Time marches on. And when you're on Mars time, as is the case for the engineers and scientists readying two robots bound for the red planet, it’s a race against the clock.

Dual Mars Exploration Rovers (MER) -- for now simply earmarked as MER-A and MER-B -- are taking shape at the Jet Propulsion Laboratory (JPL) in Pasadena, California. It has been tough and taxing work for the MER team.

While hurdles still exist in prepping the twin robots for unforgiving ship-and-shoot deadlines next year, they are starting to see light at the end of the tunnel - the reddish glow of Mars.

One comeback was to anchor a future Mars landing project in lessons learned from the successful July 1997 cheaper, better, faster Mars Pathfinder. That's the lander that unleashed the cute and cuddly Sojourner mini-rover.

What Sojourner's surface reconnaissance lacked in science snooping was made up in feel good feelings about reconnecting with Mars since the Viking landers touched down over 20 years earlier.

Blueprinting the 2003 MER project after Mars Pathfinder proved to be a tricky assignment.

"Even though we're using a heritage system we had to go back and re-qualify everything," said Z. Nagin Cox, a JPL flight system engineer on the Mars 2003 rover mission.

Long gone is the scheme of simply mimicking and scaling up the older Mars Pathfinder's entry, decent, and landing (EDL) equipment. That proved to be more than articles of faith.

Cox provided an update on the MER work during Mars Week, held earlier this month at the Massachusetts Institute of Technology in Cambridge.

For one, the MER rovers are much larger, hence heavier than Sojourner. And that's just the beginning of a string of differences.

The MER spacecraft are coming in at a different time of day than Mars Pathfinder, JPL's Cox said. That judgment was made so the twin Mars probes are in contact with Earth, radioing back status reports on how things are going as they plunge toward martian terra firma.

"For the Mars Polar Lander in '98 we had a situation where we don't know what occurred," Cox said. The ripple effect for MER was extra money spent on a radio link, as well as a decision to plummet toward the red planet at a different time of day, thus encountering a different atmospheric density than Mars Pathfinder, she added.

That decision snowballed throughout the MER project, Cox noted.

For one, tricky winds could add vertical and horizontal uncertainties during the ballistic plunge toward Mars. Extra solid rocket motors to counter wind gusts, a special look-down camera, added software smarts, along with parachute and airbag design changes - all these and other alterations high-jacked hopes of keeping things in the same engineering box as the Mars Pathfinder EDL way of getting down safely and on target.

An upbeat Cox said that the MER project is looking good. Launching of MER-A and MER-B slated for May and June, respectively, of next year. Both will land in January 2004. "It's amazing to actually see the flight hardware come together," she said.

MER team members believe that troublesome technical snags are behind them. Nevertheless, program officials face major readiness reviews later this year and in January.

They have tested, beefed up, and double-checked the EDL chain of events that need to happen, as well as nail bite over parachute and airbag revisions. Millions of extra dollars were needed to raise the odds of a safe and sound landing of each rover on Mars.

MER program costs have officially edged up to $800 million. Some insiders, however, suggest the effort's true accounting ledgers tally toward the $1 billion mark.

If successful, the MER rovers open up a new chapter in Mars surface studies.

Each rover totes a full toolbox - a set of hardware to search for signs of ancient water and climate. There's the Pancam and Mini-TES, an infrared spectrometer, built to scour Mars' surface to look for rocks and soils worthy of close-up inspection.

Three more instruments -- a Microscopic Imager and two spectrometers -- are designed for placement against select rock and soil targets.

And as everybody knows, working on Mars can be a grind. That's why the rovers each carry the Rock Abrasion Tool, tenderly known as the RAT. It will be utilized to scrape away at dust-covered rocks, permitting instruments to probe what lies beneath.

Doubling up on Mars with robots means two separate landing spots.

Matthew Golombek, JPL Mars Exploration Program landing site scientist said that selection of MER touchdown areas continues on schedule. "We're down to four sites from which the final two will be selected in the March-April time frame," he told SPACE.com.

The four sites are a patch of hematite in Terra Meridiani, Gusev crater, the low wind conditions of Elysium, and Isidis Planitia. Each site has a landing ellipse or a "margin of error" to allow for slight variations that may occur in the entry, descent and landing of each Mars rover.

"The final engineering tests are being completed. The most important for the landing sites is the airbag drop tests. So far these tests are going very well, so there are likely to be no more changes to the engineering constraints," Golombek said.

Another MER landing site workshop is set for early January. There are a series of reviews and follow-on meetings that will then lead to a final landing site review and selection of touchdown zones for the two rovers. Officials at NASA Headquarters will carry out this job.

Meanwhile, the Mars Global Surveyor and Mars Odyssey spacecraft now circling the planet continue to stream data to Earth - useful information to help engineers and scientists pick and model just where the rovers should put down.

MER teams are pushing hard and fast, and time is at a premium, said Steve Squyres, Cornell University leader of the Athena science payload that each rover will carry.

"Things overall on MER right now are looking better than they ever have. Cost, schedule, and mass are tight, but all are holding steady. Most importantly, our airbag and parachute problems are now behind us," Squyres told SPACE.com. "We have had very successful tests in both areas, and we finally feel comfortable that we know how to land this thing."

Squyres said MER work is moving at a very fast pace. "But right now I would characterize the challenges that we are facing as very much business as usual...just the normal sorts of troubleshooting and testing that you always have at this stage of a project. Our extraordinary problems, mostly dealing with EDL, have been solved," he said.

As for project morale, it’s the best that Squyres has seen.

"Maybe it's solving the EDL problems. Maybe it's just finally having real spacecraft put together. But I'd say that all of us are feeling really, really good about this project right now," Squyres said.