The first Mars Exploration Rover, beset for a time by computer troubles but now trekking on a geology adventure to an impact crater, has reached the midway mark of its primary mission.

Scheduled to last 90 martian days, or sols, Spirit completed the 45th workday on the Red Planet Wednesday morning. The six-wheeled robot is currently en route from its landing base to Bonneville Crater where it will survey the site for eager scientists on Earth, hoping to find clues about Mars' history.

This map shows Spirit's track to Bonneville Crater. The rover has reached a small depression nicknamed Laguna Hollow. Credit: NASA/JPL/Cornell Download a larger version of image here

Spirit made its hectic entry, descent and landing on Mars January 3. In the subsequent two weeks, the rover had to unfold its wheels, stand up and then prepare to drive off its landing platform.

Once rolling the surface, the craft headed for a pyramid-shaped rock named Adirondack. It was there that computer memory troubles struck on January 21, knocking out communications between the rover and ground controllers. For a time, many worried the mission could be over. But engineers struggled to regain command of the craft, diagnose the problem and nurse Spirit back to health.

"In a nutshell the problem is as simple as this:" Glenn Reeves, flight software architect, explained at a news conference earlier this month, "as we accumulated additional files in the onboard file system, the system consumed additional memory and eventually we ran out. That was problem No. 1. And problem No. 2 is our reaction to that was fairly severe and in the course of reacting we corrupted that same file system.

"The answer to get (the rover) back to its state where we are healthy and pristine was basically get all of the information out of that file system that we could and then erase the flash memory that actually holds it and to recreate the file system on the vehicle."

It took a couple of weeks, but Spirit made a full recovery and returned to the job it was sent to do -- finding proof that Mars once had a watery past. If the planet did have water, it could have sustained life.

The rover is bound for Bonneville Crater to study the rocks, soil and landscape. Unlike the drama-packed moments during the "six minutes of terror" of landing, the science-led exploration being performed now is distinguished by the discovery of never-before-seen parts of our neighbor planet.

"In some ways getting down on the ground and getting the rover off (the landing base), that's the highest pressure, highest visibility period," said Richard Cook, rover project manager. "And there is sort of almost a let down, to a certain extent, amongst a lot of people that once you get through (those events) you are now done with a lot of exciting stuff. It's interesting, I think, that the team's morale and mood has, if anything, sort of turned around after that because we are amazed about how interesting what we are seeing is and how capable these vehicles are as far being able to drive and to look at new and interesting things. In the case of science, it's fascinating to everybody."

The first 360-degree panorama from Spirit. Credit: NASA/JPL/Cornell Download a large version here.

Aside from the computer troubles that temporarily sidelined Spirit, officials are pleased at how the first half of the mission has progressed.

"I think it's have gone way better than we would have expected," Cook said. "One of our experiences on Pathfinder was that due to varying levels of problems during the mission we would not be able to effectively use like one out of every three days. We would have a problem with the vehicle wouldn't get the instructions we sent for the day or we would have a coordination problem with the (Deep Space Network communications system).

"So during our development period (for Spirit and Opportunity) we had this mantra that if we could get two out of every three days to be successful and productive then we would be doing very well.

"With the exception of the period of time when (Spirit) had the anomaly, we've been doing much better than that. We've really had, knock on wood, a very long chain of very successful days."

With Spirit is fine shape, engineers and managers are looking at plans to extend the mission well beyond the primary goal of 90 martian days.

"We are actually in the process of doing that, to talk with the Mars program both here at JPL and at NASA Headquarters about what they want to do as far as an extended mission is concerned," Cook said. "Clearly, we have these priceless assets on the surface and so we need to make the best use of them as we can. Over time, we try to become more efficient and reduce the size of the teams because we can do more with less.

"My expectation is that we will continue to operate them for quite a while longer after the prime mission."

An artist's concept of the rover exploring the landscape. Credit: NASA/JPL

Exactly how long Spirit, and twin rover Opportunity, will continue to function in the harsh Mars environment isn't clear.

"We are actually in the midst of doing an exercise to come up with that. In the case of Spirit, from a power and a thermal point-of-view, we think the vehicle will be very productive much longer beyond the 90 sols. Exactly what that number is we're still trying to come up with," Cook said.

"The issue that is sort of at the back of everyone's mind is we do go through these deep thermal cycles every day. So there is a concern that after we get past sol 200 or something that we are going to start to have the potential for some sort of problem due to the thermal cycling of the electronics and mechanical hardware. It is very hard to predict when that will occur, so we are continuing to proceed along with the thought that we should be trying to get as much out of these right now as we can."

The temperature range between the cold of night and the warmth of day swings from -95 degrees C to +10 degrees C for external components like motors, Cook said. Internal hardware and electronics experience less severe shifts, going from -10 degrees C to +30 or +40 degrees C.

"It is really the stuff outside that's more of a concern."

Still, the rovers could survive for twice their primary mission length.

"Doubling is certainly not out of the question. And Opportunity is in the same condition (as Spirit)," Cook said.

Controllers expect Spirit to reach Bonneville within two weeks. What it discovers will determine how long it spends there. For its part, Opportunity continues to collect science data inside the tiny bowl-like crater in which it landed 25 sols ago.

"It still remains very exciting to the team and they are looking forward every day to seeing what these things can do," Cook said.

Video coverage for subscribers only:
   VIDEO: RELIVE THRILL OF LANDING NIGHT IN HIGHLIGHTS MOVIE QT
   VIDEO: VIEW 360-DEGREE COLOR PANORAMA WITH NARRATION QT
   VIDEO: FLIGHT DIR. CHRIS LEWICKI GIVES OK FOR 1ST DRIVE QT
   VIDEO: MOMENT WHEN DRIVE ONTO SURFACE IS CONFIRMED QT
   VIDEO: CELEBRATIONS AT FIRST VIEW OF EMPTY LANDER BASE QT
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