From: House Committee on Science, Space, and Technology
Posted: Wednesday, April 12, 2000
NASA's Mars Surveyor Program (MSP) began in 1994 with plans to send spacecraft to Mars every 26 months. Mars Global Surveyor (MGS), a global mapping mission, was launched in 1996 and is currently orbiting Mars. Mars Surveyor '98 consisted of Mars Climate Orbiter (MCO) and Mars Polar Lander (MPL). Lockheed Martin Astronautics (LMA) was the prime contractor for Mars Surveyor '98. The Jet Propulsion Laboratory (JPL), California Institute of Technology, manages the Mars Surveyor Program for NASA's Office of Space Science.
MPL, with the two DS2 probes, was launched on 3 January 1999 for arrival at Mars on 3 December 1999. All three were mounted to a shared cruise stage, which provided Earth communications, power, and propulsion support services for the trip to Mars. All were targeted to a sector at approximately 76°S, 195°W on the edge of the Martian south polar layered terrain. The length of the planned MPL mission after landing was 90 days; the DS2 mission was two days. The probes were to be released from the cruise stage after lander-cruise stage separation, plummeting to the surface to impact about 60 kilometers from the MPL landing site.
MPL approached Mars on 3 December 1999, apparently in good health. A final trajectory-correction maneuver, TCM-5, was executed 6.5 hours before entry. At 12:02 p.m. PST, the spacecraft slewed to entry attitude. At this attitude, the antenna pointed off-Earth, and the signal was lost as expected. Lander touchdown was expected to occur at 12:14 p.m. PST, with a 45-minute data transmission to Earth scheduled to begin 24 minutes later. It was expected that the first data from the DS2 probes would be received on 4 December at 7:25 p.m. PST, about 7 hours after MPL touchdown. However, no communications from MPL or the probes were received.
On 16 December 1999, in accordance with Jet Propulsion Laboratory (JPL) policy, the Laboratory Deputy Director appointed a Special Review Board (the Board) to examine the loss of MPL and DS2. The Board included members from JPL, industry, and academia, as follows:
The Board was tasked to:
1) Determine the possible root causes for the loss of the two missions.
2) Identify actions needed to assure future success in similar Mars landings.
Given the total absence of telemetry data and no response to any of the attempted recovery actions, it was not expected that a probable cause, or causes, of failure could be determined.
ASSESSMENT OF FAILURE MODES
The JPL Special Review Board and its consultants identified a number of failure scenarios, which for convenience were organized by mission phase. The failure scenarios for MPL are presented in Section 6 and those for DS2 are presented in Section 8 of the full Report* submitted as part of this testimony.
The Board organized itself into seven Review Teams, in the areas of Environment and Landing Site, Mechanical Systems, Dynamics and Control, Communications/Command and Data Handling, Propulsion and Thermal, Avionics, and Flight Software/Sequencing. Each Review Team provided an assessment in their respective areas related to the design and test practices relevant to the hypothesized failures. The Review Teams' Findings, Process Assessments, and Lessons Learned are presented in Section 7 of the Report for MPL and Section 9 of the Report for DS2.
The Review Teams conducted their investigations through meetings and teleconferences with Mars Surveyor '98 personnel from LMA and JPL, and DS2 project personnel, throughout January and February 2000. Plenary sessions of the Board were held through the first part of March, during which the Board determined its Findings and Recommendations (see Sections 3 and 4 of the Report) and the system-level Findings, Assessments, and Lessons Learned (see Section 5of the Report).
The following failure modes were assessed as plausible by the Board:
A magnetic sensor is provided in each of the three landing legs to sense touchdown when the lander contacts the surface, initiating the shutdown of the descent engines. Data from MPL engineering development unit deployment tests, and Mars 2001 deployment tests showed that a spurious touchdown indication occurs in the Hall Effect touchdown sensor during landing leg deployment (while the lander is connected to the parachute). The software logic accepts this transient signal as a valid touchdown event if it persists for two consecutive readings of the sensor. The tests showed that most of the transient signals at leg deployment are indeed long enough to be accepted as valid events, therefore, it is almost a certainty that at least one of the three would have generated a spurious touchdown indication that the software accepted as valid.
The software - intended to ignore touchdown indications prior to the enabling of the touchdown sensing logic - was not properly implemented, and the spurious touchdown indication was retained. The touchdown sensing logic is enabled at 40 meters altitude, and the software would have issued a descent engine thrust termination at this time in response to a (spurious) touchdown indication.
At 40 meters altitude, the lander has a velocity of approximately 13 meters per second, which, in the absence of thrust, is accelerated by Mars gravity to a surface impact velocity of approximately 22 meters per second (the nominal touchdown velocity is 2.4 meters per second). At this impact velocity, the lander could not have survived.
Unlike the case with MPL, there was no one failure mode that was identified as being most probable. However, there were four failure modes that were determined to be plausible and they are listed below. Refer to Section 8 for a more detailed treatment of the DS2 failure modes.
Both the MPL and DS2 projects made noteworthy efforts to reduce the cost of implementing flight projects in response to severe and unprecedented technical and fiscal constraints. Although the MPL and DS2 missions were lost, there are valuable lessons to be learned from both, which the full Report attempts to set forth.
One lesson that should not be learned is to reject out of hand all the management and implementation approaches used by these projects to operate within constraints that, in hindsight, were not realistic. A more appropriate point of departure would be to evaluate the approaches, and improve, modify, or augment them in response to implementing the Recommendations contained herein.
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