In fact, NASA might have to consider cutting one of the two vehicles, and send only one.

The following month the space agency announced that two look-alike rovers would be hurled independently toward Mars in 2003. Each would tote identical copies of the Athena science payload.

Now, over a year into the work, JPL-led Mars Exploration Rover (MER) engineering and software teams find themselves in hyper-drive. They are racing the clock to fabricate, checkout, and prepare the twin robots and associated hardware.

MER-A's launch period is May 30-June 16, 2003 and lands January 4, 2004. MER-B's takeoff is set for June 27-July 14, 2003, reaching Mars' surface on February 8, 2004.

Meanwhile, scientists have begun picking top-notch touchdown spots for each of the wheeled rovers. Two rovers are far better than one - not only for science, but doubling the odds that at least one robot makes it to Mars.

Early in the planning of the MER mission, a quick study suggested the rovers could replicate many of the engineering approaches used in the Pathfinder way of doing business. A go-ahead was given to do just that.

"Lo and behold, as we went through it, things became a lot more complex," said Robert Braun, Deputy for Planetary Exploration in the Systems Engineering Competency at NASA's Langley Research Center in Hampton, Virginia.

Braun also serves on an independent review board of the MER project. He said that piecing the twin rover missions together has become more complicated than Pathfinder for a number of reasons.

In the first place, rover science duties in 2004 are far greater than experiments undertaken in 1997. There has been growth in the mass, power needs, and size of the new rover design. That, in turn, has impacted the MER entry, descent, and landing system. A larger parachute is necessary, extra control rockets are needed, and critical airbags are being designed and sized differently than those utilized for Pathfinder's bounce into the history books.

"Everything we did on Pathfinder is just a little bit harder on MER. That is the real challenge, and something that the agency did not understand in the beginning…and we clearly understand now," Braun said.

"It's not just a build-to-print. It's basically a new system. It just looks like Pathfinder," Braun said. Because of all the changes, there is some concern about keeping schedule, he said.

"In the January time frame, it will be pretty clear to them, and to the rest of us, that they are either going to make it or not," Braun said, in regards to flying two independent MER payloads. At that time the project will be entering a critical phase: Assembly, Test, and Launch Operations (ATLO).

ATLO is important in making the MER vehicles flight ready. First, all spacecraft subsystems undergo individual testing. These units are then integrated to the spacecraft and tested as a system. This process must take place in a very organized, prescribed, and well-documented manner to give as much assurance as possible that the hardware and onboard software will operate properly once the spacecraft have been dispatched toward Mars.

Dual processing of the MER missions through parallel facilities is slated. The procedure is tagged: "Two ready for first launch." The hope is to have both payloads ready for the first launch window in 2003.

Clearly, the pressure is on the MER team to deliver two prime-time rovers. But can they make it?

"Where they are right now, they haven't proven that they can't make it with two rovers," Braun told SPACE.com. "I think NASA is all about pushing the envelope. That's exactly what they are doing…and they ought to be given every chance to succeed," he added.

While MER spacecraft builders are busy building, Mars experts are scouting for safe and scientifically rewarding places to send the rovers.

Word has it that there are two favorites. Melas Chasma is a broad canyon that's part of Valles Marineris and is replete with a floor of busted up features. Then there is Terra Meridani, in which deposits of hematite are distributed.

What's so hot about hematite on cold-as-hell Mars?

Hematite is a ferric iron oxide mineral that forms by a variety of processes that often involve water.

Mars researcher, Phil Christensen at the Arizona State University in Tempe, notes that coarse-grained hematite can form when large amounts of hot water move through iron-bearing rocks. That rushing water dissolves the iron and carries it away in solution. As the water cools it can no longer hold as much iron and the dissolved iron is then precipitated in cracks and veins in the surrounding rock.

Christensen explains that on Earth these hydrothermal systems commonly occur around volcanic regions, such as Yellowstone.

Using the infrared spectrometer on NASA's Mars Global Surveyor (MGS) spacecraft, Christensen has been the driving force in detecting deposits of gray hematite in a region called Sinus Meridiani. That research shores up the possibility that hot springs were bubbling away in Mars' past.