Pale Blue Dot - Carl Sagan [36]
JUST BEFORE VOYAGER 2 was to encounter the Uranus system, the mission design had specified a final maneuver, a brief firing of the on-board propulsion system to position the spacecraft correctly so it could thread its way on a preset path among the hurtling moons. But the course correction proved unnecessary. The spacecraft was already within 200 kilometers of its designed trajectory—after a journey along an arcing path 5 billion kilometers long. This is roughly the equivalent of throwing a pin through the eye of a needle 50 kilometers away, or firing your rifle in Washington and hitting the bull’s-eye in Dallas.
Mother lodes of planetary treasure were radioed back to Earth. But Earth is so far away that by the time the signal from Neptune was gathered in by radio telescopes on our planet, the received power was only 10-16 watts (fifteen zeros between the decimal point and the one). This weak signal bears the same proportion to the power emitted by an ordinary reading lamp as the diameter of an atom bears to the distance from the Earth to the Moon. It’s like hearing an amoeba’s footstep.
The mission was conceived during the late 1960s. It was first funded in 1972. But it was not approved in its final form (including the encounters with Uranus and Neptune) until after the ships had completed their reconnaissance of Jupiter. The two spacecraft were lifted off the Earth by a nonreusable Titan/Centaur booster configuration. Weighing about a ton, a Voyager would fill a small house. Each draws about 400 watts of power—considerably less than an average American home—from a generator that converts radioactive plutonium into electricity. (If it had to rely on solar energy, the available power would diminish quickly as the ship ventured farther and farther from the Sun. Were it not for nuclear power, Voyager would have returned no data at all from the outer Solar System, except perhaps a little from Jupiter.)
The flow of electricity through the innards of the spacecraft would generate enough magnetism to overwhelm the sensitive instrument that measures interplanetary magnetic fields. So the magnetometer is placed at the end of a long boom, far from the offending electrical currents. With other projections, it gives Voyager a slightly porcupine appearance. Cameras, infrared and ultraviolet spectrometers, and an instrument called a photopolarimeter are on a scan platform that swivels on command so these devices can be aimed at a target world. The spacecraft must know where Earth is if the antenna is to be pointed properly and the data received back home. It also needs to know where the Sun is and at least one bright star, so it can orient itself in three dimensions and point properly toward any passing world. If you can’t point the cameras, it does no good to be able to return pictures over billions of miles.
Each spacecraft cost about as much as a single modern strategic bomber. But unlike bombers, Voyager cannot, once launched, be returned to the hangar for repairs. The ship’s computers and electronics are therefore designed redundantly. Much key machinery, including the essential radio receiver, had at least one backup—waiting to be called upon should the hour of need ever arrive. When either Voyager finds itself in trouble, the computers use branched contingency tree logic to work out the appropriate course of action. If that doesn’t work, the ship radios home for help.
As the spacecraft journeys increasingly far from Earth, the round-trip radio travel time also increases, approaching eleven hours by the time Voyager is at the distance of Neptune. Thus, in case of emergency, the spacecraft needs to know how to put itself into a safe standby mode while awaiting instructions from Earth. As it ages, more and more failures are expected, both in its mechanical parts and in its computer system, although there is no sign, even now, of a serious memory deterioration, some robotic Alzheimer’s disease.
This is not to say that Voyager is perfect. Serious mission-threatening, white-knuckle mishaps did occur. Each time, special teams