Mapping Mars: Science, Imagination and the Birth of a World
- -
- 100%
- +
Two problems make the mapping of other planets different, one conceptual, one practical. On earth, experience allows you to know what the features you are mapping are: hills, valleys, forests and so on are easily recognised for what they are. While the pictures a spacecraft’s cameras send back may be very good, this level of understanding is just not immediately available. When the first images of Mars were sent back by Mariner 4 they were initially unintelligible to Murray and the rest of the imaging team. Before the researchers even started on a physical map, they needed a conceptual one, a way of categorising what was before their eyes. How to do this – how to see what had never been seen before – was the besetting problem of early planetary exploration.
The practical problem is that unlike an earthly surveyor, you can’t wander around the surface of an alien planet making measurements at leisure. Your only viewpoint is that of a spacecraft flying past the surface at considerable distance and speed. So you not only don’t know what you’re looking at; you’re also none too sure of where you’re looking from. A spacecraft’s position is not a given, like that of a church. It is something that its controllers have to continuously work out. What they know for sure is how fast it is receding from the earth, because that causes frequency changes in its radio signals. To find out where the spacecraft actually is, this information is compared with estimates of where the spacecraft thinks it is – the primary tools here are small on-board cameras called star trackers – and calculations of where it ought to be, derived from measurements and models of all the forces – the gravity of the sun and the planets, the gentle nudges from on-board thrusters – that are shaping its trajectory. If all is going well, the calculations based on all these observations fall into line to produce a consistent picture.* But though this may be accurate enough for navigation, it is not accurate enough for map making. You don’t know precisely where the spacecraft is, or precisely which way its camera is pointing, or, for that matter, precisely where the surface of the planet is. So you can’t say exactly what bit of the planet you’re looking at in any given picture.
Working round these problems involved Davies in a huge amount of laborious cross-checking and number crunching (Airy himself would have loved it, I suspect). First he had to put together a set of clearly distinguishable features that appeared in more than one of the pictures – the centres of craters, for the most part. The precise locations of these features within the individual frames in the data sent back by the spacecraft then had to be put into a set of mathematical equations along with the best available figures for the spacecraft’s position when each picture was taken, and the direction in which the camera was pointing at the time. Then he had to add in factors describing the distortions the cameras were known to inflict on the pictures they took. Once all this was done, the whole calculation had to be fed into a computer on punch cards; the computer then ground through possible solutions until it came to one that made the values of all the variables in the equations consistent. Those values defined a specific way of arranging the set of surface features in three dimensions – imagine it as a framework of dots linked by straight lines – which came as close as possible to satisfying all the data. Effectively, the final answer said ‘if the reference points you’ve specified are arranged in just this way with respect to one another, and if the spacecraft was at these particular points at these particular times, then that would explain why the reference points appear in the positions that they do in these pictures’. That optimal arrangement of reference points was the control net.
Once Davies and his colleagues provided the control net, it could be used to position all the rest of the data. It became possible to say quite accurately where things were with respect to the planet’s poles and its prime meridian. Indeed, one of the primary functions of the control net was to define the planet’s latitude and longitude system – which is why Davies, as both maker of the control net and a member of the International Astronomical Union committee responsible for giving names to features on other planets, was able to put Mars’s Greenwich in a little round crater within the larger crater that was being named after Airy.
Since his first work on Mars, Davies has done his bit in the mapping of more or less every solid body any American spacecraft has visited. By the 1970s he had completely forsaken the black world of spy satellites for the scientific delights of other planets and the personal pleasure of exploring this one: once unencumbered by security clearances and the knowledge they bring, he was free to travel to meetings all around the world, and did so with Louise and alacrity. He’s never made headlines – I doubt he’d want to – but his contributions have been vital prerequisites for much of the work that has.
But there’s still more that Mert would like to do. The mathematics of the control net maximise its self-consistency, not its accuracy. This makes it likely that it contains errors. If you had some independent way of checking it – if you had a point in the control net the location of which you knew independently – you might be able to do something about that.
In principle, such independent measurements are possible. When I interviewed Davies in his office at RAND in December 1999, America had landed three spacecraft on the surface of Mars – the two Viking landers in 1976, and Pathfinder in 1997. The radio signals sent back from those spacecraft revealed their positions very accurately with respect to the fixed-star reference system used by astronomers. If you could find the spacecraft in images of the Martian surface that also contained features tied into the control net, you could check the position of the spacecraft with respect to the net against its absolute position as revealed by the radio signals. That would allow you to calibrate the net with new precision. Do the same for a few spacecraft and you could tie the thing down to within a few hundred metres, as opposed to a few kilometres.
The frustration is that you can’t see the spacecraft. About the size of small cars, from orbital distances – hundreds of kilometres – they are lost in the Martian deserts. The Mars Observer Camera, part of the Mars Global Surveyor spacecraft, has been trying to pick out some sign of the three spacecraft since 1997. It is by far the most acute camera ever sent to Mars. But even MOC can’t pick out the landers. Mankind has made its mark on Mars – but that mark has yet to be seen.
Lacking any proper sightings, checks on the control net using the landers’ locations have had to be indirect. From matching the features that the landers see on the horizon around them with features visible in pictures taken from orbit, it’s possible to make estimates of where the landers are, estimates that are potentially very accurate. Unfortunately, the different experts who try this sort of triangulation get different answers. When Mert and I met in 1999, various inconsistencies had convinced him that one bit of data which he had thought pretty good, and which he had used to calibrate the control net – a two-decade-old estimate of where exactly in the rubble-strewn plains of Chryse Viking 1 had landed – was, in fact, wrong. In a week’s time he was going to go and tell the American Geophysical Union’s fall meeting about the mistake and the fact that it had introduced an error of a fraction of a degree into the control net’s definition of the prime meridian. But if that was an irritation, there was also a new hope. The very next day, a new lander would be setting itself down on the Martian surface, giving MOC another man-made landmark to try to pick out. A steeple to navigate by.
* The other, according to Caltech professor Bruce Murray, is Murray’s Caltech colleague Ed Danielsen.
* In 1999, NASA’s Mars Climate Orbiter demonstrated what happens when things don’t go well. When reporting its thruster firings the spacecraft’s software used metric measurements (Newton seconds). The software on earth thought that these reports were in pound (thrust) seconds, a smaller unit, and thus underestimated the effects of the thruster firings. This meant that JPL’s model of the Climate Orbiter’s position became increasingly inaccurate and, when its controllers tried to insert the spacecraft into orbit round Mars, it was plunged deep into the atmosphere and burned up.
The Polar Lander
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.
Mariner 9
‘I think it’s part of the nature of man to start with romance and build to a reality.’
Ray Bradbury, in Mars and the Mind of Man
Mars Polar Lander was JPL’s thirteenth mission to Mars and its fifth failure. Mariner 3 died with its solar panels pinned to its side by the wrapping in which it had been launched in 1964; Mariner 8 fell into the Atlantic in 1971; Mars Observer exploded as it was trying to go into orbit round Mars in 1993; Mars Climate Orbiter burned up in the atmosphere in 1999; Mars Polar Lander made its mistake just forty metres up a few months later. An optimist might point out that each got closer to the target than the previous failure. A pessimist might point out that the frequency of failure seems to be on the increase.