Sunday, September 2, 2007

Mars Polar Lander/Deep Space 2

Credit: NASA

The third and final launch of the 1998-99 launch window was the Mars Polar Lander (MPL), which was part of the Mars Surveyor '98 program. MPL also carried the Deep Space 2 surface-penetrator mission to Mars, two microprobes that, like the Russian Mars '96 penetrators, were supposed to bury themselves underneath the Martian surface upon impact and take measurements.

MPL was to touch down on the southern polar layered terrain between 73°S and 76°S, in a region called Planum Australe, less than 1000 km from the south pole and near the edge of the carbon dioxide ice cap in Mars' late southern spring. The terrain appears to be composed of alternating layers of clean and dust-laden ice, and may represent a long-term record of the climate, as well as an important volatile reservoir. The mission had as its primary science objectives to:
  • Record local meteorological conditions near the martian south pole, including temperature, pressure, humidity, wind, surface frost, ground ice evolution, ice fogs, haze, and suspended dust
  • Analyze samples of the polar deposits for volatiles, particularly water and carbon dioxide
  • Dig trenches and image the interior to look for seasonal layers, and analyze soil samples for water, ice, hydrates, and other aqueously deposited minerals
  • Image the regional and immediate landing site surroundings for evidence of climate changes and seasonal cycles, and
  • Obtain multi-spectral images of local regolith to determine soil types and composition.

    These goals were to be accomplished using a number of scientific instruments, including a Mars Volatiles and Climate Surveyor (MVACS) instrument package that was comprised of a robotic arm and attached camera, a mast-mounted surface stereo imager, a meteorology package and a gas analyzer. Also, a Mars Descent Imager (MARDI) was planned to capture regional views from parachute deployment at about 8 km altitude down to the landing. The Russian Space Agency provided a laser ranger (LIDAR) package for the lander, which would be used to measure dust and haze in the Martian atmosphere. A miniature microphone would also be on board to record sounds on Mars. And, as mentioned above, attached to the Lander was the Deep Space 2 microprobes, named Scott and Amundsen, which were to be deployed to fall and penetrate beneath the martian surface when the spacecraft reached Mars.

    MPL and the Deep Space 2 probes were launched on a Delta 7425 rocket on January 3, 1999. After an 11-month hyperbolic transfer cruise, MPL reached Mars on December 3, 1999. A final 30 minute tracking session began at 12:45 UT (7:45 a.m. EST), and was used to determine if a final thruster correction was necessary. Final contact to retrieve data on the status of the propulsion system was made from 19:45 UT to 20:00 UT. At 20:04, 6 minutes before atmospheric entry, an 80 second thruster firing was to turn the craft to its entry orientation. The cruise stage was to be jettisoned at about 20:05 UT, and about 18 seconds later the microprobes were to be dropped from the cruise stage into the Martian atmosphere (also targeted at the southern polar layered terrain). The Lander was to make a direct entry into Mars' atmosphere at about 20:10 UT (3:10 p.m. EST). Due to a lack of communications, it is not known whether all these steps following final contact were executed, nor whether any of the descent plan took place as designed. The last telemetry from MPL was sent just prior to atmospheric entry on December 3, 1999. No further signals have been received from the Lander. The cause of this loss of communications is unknown.

    According to the investigation that followed, the most likely cause of the failure of the mission was a software error that mistakenly identified the vibration caused by the deployment of the Lander's legs as being caused by the vehicle touching down on the Martian surface, resulting in the vehicle's descent engines being cut off while it was still 40 meters above the surface, rather than on touchdown as planned. Another possible reason for failure was inadequate preheating of the catalysis beds for the pulsing rocket thrusters: hydrazine fuel decomposes on the beds to make hot gases that throttle out the rocket nozzles; cold catalysis beds caused misfiring and instability in crash review tests.

    Attempts were made in late 1999 and early 2000 to search for the remains of the MPL using images from the Mars Global Surveyor. These attempts were unsuccessful, but re-examination of the images in 2005 led to a tentative identification described in the July 2005 issue of Sky and Telescope magazine. However, higher resolution photos taken later in 2005 revealed that this identification was incorrect, and that MPL remains lost. NASA is hoping that the higher resolution cameras of the Mars Reconnaissance Orbiter, currently in Martian orbit, will finally locate the lander's remains.

    As for the fate of the two Deep Space 2 microprobes, the probes apparently reached Mars without incident; however, communications was never established after landing. The cause of the microprobes' failure is unknown. The crash review board suggests several possible causes for failure.
  • The probe radio equipment had a low chance of surviving the impact
  • The probes may have hit ground too rocky for survival
  • The batteries on the probes, which had been charged before launch almost a year earlier and not checked since then, might not have retained sufficient power.

    The recently launched Phoenix Lander carries some instruments derived from those on MPL.

    The above image shows the landing site for the Deep Space 2 microprobes, as taken by the Viking 2 Orbiter. The image is approximately 140 km across, and is located at 73° South, 210° West.
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