Читать книгу Mapping Mars: Science, Imagination and the Birth of a World - Oliver Morton - Страница 12

I can think of nothing left undone to deserve success.

Robert Falcon Scott, diary entry, November 1, 1911

On the morning of that next day, Friday, 3 December 1999, JPL in Pasadena is awash with visitors, just as it always is when one of its spacecraft is about to do something exciting. The road leading past the local high school and up to the lab is lined with outside-broadcast vans. Inside, the tree-lined plaza at the lab’s centre – the place where, at the celebration to mark Voyager 2’s successful passage past Neptune, Carl Sagan danced with Chuck Berry – is filled with temporary trailers in which the working press will work, when there is work for them to do. It’s not just journalists who are wandering around looking for gossip, coffee and companions unseen since the last such event. There are VIPs from the upper echelons of NASA and beyond, distinguished visitors from other research centres, the families and friends of people involved in the mission. And back down the freeway at the convention centre in downtown Pasadena there are hundreds of paying customers turning up for a parallel popular event held by a group called the Planetary Society, a planetary-science fan club and lobbying organisation created by Bruce Murray, Carl Sagan and a one-time JPL mission planner named Lou Friedman. The Planetfest gives the public a chance to watch the events on Mars played out on vast TV screens, to hear the findings analysed by experts, to meet their favourite science fiction authors, to admire and buy art inspired by planetary exploration, to collect toys and gaudy knick-knacks and to party the weekend away. No other scientific event – not even the sequencing of a particularly juicy microbe or chromosome – gets attention like this. But then no other science stirs the emotions like planetary science.

The absent star of the show is the Mars Polar Lander. A life-sized stand-in sits in a sandbox in the middle of the plaza at JPL, a backdrop for TV reporters from around the world. Like most spacecraft, it looks a little ungainly: three widely spaced round feet, each of them braced by a set of three legs; segmented solar panels to either side, partly folded out flat, partly flush to the spacecraft’s sloping shoulders, tilted to catch the beams of a sun low on the Martian horizon; spherical propellant tanks and rocket nozzles sit in its belly, antennae, masts and a sort of binocular periscope perch on its back. A scoop on the end of a robot arm scratches the pseudo-Martian sand.

The real Polar Lander, cameras and legs and solar panels tucked into an aeroshell that will protect them from the atmosphere, is falling towards Mars at about 22,500 kilometres an hour. The last course corrections were made early in the morning, fine-tuning the trajectory to maximise the chances of hitting the chosen landing site a bit less than 1000 kilometres from the south pole of Mars. They seem to have worked; the trajectory appears as true as if the spacecraft were running on tracks. Anyway, nothing more can be done – as Apollo astronaut Bill Anders remarked when the third stage of his Saturn V put him and his crewmates on course for the moon, ‘Mr Newton is doing the driving now.’ The spacecraft has nothing to do but obey the law of gravity. Oh, and to fire the occasional rocket, discard its heat shield at the appropriate time, deploy a parachute or two, all things that have to happen precisely at the right time and can’t be controlled from earth because it would take the commands fourteen minutes to get to Mars. Standard spacecraft stuff – only nothing on interplanetary spacecraft is standard. You can never be sure you’ve checked out all the systems and you never fly exactly the same model twice. Every mission is a sequence of hundreds of events controlled by thousands of mechanisms and circuits, any one of which could go wrong.

Because of all this – and especially because the lab’s previous Mars mission, Mars Climate Orbiter, ended in ignominious failure just a few months ago – the tension back at JPL is tangible. But it is also unfocused. There is no more to see than there is to do. An oddity of space exploration is that only very rarely do you get to see the process in action. You see the results, which are often spectacular in and of themselves, but there’s never a cut-away camera angle to let you see the spacecraft through which these wonders of the universe are being revealed. And while it’s hardly surprising that we can’t see the means by which – through which – we’re witnessing these wonders, it’s also a great pity. You don’t have to be Mert Davies, intent on refining his control net, to want to see a picture of a spacecraft on the rubble-strewn plains of Mars. You just have to be human and to want to see something human in that great emptiness where nothing human has been seen before. Such a sight would close some sort of cognitive circuit; it would make Mars a distant mirror in which we could see something of ourselves reflected. It would thicken the connections between our planets and draw Mars further into our world.

This need to close the loop explains why the most popular unmanned space mission ever was the 1997 Mars Pathfinder. Anyone with a web browser could watch as its limited little rover, Sojourner, fitfully explored the rock garden it had been landed in. It explains why the artists displaying their wares to the faithful down at Planetfest in Pasadena do not, for the most part, just paint spectacular landscapes when they paint Mars – they paint landscapes with human participation inside them: an astronaut, a rover, even an unmanned craft. One of the most popular pictures of Mars ever painted is Return to utopia by Pat Rawlings, which shows a future astronaut planting a flag – whose? we can’t see – next to the second Viking lander, simultaneously celebrating its far-flung location and pulling it back from nature into the human world.

Here’s what we’re not seeing by around lunchtime on 3 December: about ten minutes before it hits the atmosphere, Mars Polar Lander begins making its final preparations, resetting its guidance systems, prepping one of its cameras. Mars is vast in its sky, only a few thousand kilometres away, half in shadow, half in sunlight, its surface a range of browns and yellows, the red of its earthly appearance revealed from space as an atmospheric illusion. At this range you can see the craters, the streaks of dust blown by the winds, the strange changing textures of the surface, the largest of the ancient, dried-out valleys, perhaps the wispy whiteness of high dry-ice clouds. New features stream around the curve of the planet as the spacecraft catches up with its target, its trajectory taking it south and east at seven kilometres a second towards the harsh brightness of the southern polar cap. Six minutes before atmospheric entry, the spacecraft twists round so that its aeroshell heat shield is pointed forwards. A minute later a set of six explosive bolts is detonated and the lander slips away from the cruise stage that has been providing it with power and communications on the eleven-month journey from earth. From now on all the power comes from the batteries and no communication is possible until the lander’s own antennae are deployed on the ground. Once the cruise stage and the lander are safely separated, the cruise stage goes on to release two microprobes called Scott and Amundsen, spacecraft designed to survive smashing into the planet’s crust at high speed and then measure the moisture of its soil. They are so tiny that you could cup one in your hands like a grapefruit.

As lander, cruise stage and probes drift away from each other, perspectives alter. Mars stops being a vast wall in front of the spacecraft and becomes a strange new land below them; the ice-white limb of the planet barring the sky becomes a curved horizon. The outer reaches of the atmosphere begin to stroke the lander’s protective aeroshell, too thin at first to have much effect, but getting thicker by the second. Soon on-board accelerometers decide the breaking force is getting strong enough to be worth bothering about and tiny thrusters start firing to keep the aeroshell’s blunt nose cone pointed the right way. The atmosphere’s grip tightens further. Within a minute or so, the deceleration is up to 12g – the sort of force you’d feel if a cruising airliner came to a full halt in a couple of seconds. The nose cone is at 1650°C and the air around it is incandescent. The Polar Lander is a minute-long meteor in the Martian sky.

Three minutes after atmospheric entry begins, the worst is over, though the lander is still moving at 1500 kilometres an hour. A gun at the back of the aeroshell fires out a parachute and the thin air rips it open seven kilometres above the surface. Ten seconds later the charred front of the aeroshell is jettisoned and a camera pointing downwards starts to take pictures of the landscape below as it rushes upwards. If they make it back to earth, these descent images will make quite a movie.

While all this is happening I’m picking at a tuna sandwich in the JPL cafeteria. I chat to some of the scientists from other projects who are gathering round the television monitors that show what’s happening in mission control, then wander back across the plaza, past the model in the sandbox, to the press room. There’s no hurry – the probe is silent during the landing sequence and is only due to pipe up twenty-three minutes after touchdown. Even then there will be a fourteen-minute delay as the radio waves creep across the solar system at the speed of light. Plenty of time.

A quarter of a billion kilometres away, Mars Polar Lander’s legs snap out from their stowed position, ready for the ground below.

Four months later a board of enquiry decided that this was the crucial moment. When the legs snapped into position, they apparently did so with a touch more vigour than was necessary, flexing a little against the restraints meant to hold them in position. Little magnetic sensors in the spacecraft’s body seem almost certain to have interpreted this flexing as meaning that the legs had encountered resistance and were bending under the weight of the spacecraft – just as they would at the moment of touchdown. The state of these sensors was being monitored a hundred times a second by the part of the spacecraft’s software that was in charge of turning off the engines straight after landing and, since the legs took more than a hundredth of a second to reach their proper position, the sensors reported that the spacecraft seemed to have touched down on two successive checks. If it had heard this report only once, the software in charge of turning off the engines would have ignored the reading as a transient glitch. Hearing it twice, the software in charge of turning off the engines after touchdown concluded that the spacecraft had indeed touched down. Unfortunately, it was still almost four kilometres up in the air.

A bit more than a minute later, when the spacecraft’s radar said that it was only forty metres above the surface, the misinformed software had its virtual hand put on the virtual switch that controlled the engines. It turned them off straight away, unable to know or care that the spacecraft was still moving at almost fifty kilometres an hour. After falling that last forty metres, Mars Polar Lander hit the surface at something like eighty kilometres an hour, a speed it could never survive.

Back in December, no one knows any of this. About an hour after lunch on Friday, we know that the first transmission from the surface hasn’t happened, but though that’s a little disappointing, no one is really worried. Spacecraft are programmed to be flighty things and at the slightest sign of something out of the ordinary they are apt to go into ‘safe modes’, which means shutting down all non-vital systems for a set amount of time. The lander’s ability to go safe had been turned off during the descent sequence – when wilful inactivity would have been fatal – but once it got down to the ground this override would turn itself off and the spacecraft would be free to go into a silent funk if some subsystem or other had exceeded its safety levels during the landing.

Over the next few days the silence gets worse. Scott and Amundsen, the ground-penetrating microprobes, are never heard from at all. To this day no one knows what happened to them. The team running the polar lander itself methodically lists the things that could be stopping the probe from communicating and tries to work its way around them, using various different types of radio command. Is the main antenna facing the wrong way? Then send the lander instructions to scan its beam across the sky. Did it not hear those instructions? Send them over another frequency. Did it go into a different sort of safe mode, or go safe twice? Listen at the later times when it was meant to transmit. Each possibility is a branch on what the engineers call a fault tree, and every branch has to be checked out.

While all this is going on up at JPL, down at the Pasadena convention centre the Planetfest rolls on. The fact that there are no neat new pictures of the surface to be seen puts a damper on it, to be sure – but not too terrible a one. People still come to hear the assembled luminaries talk about the great future of Mars exploration. They hear from astronauts and scientists and engineers and Star Trek actors and Bill Nye me Science Guy, proselytiser by appointment to PBS. And they hear from the science fiction writers. From Larry Niven, who has just written a fantasy in which all humanity’s dreams about Mars come true at the same time; from Greg Bear, whose Moving Mars imagined the planet’s future as a backwater from which settlers watch the ever more high-tech earth redefine what is human; from Greg Benford, whose The Martian Race, published this very weekend, sets a new standard of technical accuracy for first-mission-to-Mars stories. And from Kim Stanley Robinson, whose books Red Mars, Green Mars, and Blue Mars provide the fullest picture yet attempted of life on that planet. Unlike every previous generation of science fiction writers, these men have had data from Mars orbit and the Martian surface on which to base their visions, and they are scrupulous in their use. In their hands, the physical facts of planetary science and the romance of travel to other worlds are brought as close as they yet can be.

Meanwhile, up at JPL, what seemed so close is slipping away. After each new attempt to make contact an ever more despondent flight team comes out to face an ever smaller press corps and tell us that nothing was heard. They were so excited on Friday morning – by the early hours of Sunday, some are almost in tears. On Monday morning most have had a chance to rest, but though the faces are fresher and the eyes clearer, a certain resignation has settled in. By Monday night, all the one-fault branches on the fault tree have been evaluated; it’s clear that at least two separate systems must have failed. The team will keep climbing ever more unlikely limbs of the fault tree for a week or so yet, but for the rest of us that’s it. The lander is lost. The last tents in the media caravan are folded up just after midnight; we don’t even have the ingenuity, or stamina, to find a bar.