Spirit Is One Of Two

Stop us if this sounds familiar. Come Saturday morning, if all goes according to plan, a spacecraft from Earth will speed through the Martian atmosphere in hopes of deploying its rockets, parachutes and airbags all in sequence and touching down on the Red Planet. Shortly after landing the rover vehicle will supposedly deploy and start talking to another satellite from Earth orbiting overhead.

It's the same script European Space Agency controllers hoped would unfold without incident in the Beagle 2 mission. So far, Beagle has failed to bark. European officials now hope that its mothership, Mars Express, will be able to talk with Beagle once its in an orbit low enough to facilitate communications.

No, in this case, we're talking about the first of two American probes to land on Mars this month. Spirit and Opportunity will explore different sides of the Red Planet for signs of life. They'll look in areas where geographic data indicates there was a pretty darned good chance water once existed.

Floating on parachutes, Spirit will come to within eight seconds of landing before the golf-cart-sized probe deploys a set of airbags designed to cushion its impact. "It's not the fall that kills you. It's what you hit at the end," said Pete Theisinger, NASA's Mars project manager. He says one sharp or canted rock could doom Spirit to the afterlife that has swallowed about 20 Mars probes before it.

"Entering into Mars is always very tricky, as everyone knows, and we can fail. But we want to learn from those failures, so next time -- of course, we have another rover coming three weeks later, so we do have our own next time -- we can learn from the experience so we can correct any problems," said Polly Estabrook, telecommunications lead for the entry, descent and landing portions of the joint Mars Exploration Rover mission.

In addition to acknowledging critical commands with a series of beeps, Spirit is supposed to have a little chat with the Mars Global Surveyor which has been plotting the Martian surface since 1997. Spirit is supposed to update Surveyor on the function of its critical systems about a minute before landing. The transmission will stop as the signal is blocked by those airbags, which will allow Spirit to bounce around a bit after landing on the surface. After that, they hope a second orbiter, Mars Odyssey, will pick up signs of life from Spirit when it passes overhead a couple of hours later.

If that doesn't work, the pins-and-needles crew at the JPL lab in Pasadena (CA) will have to wait until Odyssey makes another orbit.

FMI: www.marsrovers.jpl.nasa.gov/home/index.html