pale blue dot -carl sagan-第39节
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They are far cheaper； they don't talk back； you can send them to much more dangerous locales； arid； with some chance of mission failure always before us； no lives are put at risk。
〃HAVE YOU SEEN ME？〃 the back of the milk carton read。 〃Mars Observer； 6' x 4。5' x 3'； 2500 kg。 Last heard from on 8/21/93； 627；000 km from Mars。〃
〃M。 O。 call home〃 was the plaintive message on a banner hung outside the jet Propulsion Laboratory's Mission Operations Facility in late August 1993。 The failure of the United States' Mars Observer spacecraft just before it was to insert itself into orbit around Mars was a great disappointment。 It was the first post…launch mission failure of an American lunar or planetary spacecraft in 26 years。 Many scientists and engineers had devoted a decade of their professional lives to M。 O。 It was the first U。S。 mission to Mars in 17 years—since Viking's two orbiters and two landers in 1976。 It was also the first real post…Cold War spacecraft： Russian scientists were on several of the investigator teams； and Mars Observer was to act as an essential radio relay link for larders from what was then scheduled to be the Russian Mars '94 mission； as well as for a daring rover and balloon mission slated for Mars '96。
The scientific instruments aboard Mars Observer would have napped the geochemistry of the planet and prepared the way for future missions； guiding landing site decisions。 It might have cast a new light on the massive climate change that seems to have occurred in early Martian history。 It would have photographed some of the surface of Mars with detail better than two meters across。 Of course； we do not know what wonders Mars Observer would have uncovered。 But every time we examine a world with new instruments and in vastly improved detail； a dazzling array of discoveries emerges just as it did when Galileo turned the first telescope toward the heavens and opened the era of modern astronomy。
According to the mission of Inquiry； the cause of the failure was probably a rupture of the fuel tank during pressurization； gases and liquids sputtering out； and the wounded spacecraft spinning wildly out of control。 Perhaps it was avoidable。 Perhaps it was an unlucky accident。 But to keep this matter in perspective； let's consider the full range of missions to the Moon and the planets attempted by the United States and the former Soviet Union：
In the beginning； our track records were poor。 Space vehicles blew up at launch； missed their targets； or failed to function when they got there。 As time went on； we humans got ；）otter at interplanetary flight。 There was a learning curve。 The ；adjacent figures show these curves （based on NASA data with NASA definitions of mission success）。 We learned very well。 Our present ability to fix spacecraft in flight is best illustrated by the Voyager missions described earlier。
We see that it wasn't until about its thirty…fifth launch to the Moon or the planets that the cumulative U。S。 mission success rate got as high as 50 percent。 The Russians took about 50 launches to get there。 Averaging the shaky start and the better recent performance； we find that both the United States and Russia have a cumulative launch success rate of about 80 percent。 But the cumulative mission success rate is still under 70 percent for the U。S。 and under 60 percent for the U。S。S。R。/Russia。 Equivalently； lunar and planetary missions have failed on average 30 or 40 percent of the time。
Missions to other worlds were from the beginning at the cutting edge of technology。 They continue to be so today。 They 。ire designed with redundant subsystems； and operated by dedicated and experienced engineers； but they are not perfect。 The amazing thing is not that we have done so poorly； but that we leave done so well。
We don't know whether the Mars Observer failure was due to inpetence or just statistics。 But we must expect a steady background of mission failures when we explore other worlds。 No human lives are risked when a robot spacecraft is lost。 Even if we were able to improve this success rate significantly； it would be far too costly。 It is much better to take more risks and fly more spacecraft：。
Knowing about irreducible risks； why do we these days fly only one spacecraft per mission？ In 1962 Mariner 1； intended for Venus； fell into the Atlantic； the nearly identical Mariner 2 became the human species' first successful planetary mission。 Mariner 3 failed； arid its twin Mariner 4 became； in 1964； the first spacecraft to take close…up pictures of Mars。 Or consider the 1971 Mariner 8/Mariner 9 dual launch mission to Mars。 Mariner 8 Was to map the planet。 Mariner 9 was to study the enigmatic seasonal and secular changes of surface markings。 The spacecraft were otherwise identical。 Mariner 8 fell into the ocean。 Mariner 9 flew on to Mars arid became the first spacecraft in human history to orbit another planet。 It discovered the volcanos； the laminated terrain in the polar caps； the ancient river valleys； and the aeolian nature of the surface changes。 It disproved the 〃canals。〃 It mapped the planet pole to pole and revealed all the major geological features of Mars known to us today。 It provided the first close…up observations of members of a whole class of small worlds （by targeting the Martian moons； Phobos and Deimos）。 If we had launched only Mariner 8； the endeavor would have been an unmitigated failure。 With a dual launch it became a brilliant and historic success。
There were also two Vikings； two Voyagers； two Vegas； many pairs of Veneras。 Why was only one Mars Observer flown？ The standard answer is cost。 Part of the reason it was so costly； though； is that it was planned to be launched by shuttle； which is an almost absurdly expensive booster for planetary missions—in this case too expensive for two M。 O。 launches。 After many shuttle…connected delays and cost increases； NASA changed its mind and decided to launch Mars Observer on a Titan booster。 This required an additional two…year delay and an adapter to mate the spacecraft to the new launch vehicle。 If NASA had not been so intent on providing business for the increasingly uneconomic shuttle； we could have launched a couple of years earlier and maybe with two spacecraft instead of one。
But whether in single launches or in pairs； the space…faring nations have clearly decided that the time is ripe to return robot explorers to Mars。 Mission designs change； new nations enter the field； old nations find they no longer have the resources。 Even already funded programs cannot always be relied upon。 But current plans do reveal something of the intensity of effort and the depth of dedication。
As I write this book； there are tentative plans by the United States； Russia； France； Germany； Japan； Austria； Finland； Italy； Canada； the European Space Agency； and other entities for a coordinated robotic exploration of Mars。 In the seven years between 1996 and 2003； a flotilla of some twenty…five spacecraft—most of them paratively small and cheap—are to be sent from Earth to Mars。 There will be no quick flybys among them； these are all long…duration orbiter and lander missions。 The United States will re…fly all of the scientific instruments that were lost on Mars Observer。 The Russian spacecraft will contain particularly ambitious experiments involving some twenty nations。 munications satellites will permit experimental stations anywhere on Mars to relay their data back to Earth。 Penetrators screeching down from orbit will punch into the Martian soil； transmitting data from underground。 Instrumented balloons and roving laboratories will wander over the sands of Mars。 Some microrobots will weigh no more than a few pounds。 Landing sites are being planned and coordinated。 Instruments will be cross…calibrated。 Data will be freely exchanged。 There is every reason to think that in the ing years Mars and its mysteries will bee increasingly familiar to the inhabitants of the planet Earth。
IN THE MAND CENTER on Earth； in a special room； you are helmeted and gloved。 You turn your head to the left； and the cameras on the Mars robot rover turn to the left。 You see； in very high definition and in color； what the cameras see。 You take a step forward； and the rover walks forward。 You reach out your arm to pick up something shiny in the soil； and the robot arm does likewise。 The sands of Mars trickle through your fingers。 The only difficulty with this remote reality technology is that all this must occur in tedious slow motion： The round…trip travel time of the up…link mands from Earth to Mars and the down…link data returned from Mars to Earth might take half an hour or more。 But this is something we can learn to do。 We can learn to contain our exploratory impatience if that's the price of exploring Mars。 The rover can be made smart enough to deal with routine contingencies。 Anything more challenging； and it makes a dead stop； puts itself into a safeguard mode； and radios for a very patient human controller to take over。
Conjure up roving； smart robots； each of them a small scientific laboratory； landing in the safe but dull places and wandering to view close…up some of that profusion of Martian Wonders。 Perhaps every day a robot would rove to its own horizon；
