NASA's one-two punch at reactivating on-site Mars exploration is near at hand. The space agency's delivery plan is twins: two lookalike Mars Exploration Rovers -- for now tagged simply MER-A and MER-B -- are ready to wheel their way into scientific history.

Once the dual robots begin navigating the red planet, a key job is to ferret out evidence for liquid water in the planet's past. That data should help reveal just how suitable or unsuitable conditions have been, or might be today, for life on that baffling world.

Getting hardware "down and dirty" on Mars will not come easy. And it hasn't been an easy road to the red planet for MER project engineers and scientists. At first airbags didn't work. Parachute problems plagued the project too. Then there were pesky, tear your hair out worries about electronic components.

"Things look good from where I sit. The science payloads are done for both vehicles, and are in very good shape. Likewise for the rest of the rover hardware," said Steve Squyres, a planetary geologist at Cornell University in Ithaca, New York. He is principal investigator for the Athena science toolkit that each rover carries.

The MER-A launch is now slated for June 5. That liftoff is followed by MER-B on June 25th. In both cases, a Boeing-built Delta 2 booster will fling each probe into space. The first rover to fly from Florida will use a standard Delta 2. The later rover needs a tad more "oomph" to get to Mars, requiring a Delta 2 "Heavy" where larger diameter and longer strap-on rocket motors are used.

If all goes well during launch and months of cruise toward Mars, MER-A is set for landing on January 4, 2004, with MER-B following on January 25.

As now planned, the first rover is targeted to land at Gusev Crater, 15 degrees south of Mars' equator. The second is to touch down at Meridiani Planum about two degrees south of the equator and halfway around the planet from Gusev.

"We were fighting time from the very beginning," said Peter Theisinger, Mars Exploration Rover project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

One big problem encountered was a troublesome airbag landing system design. "We expected the airbags to perform a certain way. When we tested them early on, they didn't perform that way," Theisinger told SPACE.com .

Just "building to print" the Mars Pathfinder/Sojourner airbag system, the vintage 1997 model, proved unworkable. A MER is a much larger mass vehicle. The airbags needed reinforcing. Another bladder layer inside the airbags was added. Spot reinforcements were used as well, to toughen up the landing system.

Each rover uses four airbags with six lobes each that rapidly inflate around the vehicle to cushion the landing. That happens seconds before a head-on collision with Mars. From there, follow the bouncing airbags.

The first recoil from Mars' surface is likely to propel MER hardware upwards of one hundred feet (30 meters) into the thin martian air. Perhaps a dozen bounces later, the airbags roll to a stop and deflate.

"It's a very robust system. But one of the annoying features of it is that it lands more than once," observes Mark Adler, JPL Mars Exploration Rover Deputy Mission Manager. "The drivers for mission success are very simple. It’s how fast you hit. What you hit. How you bounce. And how long you live after you survive all that," he said.

It turns out -- not so much for geologists, but for spacecraft builders -- bad rocks exist on Mars.

Depending on the grazing angle that the airbags strike the planet, the inflated lobes might not perform well. Certain areas of the airbags are more sensitive to bad rocks - those with triangular edges that could poke into and damage inside bladders.

Worse yet, too hard of an impact might mean the lander impales itself on nastier, sharp-edged rocks damaging the spacecraft in its protective cocoon of airbags. Thus, at one time, the lander itself was considered an internal airbag hazard.

After numbers of fixes, and dozens upon dozens of airbag drop tests, engineering confidence in the inflatable landing system increased.

"We basically tested the heck out of them. We did far more testing on this project than Pathfinder…and we thought we did a lot then," said Rob Manning, JPL's Entry, Decent, and Landing Operations Manager for the MER program. "This project was less than three years from start to launch. Mars Pathfinder had a four-year development before launch. It has been an incredible effort," he said.

MER engineering teams ran into other technological trials and tribulations.

A series of ill-fated helicopter drop tests found that MER's huge parachute wasn't up to snuff. Its strength level was lacking.

The parachute needed is far larger than that used for Mars Pathfinder. MER's big chute had to be beefed up considerably. Even then, the parachute's total area had to be reduced, enabling it to be tightly packed to the density of a cord of wood.

There are many nail-biting phases to MER entry, descent, and landing -- a sequence of crucial events spread out over all of six minutes after months of interplanetary travel. Tough hardware and software issues have been tackled.

Wind shear is a factor. Dampening out unwanted horizontal, as well as vertical speeds is a must as the spacecraft closes in on Mars. A Decent Image Motion Estimation System (DIMES) is onboard, ready for action to assess horizontal velocity of a MER en route to its target zone.

Central to spacecraft survival during this swift swan dive is precision use of small and large rocket motors to help orient MER hardware properly during the fall toward Mars. A chain of small explosive devices must also fire in rapid order.

During the last leg of descent, the lander is positioned at the end of a long bridle. A radar altimeter unit determines the distance to the martian surface. Those radar measurements settle on the timing sequence for airbag inflation and the exact moment to cut the bridle for the lander's fall onto Mars.

Meanwhile, as all this takes place, the individual MER landers will broadcast a series of radio tones. Each of the ten-second beeps will inform ground controllers that a specific milestone in the entry, descent, and landing (EDL) has been carried out.

Early last year, gluing all those EDL functions together was a big development threat to the MER project. "I believe that's past," said JPL's Manning. "We're testing and debugging the EDL software until we drop. That's another kind of drop test," he said.

Now just weeks away from the liftoff of the first Mars Exploration Rover, Squyres of Cornell said he's feeling a mixture of happiness, pride, and relief. "It looks like we're really going to fly these things," he added.

"Spacecraft missions to other planets are incredibly tough things to make happen. You can talk and plan

all you want. But at the end of the day the only thing that really matters is good flight hardware, on time and on the launch pad. And we're very close now to having done that," Squyres said.

"We still have a long way to go in testing and training for what we'll do on the martian surface. But there's time for that. So I like our chances," Squyres told SPACE.com.

"We still have a huge amount of operations work to do before we land, of course. And the JPL and Boeing guys down in Florida are still busting their tails to get both birds to the pad on time. But for now I'm trying to savor the feeling of a hardware development job that was done well, by a very good team in the face of some very tough odds," Squyres said.

For James Garvin, Mars Lead Scientist at NASA Headquarters, the effort to dispatch the twin rovers to the red planet musters up Apollo-like feelings from the 1960s.

"This MER mission is a fabulous study in how the new NASA of today can get things done almost as if it were the old Apollo days…with a 'can do' attitude amidst the many levels of scrutiny now required," Garvin noted.

"Landing on another planet is never simple," Garvin added, "and Mars has challenged everyone for 30-plus years. But to be going to the martian surface with the ultimate in robotic geologist tools is 'way cool' as they say."

Garvin said he views the intrepid rovers wheeling about Mars as the true "rock stars" of this era. "And when they succeed in their explorations of the local materials and unravel the history of water at two places on Mars, we will all look back at this time and wonder how cleverly Mars fooled us for all these years."

Like everyone on the MER effort, JPL's Mark Adler has also been one busy person over the years.

"With all the ongoing testing, training, verifying requirements, closing action items, preparing for reviews, etc., it didn't even occur to me to pull my head out of the water to take a look around and see where we are," Adler admitted.

His assessment: "It's almost surreal to think that we're going to launch in a few weeks…and then again a few weeks after that. But I take some measure of comfort that in facing that prospect, I have no trepidation. I feel that we're ready."

Adler said there were times he wondered if the MER team would get to where they are today. "But we have," he quickly added.

"There's more development work to do during the transit to Mars to prepare for the surface missions, but with all we've been through, it's hard for me to get concerned over that to-do list given the time to landing. We have some pretty amazing vehicles and a damned fine team. This is going to be incredible," Adler concluded.