Scientists and astronomy enthusiasts are coming together to celebrate two spacecraft, now billions of miles beyond Earth, as they head for the stars.
Voyager 1 left Earth on September 5, 1977, preceded by Voyager 2 on August 20th, on a quest to study the outer solar system. Today NASA and the Smithsonian National Air and Space Museum are celebrating the 40th anniversary of this history-making mission.
Let's take a look at how these spacecraft achieved their astronomical feats.
Mathematics Lets the Grand Tour Take Flight
Throughout the 1960s, NASA had focused on sending astronauts to the Moon. But by the 1970s, as the Apollo era ended, the agency's focus shifted toward robotic missions to the planets, as well as developing the Space Shuttle program for delivering payloads to Earth orbit.
In 1964, with Apollo 11's landing still a half decade away, Caltech graduate student and Jet Propulsion Laboratory intern Gary Flandro was working to develop feasible trajectories for a mission to the outer planets. He turned his attention to the relatively new idea of gravity assist, whereby a spacecraft passing close by a planet steals some of its orbital speed, accelerating without expending any 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 such that one spacecraft could visit all four in a single mission if it launched by 1977. The craft would slingshot around each planet in succession, completing a "Grand Tour" in only 10 to 12 years. By comparison, sending a dedicated spacecraft to only Neptune would take 40 years without passing any other planets along the way.
Sky & Telescope’s Kelly Beatty recently spoke with Flandro about his landmark discovery — watch the interview here:
NASA's engineers and planetary scientists 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, Congress didn't approve The Grand Tour. Instead, it opted for a cheaper, bare-bones mission that would travel just to Jupiter and Saturn.
The Scientists' Secret
This announcement didn't deter engineers 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. Each 1,592-pound (722-kg) spacecraft consisted of the following:
- Main bus (the core structure, which included a fuel tank, thrusters, some instruments, and electronics)
- High-gain antenna used to communicate with Earth
- Two booms to hold scientific instruments, as well as one boom dedicated to carrying the craft's radioactive power supply
- Two additional antennas
The scientific instruments onboard Voyagers 1 and 2 include a cosmic-ray detector, infrared spectrometer/radiometer, ultraviolet spectrometer, and two 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 source: plutonium-238, which releases heat as it decays to more stable isotopes. Relayed through a series of thermocouples, which convert the heat into electrical current, plutonium's decay heat provided power for the satellites’ equipment.
Since plutonium-238 has a half-life of 87.7 years, both Voyager spacecraft now operate with about half of their initial energy supply. While that isn't enough to keep all equipment operational, it will successfully power the essentials through 2020 before becoming 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 radio signals to reach Earth, so onboard 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 Voyager 2's trajectory allowed for the possibility of more. Seeing the mission's popularity and wealth of scientific results, Congress had a change of heart, approving funding for Voyager 2 to continue beyond Saturn to visit Uranus and Neptune as well. Meanwhile, 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.
What We Learned
Voyagers 1 and 2 are the paragon of planetary exploration. Their images and other observations would influence science textbooks and curricula for years to come. They even inspired the future Galileo, Cassini, and Juno missions. The Voyager missions taught us many things:
- Both Voyagers offered a closer look at Jupiter’s weather patterns, including its famous Great Red Spot.
- Voyager 1 spotted a thin ring system around Jupiter, and Voyager 2 recorded the dark, dusty rings around both Uranus and Neptune.
- Voyager 1's camera discovered active volcanoes on Jupiter's moon Io.
- Voyager 2 explored the strange magnetic field generated in Uranus — unlike other planets, the field's axis doesn't pass exactly through the planet's center, and it's aligned more with the planet's equator than its poles.
- The Voyager probes revealed that Titan’s atmosphere is denser than Earth's.
- Voyager 2 spotted Neptune's Great Dark Spot, a giant cyclonic storm as big as Earth.
- Voyager 1 became the first human-made object to exit the solar system in 2012, when it crossed into the heliosheath and entered the boundary that separates our solar system from interstellar space.
By 1989, NASA had received enough information to fill the Encyclopedia Britannica 6,000 times over!
Today, the spacecraft are still traveling in different directions. Their slingshot trajectories have them headed toward the stars. Voyager 1 left the ecliptic plane following its visit to Saturn in 1980 (in order to pass close to its big moon Titan), and the probe officially made it into interstellar space in 2012. Voyager 2 passed all four planets and is expected to enter the heliosheath in the next few years. While the spacecraft will stop transmitting messages to Earth around 2020, they’ll keep flying long after that.
One thing is for certain, our enthusiasm for exploring the universe will continue until one day, these silent sailors from Earth will be the last to carry our message: “We were here."