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Last Updated: Sunday, 25 January, 2004, 12:45 GMT
Q&A: What can Mars rovers expect?
The second US rover - Opportunity - landed on at Mars at about 0505 GMT on Sunday. The "robotic geologist" joins its twin Spirit, which touched down on the Red Planet on 4 January.

Dr Steve Squyres, AP
Steve Squyres has much to look forward to
Their mission is to examine their landing sites for past environmental conditions that may have been conducive to life.

Dr Steve Squyres, of Cornell University, is principal investigator for the suite of science tools on each rover.

How do Spirit and Opportunity differ from the first Mars rover?

They are much larger and much more capable. The rover that flew to Mars in 1996-7 on the Mars Pathfinder mission was really just an engineering demonstration vehicle; it didn't have much of a science payload on it or the capability to go very far from its lander. It never got more than about 10 or 20 metres away from its lander.

This time we are flying a much, much larger vehicle. It's more than 180 kg in weight. It's able to travel tens of metres in a day across the Martian surface and it carries an entire suite of scientific instruments with it. It's a real robotic explorer.

Spirit and Opportunity: Nasa space robots' Mars exploration mission

What are Spirit and Opportunity going to be looking for?

The purpose of the mission is to try to find out whether or not Mars was once the kind of place that could have supported life. Mars is a really cold, dry, miserable place today. It is not an environment at its surface that is very well suited to life.

And yet when we look down from orbit with cameras and spectrometers we see tantalising clues that it used to be different. We see dried up river beds, we see dried up lake beds, we see the kinds of minerals that one might find in a hot-spring environment.

We're trying to go to two places where we think it might have been warmer and wetter and more Earth-like in the past and to try to read the geologic clues there and see if these really were places that would have been suitable for life.

And to look if there is still life there?

No. We're not actually looking specifically for life. What we are doing is looking for evidence that life could have been supported. This is not a life search or life detection mission.

Spirit landed at Gusev Crater, possible ancient lake feature
Opportunity touched down on Meridiani Planum, which contains minerals often associated with water
Spirit and Opportunity weigh about 17 times as much as the 1997 Sojourner rover
Both carry cameras and tools to grind and analyse rocks
Why do probes to Mars have such a high failure rate?

It's an incredibly long distance away. Landing in particular is very, very hard because you are going into a very poorly known environment. We don't yet have the technology to do a controlled landing at a pinpoint location on the Martian surface.

So at the moment you just plummet towards the surface into what could be a rock field or some other type of terrain. You have got to design a vehicle that can withstand anything it might hit, any environment it might run in to. It's not surprising that there have been a significant number of failures.

Is there anything you can learn from the Beagle experience?

The two vehicles are very different and so I'm not sure there is much we can learn from their experience. It is hard to tell what has happened to Beagle and it will be until Colin Pillinger and his team hear from their spacecraft.

And there is no opportunity to look for Beagle 2?

No. We're going to two very different places. The landing sites we've picked are quite far from the Isidis Planitia site where Beagle landed.

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