Voyager 1 left Earth on September 5, 1977, preceded by Voyager 2 on August 20th, with a joint mission to study the outer solar system. This September 5, 2017, NASA and the Smithsonian National Air and Space Museum will celebrate the 40th anniversary of the Voyager mission.
Mathematics Lets the Grand Tour Take Flight
Throughout the 1960s, NASA had focused on crewed spaceflight to the Moon, but by the 1970s, focus shifted toward robotic missions to the planets, as well as developing the space shuttle program for low-Earth orbit.
During this transitional period, NASA intern Gary Flandro was working to develop feasible trajectories for a mission to the outer planets. He turned his attention to the idea of gravity assist, whereby a spacecraft passing close by a planet steals some of its orbital speed, accelerating it without additional rocket fuel.
Flandro’s pencil-and-paper plots of the outer planets revealed that Jupiter, Saturn, Uranus and Neptune would align in the late 1970s, enabling a NASA mission to explore these planets in a single mission if it launched by 1977. The craft would slingshot around each planet, visiting all four in only 10 to 12 years. By comparison, sending one spacecraft to only Neptune would take 40 years without passing any other planets along the way.
NASA had one shot to make this plan of action happen — the next opportunity wouldn’t arrive for 176 years. It was now or never. However, due to budgetary constraints at the time, Congress didn’t approve The Grand Tour. Instead, they opted for a cheaper, bare bones mission that would travel to Jupiter and Saturn.
The Scientists’ Secret
This announcement didn’t deter scientists at the Jet Propulsion Laboratory. They built two identical spacecraft, both with the capability of making it to Neptune — even if they were as yet only funded to visit Jupiter and Saturn.
The twin Voyagers had to be tough in order to successfully survive and transmit data from extreme distances.
The scientific instruments onboard Voyager 1 and 2 went on to record astounding amounts of data. The most notable equipment includes the cosmic ray detector, infrared interferometer, ultraviolet spectrometer, and the cameras. Based on the previous Pioneer 10 and 11 missions to Jupiter, scientists knew the spacecraft would encounter intense radiation environments near the outer planets, so they added radiation-hardened electronics.
The key to the Voyagers’ long and successful mission was their power supply: radioisotope thermoelectric power. The power supply contains plutonium-238, which releases heat as it decays to more stable isotopes. Relayed through a series of thermocouples, which generate current by separating the heat source from a cold sink, plutonium’s decay heat provided power to the satellites’ equipment.
Since plutonium-238 has a half-life of 87.7 years, both Voyager spacecraft have about half of their energy supply remaining. While that isn’t enough to keep all equipment operational, it will successfully power the essentials through 2020 before the power supply becomes too weak to send signals back to Earth.
Another key to the satellites’ mission was their self-repairing computer systems. The farther the machines travel, the longer it takes for their signals to reach Earth, so their systems had to be able to overcome minor faults without Earthbound intervention. Each computer consists of multiple modules that compare data received; if one module differs from the rest, it’s considered faulty and replaced with a backup module.
Both probes visited Jupiter and Saturn, as planned (and funded), but their trajectories were plotted to allow for the possibility of more. Seeing the popularity of the Voyager mission, Congress had a change of heart, approving funding for Voyager 2 to continue beyond Saturn to visit Uranus and Neptune as well. While Voyager 1’s trajectory took it out of the ecliptic plane, where the planets orbit, funding was also approved for its continued operation and study of interplanetary space.
Source: Sky & Telescope