Оливер Мортон – Mapping Mars: Science, Imagination and the Birth of a World (страница 9)
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
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.
‘I think it’s part of the nature of man to start with romance and build to a reality.’
Ray Bradbury, in
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.
It’s hardly surprising that, with so few missions, everything that has not been a failure has been counted a terrific success. Mars exploration is still too new for there to have been any hey-ho, business-as-usual missions. But among all these successes one stands out: Mariner 9. Mariner 9 was the first American spacecraft to go into orbit round another planet. It was the first interplanetary probe to send back data in a flood, rather than a trickle. It was the first mission to Mars to provide images of the entire surface and record the full diversity of its landscapes. It was the first spacecraft to see a planet change dramatically beneath its eyes, to watch weather on another world. Mariner 9 revealed a Mars that was fascinating in its own right, rather that disappointing in the light of previous earthly expectations. And Mariner 9 allowed a small team of artists and artisans to make the first detailed, reliable maps of another planet.
There were two big differences between Mariner 9 and its earlier siblings (two of which, Mariner 2 and Mariner 5, went to Venus, not Mars). One was that Mariner 9 had a largish rocket system on board, its cluster of spherical fuel tanks hiding the distinctive octagonal magnesium body that all the Mariner family shared. This engine was needed to slow the spacecraft down when it got to Mars, thus allowing it to go into orbit round its target rather than flying past it at breakneck speed, as the previous probes had. The other, less visible, difference was that Mariner 9 would have the opportunity to send back serious amounts of data.
When Mariner 4 flew past Mars in 1965, it seemed extraordinary that the signal it sent back could be heard at all. Mariner 4’s radio transmitter had a power of ten watts; it had to send data back to a target – the earth – much less than an arc minute across (an arc minute is a sixtieth of a degree). Only a small fraction of the spacecraft’s ten-watt beam actually hit the earth, and only one ten-billionth of that fraction hit the actual receiver – a steerable radio telescope sixty-eight metres in diameter built specifically for the Mars missions at a site a couple of hours’ drive into the Mojave Desert from JPL. But the power of electronic engineers to decode such staggeringly faint signals has been one of the least celebrated wonders of the space age.* It’s an ability at least as wonderful as that of actually launching things into space, and compared with rocketry it’s both grown in capability far faster and been a good sight more dependable. That sixty-eight-metre Goldstone dish in the Mojave, along with companions near Madrid and Canberra, now brings data back from the edges of the solar system, a hundred times further away than Mars, and handles data rates as high as 110,000 bits per second. Even in the early days of Mariner 4 the limiting constraint on the rate at which data could be sent back was not the radio link, but the speed at which the tape recorder which stored the data on board the spacecraft could play it back. And that was staggeringly slow: eight bits per second. It took weeks to send back data recorded in minutes.
Mariner 4’s pictures each contained less than a thousandth of the data in a nine-inch aerial photograph. The frames were just 200 pixels wide by 200 pixels deep; the brightness of each pixel was recorded as six bits of data, providing sixty-four gradations of tone between black and white. The total amount of data in every frame (thirty kilobytes) was just a little bit more than the amount of disk-space taken up by an utterly empty document in the version of Word with which I am writing this book. In principle I could download the equivalent of Mariner 4’s entire twenty-two-image data-set from the Internet in a matter of seconds using my utterly unexceptional modem. In 1964, though, it took eight hours to get each picture back to JPL. The process was so slow that the waiting scientists printed out the numerical value for each pixel on a long ribbon of ticker tape, cut the ribbon into 200-number-long strips and then coloured each pixel in with chalk according to its numerical value. Every two and a half minutes another strip could be added to the picture. The first space-age image of Mars, taken by the first entirely digital camera ever built and transmitted over 170 million kilometres of empty space, was put together like an infant school painting-by-numbers project.
By the time Mariner 6 and Mariner 7 flew past Mars in 1969, communications were far faster (though the on board tape recorders, which outweighed the cameras whose data they stored, were still a problem). Each of the 1969 Mariners returned a hundred times more data to earth than Mariner 4 had four years earlier. In 1971 Mariner 9 – with a data rate 2000 times that of Mariner 4 and a year in which to transmit, rather than a week – did 100 times better still. And this meant that the whole scale of the operation was different. The ‘television teams’ – so called because their instrument was basically a TV camera – on Mariners 4, 6 and 7 had been small: Leighton, who masterminded the camera design; a few other Caltech faculty members; some JPL people; and a few select outsiders, such as Mert Davies. But Mariner 9 was going to provide far more data than such a team could digest and the data were to be used not just for analytical science but for the practical business of mapping. Among other things, America was committed to landing robot probes on Mars to look for life in 1976. Those probes – the Vikings – needed landing sites, and choosing landing sites required maps.
NASA would have been happy to make the maps itself. But in the mid-1960s Congress noticed that almost every government agency had its own map makers and decided that the money-hungry, fast-growing space agency would be an exception to this rule. So the mapping of the planets was instead made the duty of the United States Geological Survey. This was not entirely arbitrary; the USGS already had an astrogeology branch, headquartered in Flagstaff, Arizona, which was deeply involved in the study of the moon and was helping to train the Apollo astronauts. The USGS gave primary responsibility for its study of Mars to a team of five geologists, three from Flagstaff, two from the survey’s California centre in Menlo Park, south of San Francisco. The senior member of the USGS team was a man called Hal Masursky: in part because Murray was at the same time working on a mission to Venus and Mercury, Masursky became one of the television team’s two principal investigators. The other PI was a young man called Brad Smith, a highly rated expert on Mars as observed through telescopes who had yet to complete his PhD.