Читать книгу Mapping Mars: Science, Imagination and the Birth of a World - Oliver Morton - Страница 11

There is a passage in the oeuvre of William F. Buckley Jr, in which he remarks that no writer in the history of the world has ever successfully made clear to the layman the principles of celestial navigation. Then Buckley announces that celestial navigation is dead simple, and that he will pause in the development of his narrative to redress forever the failure of the literary class to elucidate this abecederian technology. There and then – and with awesome, intrepid courage – he begins his explication: and before he is through, the oceans are in orbit, their barren shoals are bright with shipwrecked stars.

John McPhee, In Suspect Terrain

It was Merton Davies who put Airy in his prime position. Mert is a kindly man, tall and thin, dignified but rather jolly. Everyone who knows him speaks fondly of him. You might imagine him embodying decent reliability in a Frank Capra film even before learning that he has worked for the same outfit over more than fifty years. But it’s hardly been a small-town life. Mert Davies was one of the pioneers of spy satellites, one of the small cadre of technical experts who changed the facts of geopolitical life by letting cold warriors see the world over which they were at war from a totally new perspective. After that, he became one of only two people to have played an active role in missions to every planet save Pluto.^* He has reshaped – quite literally – the way that earthlings see their neighbours in space. Davies is the man chiefly responsible for the ‘control nets’ of most of the solar system’s planets and moons – complex mathematical corsets that hold the scientific representations of those planetary surfaces together.

The first control net that he created served as the basis for the first maps of Mars made using data from spacecraft, rather than observations from earth. Compiled from fifty-seven pictures sent back when Mariner 6 and Mariner 7 flew past the planet in 1969, that first net tied together 115 points. When I met Davies in his office in Santa Monica thirty years later, his latest Martian control net held 36,397 points from 6320 images. Well into his eighties, Davies was still hard at work augmenting it further.

Davies had been interested in astronomy since boyhood, an interest he had shared with those close to him. In 1942, when he was working for the Douglas Aircraft corporation in El Segundo, California, he started courting a girl named Louise Darling. His interests made their dates a little unusual. Davies had started making a twelve-inch telescope, a demanding project. ‘I had a hard time finishing it,’ he recalls. ‘The amount of grinding it took and the difficulty of polishing that big a surface was a little bit over my head. I would take her with me to polish.’ And so she entered the world of grinding powder and the Foucault test, a simple but wonderfully precise way of gauging a mirror’s shape, which allows an amateur with simple equipment to detect imperfections as small as 50 billionths of a metre. Unorthodox courtship, but it worked. When I met Mert in 1999 he and Louise had been married for more than fifty years.

Just after the war, Davies heard that a think tank within Douglas was working on a paper for the Air Force about the possible uses of an artificial satellite. He applied to join the team more or less on the spot. The think tank soon became independent from Douglas and, as the RAND Corporation, it went on to play a major role in defining America’s national-security technologies and strategies throughout the Cold War. In the early 1950s Davies and his colleagues looked at ways to use television cameras in space in order to send back images of the Soviet Union. Then they developed the idea of using film instead of television – experience with spy cameras on balloons showed that the picture quality could be phenomenal – and returning the exposed frames to earth in little canisters. The idea grew into the Corona project, which after a seemingly endless run of technical glitches and launch failures at the end of the 1950s became a spectacularly successful spy-satellite programme.

While Corona was in its infancy, Davies was seconded to Air Force intelligence at the Pentagon, where he used the new American space technology to try to figure out what Russian space technology might be capable of. When he returned to Santa Monica in 1962, he was ready for a change. Spy satellites were no longer exciting future possibilities for think-tank dreamers, but practical programmes controlled by staff officers and their industry contractors. And there was another problem. ‘A lot of the work at RAND was going into Vietnam – my colleagues were working on reconnaissance issues there – and I wanted no part of that.’

Happily, an alternative offered itself in the form of Bruce Murray, an energetic young professor from the California Institute of Technology in Pasadena, on the other side of Los Angeles. Murray was an earth scientist, not an astronomer. His first glimpse of Mars through a telescope wasn’t a childhood epiphany in the backyard. It was a piece of professional work from the Mount Wilson Observatory. Late as it was, though, that first sight provided emotional confirmation for Murray’s earlier intellectual decision that the other planets were something worth devoting a lifetime’s study to. When Murray looked at Mars through the world-famous sixty-inch telescope, he was not just seeing an evocative light in the sky; he was seeing a world’s worth of new geology, a planet-sized puzzle that he and his Caltech colleagues were determined to crack. Their tool was to be the Jet Propulsion Laboratory, a facility that Caltech managed on behalf of the federal government. JPL, in the foothills of the San Gabriel mountains, had been a centre for military aerospace research since the war. In 1958 the Army ceded it to the newly founded National Aeronautics and Space Administration, as part of which it would become America’s main centre for planetary exploration. By 1961, JPL was planning NASA’s first Mars mission, Mariner 4. The man in charge of building a camera for it was Robert Leighton, a Caltech physics professor. He asked a geologist he knew on the faculty, Bob Sharp, to help him figure out what the camera might be looking at. Sharp asked his eager young colleague Murray to join the team.

Murray and Davies met in 1963; with three young children to support, Murray was keen for some extra income and so found consulting for RAND congenial. He and Davies quite quickly became close friends and Mert started to think he might want to get involved in Murray’s end of the space programme. After all, he had the right credentials: he had been in the space business since the days of the V2 and he had some experience in interpreting images of both the earth and moon as seen from orbit. (At the Pentagon he had analysed Russian pictures of the far side of the moon to see whether they might be fakes.) When Mariner 4’s television camera sent back its image-data – a string of twenty-one grainy pictures covering just 1 per cent of the planet’s surface – Davies was as surprised as almost everybody else to see that it looked not like an earthly desert but like the pock-marked face of the moon, or the aftermath of a terrible war. The space programme was important (Murray and his colleagues would brief the president) but it was also open (they briefed him in front of the cameras). Out among the planets there was no risk of finding yourself in a conflict you wanted no part of, or of having to keep work secret from all but your closest colleagues.

By the time Mariner 6 and Mariner 7 were sent to Mars four years later, in 1969, Davies was a key part of the team dealing with the images they sent back. His particular contribution was to work on the mathematical techniques needed to turn the disparate images into the most reliable possible representation of the planet.

Since the seventeenth century, when Willebrord Snell of Leiden first refined the procedure into something like its modern form, earthbound map makers have turned what can be seen into what can be precisely represented through surveying. Decide on a set of landmarks – Snell and his countrymen liked churches – and then, from each of these landmarks, take the bearings of the other landmarks nearby. From this survey data you can build up a network of fixed points all across the landscape. Plot every point on your map according to measurements made with respect to things in this well-defined network and it will be highly accurate. If, unlike Holland, your country is large, mountainous and only sparsely supplied with steeples, setting up a reliable network in the first place can be hard work – the United States wasn’t properly covered by a single mapping network until the 1930s, when abundant Works Progress Administration labour was available to help with the surveying. But the principle of measuring the angles between lines joining landmarks has been used in basically the same way all over the earth.

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.