The U.S. civil space program closed a chapter of space exploration when NASA officially ended all attempts at contacting the Mars Polar Lander Jan. 17 after a month-long effort in recovering the $165 million spacecraft.

"We have not heard anything back," said Richard Zurek, the mission’s chief scientist at the agency’s Jet Propulsion Laboratory in Pasadena, Calif. "So at this point, it is time to stop attempts to establish contact."

The spacecraft was scheduled for a 90-day mission starting Dec. 3, 1999, but never established contact with Earth once it entered the red planet’s atmosphere. An investigation team will report its findings this spring and make recommendations for improving interplanetary success rates for the series of Mars missions planned through 2005. Engineers are speculating the probe may have crashed into the planet due to a possible malfunction of the landing system or that the probe may have settled on rough ground, causing it to tip over.

"We’re trying to deal with the possibility that we may not learn anything more than what we know now," Zurek added. "And that’s not very satisfactory with regard to how we recover."