麻豆传媒

THIS week鈥檚 stunningly successful landing on Mars by Spirit, the first of two NASA rovers due to arrive this month, has given a needed boost to the space agency still reeling from last year鈥檚 Columbia disaster. It may even start to reverse the Red Planet鈥檚 grim record as a killer of space probes.

So far only 1 in 4 landers sent to Mars have made it to the surface successfully (see Table). But Spirit鈥檚 unexpectedly smooth arrival on 4 January, hard on the heels of the apparent loss of Europe鈥檚 Beagle 2 lander, was so dramatic that NASA鈥檚 chief scientist, Ed Weiler, vowed: 鈥淚 will never again refer to Mars as the death planet.鈥�

Past failures demonstrate just how hard it is to land safely on Mars. NASA engineers say it is far more complex than landing on the moon 鈥� mainly because of the planet鈥檚 atmosphere. The heat of atmospheric friction can easily burn up a poorly designed lander, while winds or dust storms during entry can drive it off course.

The flawless descent and landing of Spirit depended on a lengthy sequence of events and equipment that all had to work perfectly, says Rob Manning, who managed the craft鈥檚 entry, descent and landing. In all, 37 pyrotechnic devices fired during the descent to separate the craft from its carrier, deploy the parachute, jettison the heat shield, and inflate the airbags. In just 6 minutes, the hurtling craft had to withstand loads of up to 20 g as it slowed from 20,000 kilometres per hour to a safe standstill on the surface. 鈥淭hat鈥檚 a harsh environment from a human perspective,鈥� says Manning. 鈥淏ut this is what it鈥檚 designed to do.鈥�

The craft is a direct descendant of the successful Pathfinder lander, but several key changes were made to its landing system. Like Pathfinder, Spirit used a combination of parachute and retrorockets to slow its descent, then cushioned its final touchdown with airbags, bouncing across the rock-strewn surface for several minutes before coming to rest. But after years of rigorous drop tests, the airbags were strengthened and the rockets supplemented with smaller sideways-pointing control rockets to compensate for possible strong winds during descent 鈥� the thing the team was most worried about.

After days of downplaying their expectations, the team was clearly ecstatic at just how well everything worked during the landing. At mission control in Pasadena, California, NASA administrator Sean O鈥橩eefe, who spent much of last year fielding tough questions about the loss of the shuttle Columbia, was beaming as he declared: 鈥淭his is a big night for NASA. We鈥檙e back, and we鈥檙e on Mars.鈥�

The mission鈥檚 principal investigator, Steven Squyres of Cornell University, New York, was overcome with emotion as soon as it was clear the landing had succeeded. As others jumped in the air and hugged each other, he fell to his knees in the control room, burying his head in his hands. 鈥淭hese are much more than just machines to us,鈥� he said later. 鈥淭hey are our hopes, passions, dreams and aspirations.鈥�

After the exhilaration of the landing, the real work has just started. First comes a long list of tests and activities needed to deploy the 1.5-metre-high rover and get it to drive off its landing platform, a process that should take about 10 days. Only then can Spirit begin its primary mission, getting up close to rocks on the Martian surface and studying them with a variety of instruments.

The craft landed with its base down, almost perfectly level and with its roll-off ramps closer to the ground than in any of the practice runs on Earth. As a result, the roll-off, planned for sometime during the second week, is expected to be easy. Even on the mission鈥檚 first day, Squyres had enough data to declare the site in Gusev Crater everything he hoped for. The lander鈥檚 images of its surroundings reveal a plain strewn with far fewer and smaller rocks than around the three previous successful Mars landings 鈥� the two Viking craft in 1976 and Pathfinder in 1997. The scene shows plenty of rocks for the geologists to study, but not enough to hinder the rover鈥檚 movements. 鈥淚t looks a lot easier to drive on,鈥� says Squyres. 鈥淚鈥檇 say we nailed it.鈥�

Besides the rocks, the initial images reveal several smooth-surfaced, steep-sided depressions that might be small impact craters. If so, the exposed strata along their flanks could be a geologist鈥檚 dream: a cross section of the area鈥檚 geological history. But though they are tempting destinations for the rover, they could be risky since the surface there looks like fine, powdery material into which the vehicle鈥檚 wheels might sink and get stuck.

By the time the vehicle starts to explore, the first colour 3D picture of the area will be complete, giving the scientists a detailed view of potential sites to study. Spirit is expected to explore for at least 90 days, as is its twin, Opportunity, set for landing on 24 January.

Meanwhile, the Mars Express orbiter 鈥� the European Space Agency鈥檚 first mission to Mars 鈥� is also going according to plan. Several engine burns this week will nudge the spacecraft into its operational orbit, and it will begin mapping the composition of the planet鈥檚 surface and atmosphere in mid-January. One instrument will probe up to 4 kilometres beneath the surface by bouncing radio waves off the planet.

As 麻豆传媒 went to press, Mars Express was also gearing up to listen out for the Beagle 2 probe, which landed on Mars on Christmas day to look for signs of life. NASA鈥檚 Mars Odyssey spacecraft and the UK鈥檚 Jodrell Bank鈥檚 76-metre radio telescope in Cheshire have so far heard nothing from the lander.

Experts thrashing out possible reasons for Beagle 2鈥檚 silence say the probe may have crash-landed and broken up. Or the solar panels that charge its battery may have failed to unfold. A remote possibility is that it fell into a crater, where sunlight can鈥檛 recharge its battery.

Or a software glitch may have confused the lander鈥檚 on-board clock, which tells Beagle 2 when to transmit signals to Mars Odyssey or Mars Express as they pass overhead. Mission manager Mark Sims says he wishes Beagle 2 had been programmed to transmit signals more continuously following touchdown. But what frustrates him most is that the team might never know what really happened. 鈥淭hat鈥檚 the worst thing 鈥� Beagle could be sat quite happily working on the surface of Mars, but for some reason we don鈥檛 understand, it鈥檚 not talking to us,鈥� he says. 鈥淚n the ideal world, if we鈥檇 got another 5 kilos, we could have put on a beacon or a black-box recorder, but Europe didn鈥檛 have the resources.鈥�

Squyres says that within hours of Spirit鈥檚 landing, he called the Beagle team to offer his support. 鈥淚 haven鈥檛 given up on Beagle 2 yet, and neither have they,鈥� he says. 鈥淭here鈥檚 a whole bunch of people on this side of the Atlantic rooting for them.鈥� The team say they won鈥檛 give up on the mission until mid-February, when Mars Express will have had plenty of chances to make contact with the lander.

But for now, attention is focused on Spirit鈥檚 views of a new landscape on Mars. They are the first images we have seen of a place that was probably a lake bed sometime early in Martian history, and they may contain important clues to how much water Mars once had, and for how long.

Additional reporting by Hazel Muir, London