Q: What's the purpose of sending these rovers to Mars?

A: Primarily, the Mars Exploration Rover missions are expeditions to learn more about Martian geology. Scientists are particularly interested in focussing on the period of time when Mars had water. If conditions were suitable for long enough, scientists suspect life may have taken root. So far, the only proof of life in the universe is on Earth.

Q: Why are there two rovers? Are they identical?

A: First off, to better the chances of a successful mission. Two-thirds of all Mars probes fail. Scientists not-so-jokingly call it the "Death Planet." And yes, the rovers are identical, which saved time and expense during manufacturing, testing and mission design.

Q: Do the Mars Exploration Rovers have names?

A: Yes. NASA conducted an essay contest earlier this year to name them. The winner was Sofi Collis, 9, from Scottsdale, Ariz. She announced the winning names — Spirit and Opportunity — with NASA Administrator Sean O'Keefe on June 8, 2003.

Q: How long will the MER mission last?

A: Each of the two rovers is designed to operate for at least 90 Martian days, or "sols." But if the solar panels get particularly dusty, science operations may end early. The Martian day is 24 Earth hours and 37 Earth minutes long.

Q: Why can't a brush or wiper be used to clean off the panels and keep the mission going?

A: The solar arrays are fairly large and, subsequently, the brushes or wipers would also have to be large. A brush or wiper system would require too much mass and probably wouldn't do a very good job of getting rid of Martian dust. The particles are only about 1-2 micrometers in size.

Q: What sort of solar panels are being used for the rovers?

A: The rovers use gallium arsenide solar cells. At the beginning of the mission, the solar arrays will generate about 140 watts of power for up to 4 hours per day, but by the end, dust on the panels, weaker sunlight, seasonal change, and battery wear will reduce that amount to about 50 watts, not enough power for the rover to drive or keep warm at night.

Q: How will engineers know when the rovers have landed?

A: During key events — entry into Mars orbit, the descent through the atmosphere and landing — the rovers will transmit information directly to Earth using a series of predetermined tones to let engineers know when particular milestones have been reached. Engineering data also will be relayed through the Mars Global Surveyor, which currently is in orbit around Mars, during part of the time.

Q: Will the rovers' science instruments directly detect any ice or water?

A: They could, but it's highly unlikely there will be any on the surface of the planet near the equator where the rovers will land.

Q: How good will the rovers pictures be?

A: The resolution of the imager is 30 micrometers per pixel. This means that sand-sized grains (0.1 mm diameter or larger) will be visible.

Q: Where will the rovers land?

A: Spirit is due to touch down the night of Jan. 3. Its landing site is near the center of Gusev Crater, which may have once held a lake. Three weeks later on Jan. 24, Opportunity will reach the Meridiani Planum, a region containing exposed deposits of a mineral that usually forms under watery conditions.