1977

Voyager Spacecraft

Think about the greatest engineering achievements of humankind over the years. Many of them are gigantic, or exceptionally powerful. But the two Voyager spacecraft are different: they are impressive pieces of scientific equipment that have been operating on their own for decades. The most surprising stat is this: The Voyager 1 spacecraft, launched September 5, 1977, is the first human-made object to leave our solar system. And we can still communicate with it.

How does an engineer even begin to think about the design of a complex object that must last for decades without any chance for repair or refueling, and that will need to communicate with Earth even when it is more than ten billion miles away?

One thing to think about is power. Without power for the electronics, computers, radios, and heaters, the spacecraft is dead. Solar power is out because the sun is a tiny pinpoint at that distance. The engineers went nuclear, with a device called a Radioisotope Thermoelectric Generator. Inside the RTG are plutonium-238 oxide spheres that naturally produce lots of heat as they decay (with a half-life of 87 years). Thermocouples arranged in the casing surrounding the spheres convert the heat directly to electricity. Initial output of the RTG was about 480 watts. By 2025, decay of the plutonium will cause the spacecraft to run out of power.

Another consideration is communication. The spacecraft itself has a 23-watt radio paired with a directional dish antenna 12 feet (3.7 meters) in diameter. The signal is incredibly faint once it reaches Earth. But on Earth, the receiving antenna is 100 feet (30.5 meters) in diameter. By using frequencies rarely used by humans, the weak signal makes it through.

With power and a radio the spacecraft is able to communicate. The computer gathers data from the eleven instruments and does the communicating. How can a computer run so long? Step one is to engineer the system using radiation-hardened parts. Step two is to engineer redundant systems. There are actually three separate computers on board, and two copies of each one. Step three is for the software engineers to write the code very carefully. In addition, engineers have reprogrammed the Voyagers many times during the mission.

SEE ALSO Radio Station (1920), ENIAC—The First Digital Computer (1946), Space Shuttle Orbiter (1981).

An artist’s concept of the Voyager spacecraft.