A U.S. spacecraft named Spirit is to land on Mars this week. It is to be followed in three weeks by an identical spacecraft named Opportunity, that is to land in a different location. Once on Mars they are to deploy rovers to search for water to determine if the cold, barren planet could once have harbored simple life forms. But the mission is fraught with risks.
The odds of arriving at Mars successfully are just one in three, based on the record of about 30 missions since the early 1960s. The recent record is dismal. The United States lost an orbiter and a lander in 1999. In early December, a Japanese spacecraft was off course and low on fuel, so engineers abandoned their plans to orbit Mars. British scientists are still awaiting a signal from the Beagle, a rover that supposedly landed on Christmas. Its mother ship has successfully entered orbit.
"Mars has been a most daunting destination," said Ed Weiler, chief of space science at the U.S. Space agency NASA. "Some, including myself, have called it the death planet."
"Just getting to Mars is hard, but landing is even more so," he said. "We could have done everything perfectly, but all it will take in the last few seconds is a strong gust of wind and the mission could be over."
Heavy winds might tip the lander over, causing its antennas to point in the wrong direction, making it unable to communicate, even if it correctly performs its complicated maneuvers on the way down.
These include a fiery high-speed dive into the atmosphere, deployment of a parachute and braking rockets to slow its descent, and inflation of airbags to cushion its fall. Despite all this, project officials say a sharp rock could mean doom.
Yet it all worked for the Mars Pathfinder landing in 1997. Mr. Weiler says mission planners have done all they could possibly do this time to repeat that success and avoid a failure like that of 1999.
"I do not know what else humans could have done to make these two rovers successful in terms of testing, in terms of communication, in terms of independent reviews, in terms of money," he said. "Nobody in this organization or the contractors can say they were shortchanged. They got all the money they needed to make this right."
The lack of money was part of the cause of the 1999 failures, according to an investigation by an independent review panel. As a result, staffing was weak and scientists and engineers were overworked. The lander crashed on Mars when flawed computer software prematurely shut off braking rockets. Its sister orbiter burned in the atmosphere because the computer program guiding it was written in English measure while ground controllers believed it to be metric.
The director of the NASA branch operating the twin landing missions, Charles Elachi, says the agency has learned from that experience.
"We applied all the lessons learned from the Mars '99 failure and the report which resulted," he said.
Mr. Elachi says that besides providing adequate resources, NASA carefully chose its mission team from within the agency, academia, and private contractors. It has also paid close attention to the progress of activities and the people working on them.
"We conducted an unprecedented number of internal and external peer reviews," Mr. Elachi said. "We encouraged everyone who worked on the project [that] if they had any concerns, to make sure they express them, and I personally met with the team on a regular basis to make sure all their concerns were immediately being addressed."
Moreover, project scientist Joy Crisp says the groups controlling each lander have conducted repeated remote control tests on identical rover models in a big sandbox to assure that they could maneuver them as expected.
"We have learned a tremendous amount by doing these tests, worked on our procedures and how to operate all these computers and command the rover and work as a team," she said.
Another precautionary strategy called for two landers rather than just one as in past attempts, each a backup for the other.
Ed Weiler says the potential scientific payoff from the missions is worth the high risk and the $820 million expense, almost three times the cost of the 1999 project.
"These two spacecraft could be absolutely perfect, but Mars will determine whether we succeed now," he said.