Last month, NASA gave the go-ahead for both Mars Exploration Rovers -- Spirit and Opportunity -- to keep on rolling.

Each robot has been handed up to five months of overtime assignments after completing their individual three-month prime mission.

"There are very definitely strong emotional ties to the rovers," said Steve Squyres of Cornell University in Ithaca, New York, and principal investigator for the science payload carried on each of the rovers.

"We poured our hearts and souls into these things for years, so how wouldn't there be? In fact, for me personally, it was actually a little hard just to say goodbye to them at launch. It's going to be very hard to say goodbye to them forever," Squyres told SPACE.com.

As to how Squyres and his colleagues will deal with the eventual loss of the Mars machinery, he responded: "I don't know... ask me that when we get to that point. The sadness from losing the vehicles will be balanced, in part, by a big plus. We'll get our lives back! Flight operations have been exhausting, involving a lot of time away from our families. So, in that regard, life will certainly get easier after they're gone."

Squyres added that the impact of saying so long to the revered rovers is lessened by a central fact.

"The mission has been so successful. If we lost them before they had accomplished muchthat would be very painful. Losing them after they have led such long and productive lives will not be as bad. But still, it's going to be hard," Squyres admitted.

The newly approved extension provides $15 million for operating the rovers through September -- added on top of the $820 million already spent.

Those two robot Mars pioneers are a technological brew of circuit boards, computer, battery, solar panels, telecommunications hardware and science instruments. Each the size of a golf-cart, Spirit and Opportunity must wheel and deal with the planets exotic environment, treacherous terrain, and night/day temperature swings.

Despite being put together with tender loving care, the robots will eventually succumb be it by equipment breakdown or external forces. What could go kaput first?

"That's a question we don't know the answer to," Squyres explained. "One obvious possibility, of course, is that the gradual buildup of dust on the solar arrays will slowly decrease the power output to the point that we can't keep them alive any more. In that case, it'll be a very long, slow decline in capabilities, and we'll see the end coming.

Another possibility, Squyres added, is that a rover would have a sudden, abrupt failure of some critical component. "One day the thing simply wouldn't work any more. I really can't guess which it'll be."

There has already been a near-death experience within the family of Mars rovers. For Spirit at Gusev Crater, it balked down early in its mission due to computer conniptions. The robots troubles quickly de-spirited JPLs Jennifer Trosper, Spirit mission manager.

"I remember talking to my husband on the phone and I asked him first how his day was. He said it was okay. And then he asked me how my day was going," Trosper said. "WellI think Im personally responsible for the loss of a $400 million national asset," she recounted.

Spirits woes, however, were eventually stabilized and the robots software was debugged. Today, Trosper is happy to report that the rover is "more than a 100 percent of what it was on landing day."

Busy at work on Mars, Spirit has been steadfastly clicking off wheel turns as it heads for the base of "Columbia Hills", far off in the distance.

On the other side of red planet, Opportunity at Meridiani Planum is now pulled up to the western rim of "Endurance Crater". The robots camera gear has begun relaying spectacular new views.

Meanwhile, back at JPLs Mars Exploration Rover (MER) mission control, things have definitely gotten calmer and quieter. For one, rover teams have switched from Mars-clock schedules to Earth-clock agendas designed to be less stressful and more sustainable over a longer period.

"Its a strange feeling actually. For a long time we have been in a different modeso its nice to be here," said Matt Wallace, JPLs Opportunity Mission Manager. "Its starting to get a different feelmore of a settling in for the long-haul type of feel," he told SPACE.com.

In many ways, tasking the rovers has become routine and, in some cases, repetitive. But Wallace is quick to add: "Its never a run-of-the-mill job when youre operating on Mars."

For the MER team, Wallace observed, having the rovers operate so well on Mars is a by-product of people investing years of time. "Theres a lot of emotional energya lot of hard work."

JPLs Wallace managed the assembly and integration of Spirit and Opportunity. "We watched these vehicles grow upget launched...land and operate," he said.

Although the NASA robots are mechanized twins at birth, each has a distinct personality, Wallace observed.

Undergoing testing here on Earth prior to Mars sendoff, Wallace felt that Spirit exhibited a tendency to be less well-behaved, the more adventurous of the two vehicles. "Opportunity tended to kind of toe the linea little more staying inside the lines," he related.

Both robots have performed exceptionally well on Mars, from an engineering and scientific perspective with more to come, Wallace said. "I cant really say that one or the other has become a favorite, or stolen the showat least not from our perspective."

Nevertheless, there will come a time when the Mars machines move no more.

Spirits long-duration trek to Columbia Hills will be tough, but appears doable, Wallace said.

Getting Spirit through the September time frame and into the fall at Gusev Crater will be a challenge. The rover faces minimal power draw from the Sun and will experience the coldest temperatures being farther from the equator than Opportunity at Meridiani Planum.

Wallace said its hard to predict how the rovers will eventually be silenced.

First of all, there are scads of moving parts on each robot. Yet, even then, a failed wheel or steering actuator, even with some breakdown of communications equipment -- a rover could still remain perky.

"There are some good, graceful degradation modes on the outside of the rovers," Wallace said. Inside the robots, however, its a different story. There are a number of "single-string" components -- such as power converters, electronic boards -- that could bring a rover to full-stop and mean end-of-life.

"You never really hit the wall. Theres always something that happens and you try to recover for some period of time. You hold on, and hold on, to hope," Wallace said.

When the day comes that Spirit and Opportunity pass away on Mars, Wallace said, you have to step back and say: "Hey, this is a machine. Thats why we send machines to these hazardous environments before we send humans."

"But its always tough at the endno matter how it comes or when it comes," Wallace concluded.