Contractor takes blame for math goof that crashed Mars probe

Contractor takes blame for math goof that crashed Mars probe

By PAUL RECER

WASHINGTON (November 10, 1999 6:02 p.m. EST http://www.nandotimes.com) - For nine months, the Mars Climate Orbiter was speeding through space and speaking to NASA in metrics. But the engineers on the ground were replying in non-metric English.

The mathematical mismatch that was not caught until after the $125 million spacecraft, a key part of NASA's Mars exploration program, was sent crashing too low and too fast into the Martian atmosphere. The craft has not been heard from since.

"We were on the wrong trajectory and our system of checks and balances did not allow us to recognize that," Edward Stone, director of the Jet Propulsion Laboratory, said Wednesday. The NASA center in California was in charge of the Mars mission.

Noel Henners of Lockheed Martin Astronautics, the prime contractor for the Mars craft, said at a news conference that his company's engineers were responsible for ensuring that the metric data used in one computer program were compatible with the English measures used by another program. The simple conversion check was not done, he said.

"It was overlooked," Henners said.

The Mars Climate Orbiter was launched Dec. 11 and spent nine months coasting toward Mars.

Art Stephenson, director of the Marshall Spaceflight Center and head of a NASA investigation team, said that the spacecraft was not symmetrical and that pressure from the sun caused it to slowly twist or roll as it sped along. On-board gyroscopes partially controlled the motion, but eventually rocket firings were needed to stabilize the craft, he said. This happened 12 to 14 times a week over the nine-month voyage.

Engineers on the ground calculated the size of the rocket firing using feet-per-second of thrust, a value based on the English measure of feet and inches. However, the spacecraft computer interpreted the instructions in Newtons-per-second, a metric measure of thrust. The difference is 4.4 feet per second.

"Each time there was a burn (rocket firing) the error built up," said Stephenson.

As the spacecraft approached its rendezvous with Mars and the engineers prepared for a final rocket firing, there were indications that something was seriously wrong with the navigation, but no corrective action was taken, Stephenson said.

When the Mars Climate Orbiter did fire its rockets, the craft went too low into the planet's atmosphere instead of into a safe orbit. Communication signals stopped when the craft passed behind Mars and have not been heard since.

"We entered the Mars atmosphere at a much lower altitude (than planned)," said Ed Weiler, NASA's chief scientist. "It (the spacecraft) either burned up in the Martian atmosphere or sped out (into space). We're not sure which happened."

Stephenson said that the problem was not with the spacecraft, but with the engineers and the systems used to direct it.

"The spacecraft did everything we asked of it," said Stephenson. He said the mathematical mismatch was "a little thing" that could have been easily fixed if it had been detected.

"Sometimes the little things can come back and really make a difference," he said.

Stone said that problems found in the Mars Climate Orbiter loss have led to major changes in control and operation of a sister spacecraft, the Mars Polar Lander, that is scheduled to land on Mars on Dec. 3.

"There were hard lessons to learn from the loss of the Mars Orbiter, but learn them we will," said Stone. "We must learn and learn quickly" because the Mars landing is just over three weeks away, he said.

Following recommendations from Stephenson's investigation board, Stone said the team directing the Mars Polar Lander has been reorganized and strengthened. He said senior engineers have been added to the group and a new system of review and evaluation has been installed. He said that navigation of the Mars craft now will be checked by two independent means to make sure it is on target for the landing.

Loss of the Mars Climate Orbiter is a serious blow to NASA's exploration program. The spacecraft was to orbit Mars and act as a radio relay for signals from the Mars lander. Those signals now will be routed through another spacecraft.