints, radiators and heater circuits. Communications with Earth were in X band using Cassini Deep Space Transponders and 15 W RF solid-state power amplifiers through the 1.3 m high gain antenna for both uplink and downlink, a medium gain transmitting antenna, and a low-gain receiving antenna. A 10 W UHF system was to be used for 2-way communications with the Mars Polar Lander. A RAD6000 processor was used for on-board command and data handling.

Mission profile

Mars Climate Orbiter was launched on a Delta II 7425 (a Delta II "Lite" launch vehicle with four strap-on solid rocket boosters and a Star 48 (PAM-D) third stage). Launch was at 18:45:51 UT (1:45:51 p.m. EST) on December 11, 1998 from Pad A of Cape Canaveral Air Force Station Space Launch Complex 17 in Florida. After a brief cruise in Earth orbit, the Delta II 3rd stage put the spacecraft into trans-Mars trajectory and about 15 days after launch the largest trajectory correction maneuver (TCM) was executed using the hydrazine thrusters. During cruise to Mars, three additional TCM's using the hydrazine thrusters were performed on March 4, July 25, and September 15, 1999.

The spacecraft reached Mars and executed a 16 minute 23 second orbit insertion main engine burn on September 23, 1999 at 09:01 UT (5:01 a.m. EDT) Earth received time (ERT, signal travel time from Mars was 10 minutes 55 seconds). The spacecraft passed behind Mars at 09:06 UT ERT and was to re-emerge and establish radio contact with Earth at 09:27 UT ERT, 10 minutes after the burn was completed. However, contact was never re-established and no further signal was ever received from the spacecraft. Findings of the failure review board indicate that a navigation error resulted from some spacecraft data being reported in Imperial units instead of metric. This caused the spacecraft to miss its intended 140–150 km altitude above Mars during orbit insertion, instead entering the martian atmosphere at about 57 km. The spacecraft would have been destroyed by atmospheric stresses and friction at this low altitude.

The burn would have slowed the spacecraft and put it into a 14 hour elliptical (~150 × 21 000 km) capture orbit. The orbiter was to begin aerobraking, using the solar panel to provide resistance and continue until a 90 × 405 km orbit was achieved, nominally on 22 November 1999, with periapsis at 89 N. The hydrazine thrusters would then have been used to change the orbit to a 2-hour, 421 km near-circular polar science mapping orbit on 1 December 1999. The orbit was to be nearly Sun-synchronous, crossing the daytime equator at about 4:30 p.m. local time. The first phase of the mission was to support the Mars Polar Lander (which would also become a failure) from its landing on Mars on 3 December 1999 to the end of the lander primary mission on 29 February 2000. The orbiter would pass over the lander site 10 times per martian day for 5 to 6 minutes each time, communicating via the UHF 2-way relay link at 128 kbit/s. Mars science operations and mapping, involving operation of the MARCI and PMIRR, would initiate on 3 March 2000 and continue for one martian year (687 days). At the end of the mapping mission on 15 January 2002, the orbiter was to be placed in a stable orbit and function as a UHF relay for the Mars 2001 mission.

The metric/imperial mix-up

The loss of the craft was the result of several factors, including a design flaw that resulted in asymmetric torque, software errors in the small forces model, and management inattention.

The metric/imperial mix-up that destroyed the craft was caused by a human error in the software development, back on Earth^[1]. The thrusters on the spacecraft, which were intended to control its rate of rotation, were controlled by a computer that underestimated the effect of the thrusters by a factor of 4.45. This is the ratio between a pound force (the standard unit of force in the imperial system) and a newton (the standard unit in the metric system). The software was working in pounds force, while the spacecraft expected figures in newtons; 1 pound force equals approximately 4.45 newtons.

The software had been adapted from use on the earlier Mars Global Surveyor, and was not adequately tested before launch. The navigation data provided by this software was also not cross-checked while in flight. The Mars Climate Orbiter thus drifted off course during its voyage and entered a much lower orbit than planned, and was destroyed by atmospheric friction.

Multiple calculations which indicated that the probe was off course were ignored.

Project cost

Total project cost was $327.6 million for both orbiter and lander (not including Deep Space 2). Out of this, $193.1 million were for spacecraft development, $91.7 million for launch, and $42.8 million for mission operations.^[2]