Traffic jam on Mars

When NASA launched a pair of rovers to Mars more than three years ago, no one ever thought the darn things would still be working by now, says Cornell astronomer Steve Squyres, the top scientist for the Red Planet rover missions. The proof of that lies in the fix that the Mars program finds itself in today, with two separate missions transmitting on exactly the same frequency.

The data traffic jam isn't insurmountable, Squyres says, but it just goes to show that even a smashing success can carry complications.

The Spirit and Opportunity rovers were launched in the summer of 2003, and landed on opposite sides of the Red Planet in January 2004. Since then, the machines have had their glitches, but they're still producing piles of imagery and other scientific data.

"I never dreamed it was going to last this long," Squyres said.

Squyres admits that some of his colleagues have wondered whether he was really all that surprised. He must have known the rovers' ride was going to last more than the originally planned 90 days, right?

"When you look now, 940 days into what was supposed to be a 90-day mission, it'd be easy to convince yourself that we knew all along that it was going to be successful," he told me Monday. "And in fact, it was nothing like that. We lurched from disaster, to disaster, to disaster."

The evidence of that can be seen in "Mars Dead or Alive," a TV documentary being re-aired on PBS stations tonight. The program traces the preparations for the Mars missions, showing that at several points even before launch, Squyres and his team weren't sure whether the rovers would get off the ground.

In a backhanded way, the current data traffic jam provides still more evidence, Squyres said.

When Spirit was being prepared for flight, engineers built an extra X-band communications transponder as a spare. It turned out that the spare transponder wasn't needed for the Spirit rover - and instead, the equipment was put to use on another probe, the Mars Reconnaissance Orbiter, or MRO, which was launched to Mars last year.

At the time, the arrangement was seen as a clever money-saver. "The things cost a million bucks apiece," Squyres explained.

But there's a slight downside: The frequency settings are hard-wired into the transponders and can't be changed on the fly. "It turns out - and this is causing us all kinds of headaches now - that Spirit and MRO communicate with Earth at exactly the same frequency," Squyres said.

In this respect, the fact that Spirit is still alive and kicking is just a tad inconvenient.

"What it meant was the unthinkable occurrence that both spacecraft were alive at the same time," Squyres said. "Nobody expected that to be an issue, because everybody knew that Spirit would be dead by the time MRO got to Mars. And now it's come around to bite us."

Not that anyone resents Spirit's long life. Squyres said the mission teams for the rover and the orbiter "can work around the problem operationally," by orchestrating the interplanetary conversation so that Spirit's X-band is quiet when MRO is talking, and vice versa. Spirit also can use an alternate UHF antenna for communications with Earth through a Mars Odyssey relay.

Eventually, the Mars Reconnaissance Orbiter could serve as a data relay as well for Spirit as well as Opportunity.

So just how long can Spirit and Opportunity last? Obviously Squyres can't answer that question. But he does say both rovers are on the verge of a new spurt of exploration.

For now, Spirit is staying put in the Columbia Hills, with its solar panels oriented to make the most of the winter's wan sunlight. It recently weathered the southern hemisphere's winter solstice, and is conducting surveys of its surroundings while it waits for spring.

"The power will start very slowly creeping upwards before long, and boy, it's going to be good to see that," Squyres said. In six weeks or so, Spirit should be able to start rolling again.

Opportunity, meanwhile, is about 220 meters - a little more than twice the length of a football field - away from the rim of a half-mile-wide (750-meter-wide) Victoria Crater, which could serve as the stage for the rover's climactic scenes.

"Chances are that Victoria Crater is going to be Opportunity's final resting place," he said. "I don't expect we're ever going to leave the vicinity of Victoria Crater."

There are at least two reasons for that:

One is that the deep crater is likely to contain the best record of Mars' geological history ever found, and that it's likely to take more than a year (an Earth year, that is) to unravel that history. Even before Opportunity reaches Victoria's rim, NASA scientists want to check out a couple of smaller craters that could give them a foretaste of what Victoria itself will reveal.

"If we find that it's just like stuff that we've seen elsewhere, then we can move on," Squyres said. "But if we find that it's different, then we've got to take a little more time."

The other reason is that once you've seen Victoria, there's not much left to see, Squyres said: "You look around, and the next crater onward that's as big or bigger than Victoria is 25 kilometers (16 miles) away. ... This is it. Around us in every direction is a whole bunch of nothing."

But Squyres doesn't intend to end this adventure early, even if all that's left to see is a whole bunch of nothing. He's already had plenty of opportunities to immerse himself in other space missions. But as long as Spirit and Opportunity are willing, Sqyures intends to make them his first scientific priority.

"I've got to stay with these rovers," Squyres said. "Until the day they die, I'm going to be working with these rovers."

Keep posted on future chapters by checking out our "Return to the Red Planet" coverage as well as NASA's Web site for the Mars Exploration Rovers, Cornell's news archive and Squyres' own "Mission Update" page.