Оливер Мортон – Mapping Mars: Science, Imagination and the Birth of a World (страница 7)
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.
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.