Pale Blue Dot - Carl Sagan [86]
According to the Commission 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 better 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 are 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 have done so well.
We don’t know whether the Mars Observer failure was due to incompetence 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, and 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 and 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