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Thu, Oct 30, 2008

NASA's Phoenix Mission Faces Survival Challenges

Agency Works To Extend Lander's Lifespan

In a race against time and the elements, engineers with NASA's Phoenix Mars Lander mission hoped to extend the lander's survival by gradually shutting down some of its instruments and heaters, starting this week... but time may be running out for the erstwhile spacecraft.

On Wednesday, Phoenix entered safe mode in response to a low-power fault brought on by deteriorating weather conditions. While engineers anticipated that a fault could occur due to the diminishing power supply, the lander also unexpectedly switched to the "B" side of its redundant electronics and shut down one of its two batteries. During safe mode, the lander stops non-critical activities and awaits further instructions from the mission team.

Within hours of receiving information of the safing event, mission engineers at NASA's Jet Propulsion Laboratory, Pasadena, CA and at Lockheed Martin in Denver, were able to send commands to restart battery charging. It is not likely that any energy was lost... but the switch to safe mode may prove to be an ominous portent.

Weather conditions at the landing site in the north polar region of Mars have deteriorated in recent days, with overnight temperatures falling to --141F (-96C), and daytime temperatures only as high as -50F (-45C), the lowest temperatures experienced so far in the mission. A mild dust storm blowing through the area, along with water-ice clouds, further complicated the situation by reducing the amount of sunlight reaching the lander's solar arrays, thereby reducing the amount of power it could generate. Low temperatures caused the lander's battery heaters to turn on Tuesday for the first time, creating another drain on precious power supplies.

Science activities will remain on hold for the next several days to allow the spacecraft to recharge and conserve power. Attempts to resume normal operations will not take place before the weekend.

"This is a precarious time for Phoenix," said Phoenix Project Manager Barry Goldstein of JPL. "We're in the bonus round of the extended mission, and we're aware that the end could come at any time. The engineering team is doing all it can to keep the spacecraft alive and collecting science, but at this point survivability depends on some factors out of our control, such as the weather and temperatures on Mars."

Originally scheduled to last 90 days, Phoenix has completed a fifth month of exploration in the Martian arctic. As expected, with the Martian northern hemisphere shifting from summer to fall, the lander is generating less power due to shorter days and fewer hours of sunlight reaching its solar panels. At the same time, the spacecraft requires more power to run several survival heaters that allow it to operate even as temperatures decline.

"If we did nothing, it wouldn't be long before the power needed to operate the spacecraft would exceed the amount of power it generates on a daily basis," said Goldstein earlier this week. "By turning off some heaters and instruments, we can extend the life of the lander by several weeks and still conduct some science."

Over the next several weeks, four survival heaters will be shut down, one at a time, in an effort to conserve power. The heaters serve the purpose of keeping the electronics within tested survivable limits. As each heater is disabled, some of the instruments are also expected to cease operations. The energy saved is intended to power the lander's main camera and meteorological instruments until the very end of the mission.

The Phoenix team has parked the robotic arm on a representative patch of Martian soil. No additional soil samples will be gathered. The thermal and electrical-conductivity probe (TECP), located on the wrist of the arm, has been inserted into the soil and will continue to measure soil temperature and conductivity, along with atmospheric humidity near the surface. The probe does not need a heater to operate and should continue to send back data for weeks.

Throughout the mission, the lander's robotic arm successfully dug and scraped Martian soil and delivered it to the onboard laboratories. "We turn off this workhorse with the knowledge that it has far exceeded expectations and conducted every operation asked of it," said Ray Arvidson, the robotic arm's co-investigator, and a professor at Washington University, St. Louis.

When power levels necessitate further action, Phoenix engineers will disable a second heater, which serves the lander's pyrotechnic initiation unit. The unit hasn't been used since landing, and disabling its heater is expected to add four to five days to the mission's lifetime. Following that step, engineers would disable a third heater, which warms Phoenix's main camera -- the Surface Stereo Imager --and the meteorological suite of instruments. Electronics that operate the meteorological instruments should generate enough heat on their own to keep most of those instruments and the camera functioning.

In the final step, Phoenix engineers may turn off a fourth heater -- one of two survival heaters that warm the spacecraft and its batteries. This would leave one remaining survival heater to run out on its own.

"At that point, Phoenix will be at the mercy of Mars," said Chris Lewicki of JPL, lead mission manger.

Engineers are also preparing for solar conjunction, when the sun is directly between Earth and Mars. Between November 28 and December 13, Mars and the sun will be within two degrees of each other as seen from Earth, blocking radio transmission between the spacecraft and Earth. During that time, no commands will be sent to Phoenix, but daily downlinks from Phoenix will continue through NASA's Odyssey and Mars Reconnaissance orbiters. At this time, controllers can't predict whether the fourth heater would be disabled before or after conjunction.

FMI: www.nasa.gov/phoenix

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