CAPE CANAVERAL, Fla. -- Anxious scientists are awaiting the planned liftoff Saturday of NASA's second of two Mars Exploration Rovers -- this one named Opportunity.

"This is an emotionally overwhelming experience," said Steve Squyres, Mars Exploration Rover principal investigator at Cornell University "Being here at the Cape, with all the history. If you want to leave this planetthis is where you do it. This is where it happens."

For those who have lived with readying the rovers over the years, there has been an incredible mix of emotions and experiences, Squyres said. "I can't disentangle them."

Squyres said that, while Spirit and Opportunity are billed as identical twin rovers, "each rover has a personality. You've got to know your hardware."

"Frankly, of the two, I think Spirit has been the problem child," Squyres said. "You know, it's the first one, right?"

Spirit had a few more "funnies," Squyres revealed, as it was going through its build-up, more so than Opportunity. On the other hand, the soon-to-be-launched rover has some temperamental technology onboard too. For one, a gearbox that positions a pointing mirror for one of the instruments is slightly sticky.

"So we'll have to goose it with a little bit more heat to make it move," Squyres said. "There's a whole bunch of little things like that."

Getting two healthy rovers on their way to Mars is top priority. But in the event that Opportunity suffers a mishap or is lost en route, the first rover hurled toward Mars would be re-targeted.

"If we lost Opportunity during the launch, then we would retarget Spirit to Meridiani Planum since it is considered safer for landing than Gusev. Let's hope this doesn't happen, though," said Cathy Weitz, Mars Exploration Rover program scientist at NASA Headquarters in Washington, D.C.

By crossing seven months of mileage between Earth and Mars, there is no assurance that all will work as planned. Nearly half of all Mars probes have failed.

"We've done everything we can to improve the chances for a successful landing, but Mars can always throw us a curveball and spoil the victory," Weitz told SPACE.com.

"Of course, the launch is just one step on a long, arduous trip to Mars. Both spacecraft still have to make it all the way around the track and cross the finish line before we can declare the mission a success. And we will need to land safely on Mars in order to cross that finish line, which makes this a very difficult race to complete," Weitz said.

"There's been a huge amount of work to get to this point," said Squyres, who championed the work on the Athena science payload carried by each rover. The Mars science community has made an immense effort to pick the two scientifically best landing sites that we can safely get to and safely explore with these two rovers, he told SPACE.com.

"Nevertheless, there is some probability -- which I can't quantify, but I'm sure it's not small -- that Mars has faked us out," Squyres said.

Squyres said that Opportunity's landing site, Meridiani Planum, is loaded with hematite. "Almost all of the mechanisms that we know for forming that mineral on Earth involve the action of liquid water. The key word there is almost, as there are mechanisms for forming hematite that don't involve liquid water at all. We could get there and, quite literally, come up dry," he said.

"None of us can guarantee what we're going to find at the two sites," Squyres said.

Having Opportunity head for the hematite is a dream come true for Philip Christensen, planetary geologist at Arizona State University in Tempe.

"I've waited a long time to see if our ideas about the formation of the hematite are correct. It is a very interesting feeling to have argued for that site and now feel both the excitement and the pressure of knowing we are going to see if I was right," Christensen said. "The excitement outweighs the tension because it really is the ultimate in Mars exploration to be able to test an idea you have," he told SPACE.com.

For John Grant, a planetary scientist at the Smithsonian Institution's Center for Earth and Planetary Studies in Washington, D.C., having the rovers wheeling about means doing virtual field work on Mars. Grant was co-leader of a lengthy process to whittle down a mega-list of prospective landing sites to the two top-notch sites.

"It is a planetary geologist's dream come true. And given the nature of the two landing sites, the potential for unraveling the history of water in the geologic evolution of the two sites is high. I can't wait!," Grant said.

Everyone is energized with anticipation, said Jim Garvin, lead scientist for the Mars Exploration Program.

"A lot of our science 'future' depends on what Spirit and Opportunity find, as well as the Beagle-2 onboard Europe's Mars Express. Three landing attempts in a one-month-long period is an unprecedented exploration step for the people of our planet and we will learn much from the experience," Garvin said.

Garvin said that his expectation level is on high. "But I'm frightened by the perils that lie ahead as we return to the 'forests of rocks' on Mars and try to read their story."

Squyres underscores the fact that, historically, the odds of successfully voyaging across the interplanetary void, land upon Mars, then operate on the planet are not favorable. "Mars is a spacecraft graveyard," he said.

"If the only reason you are doing this is for the data, the press conferences, and the excitement after you landyou are doing it for the wrong reason," Squyres said. "You've got to enjoy the processthe day to day process of creating something that has never been attempted before."

"If you can't take satisfaction from that process, then you're in the wrong business," Squyres said.

What if early next year, the twin robots smash, crash and fail to wheel themselves across Mars?

"If, heaven forbid that both vehicles should fail and we come away with no data I will still treasure every moment of this experience. It's been the adventure of a lifetime," Squyres said.