An artist's concept of Mars Reconnaissance Orbiter. Photo: NASA/JPL

The Mars Exploration Rovers represent the next step in an ambitious, on-going program to explore the Red Planet, to map out its structure, composition and meteorology and to determine whether it ever harbored life.

NASA plans to follow the 2003 rover missions with launch of the Mars Reconnaissance Orbiter in August 2005, a 1,900-kilogram (4,200-pound) spacecraft loaded with nine state-of-the-art instruments and cameras that are "truly an order of magnitude beyond that which we've done with Odyssey and MGS," said James Garvin, NASA's chief Mars scientist.

In 2007, NASA will launch a so-called Scout mission. Four such missions were under consideration: A lander built with spare parts left over from the scrapped 2001 mission; a rocket-powered airplane; an orbiter to look for trace gases in the atmosphere that are indicative of biological processes; and a mission to bring a sample of the martian atmosphere back to Earth for detailed analysis.

In August, the space agency selected the "Phoenix" mission that uses the 2001 lander to explore the martian north pole. "The science instruments that Phoenix brings to Mars are chosen for their ability to analyze the ice and soils of the arctic region," said Peter Smith of the University of Arizona Lunar and Planetary Laboratory that will head the mission.

The Phoenix lander on Mars. Credit: NASA/JPL/LPL

In 2009, NASA hopes to launch the Mars Science Laboratory, a large nuclear-powered rover that will "carry with it the most sophisticated, carbon-sniffing gear invented by humans," Garvin said. The MSL would operate for a year or more.

"We believe there is missing carbon and once we understand it, what it's like, where it came from, we'll understand more about the possibilities of life, or at least of there having been life on Mars at at least one extremely high priority site."

Following the 2009 launch, NASA will either press ahead with a sample return mission or additional missions to further refine potential landing sites. Ultimately, Garvin hopes, humans will follow.

"We think we have a hell of a program," Garvin said. "It's going to be exciting. I think we're going to find some remarkable stuff."