Artist’s drawing of the Voyager 1 spacecraft, which launched on September 5, 1977.
Photo Credit: NASA (Click image for full size.)
Thirty-five years after the Voyager program started, the twin spacecraft have surpassed all expectations.
The Voyager program began with a specific mission: to explore Jupiter and Saturn using two identical spacecraft. Not only did the program foster groundbreaking scientific understanding of Jupiter and Saturn, it did the same for Uranus and Neptune, giving scientists a new perspective on the solar system. But the best may lie ahead: within the next decade, Voyager 1 and Voyager 2 are expected to be the first spacecraft to broach interstellar space.
How did the program—initially designed to last five years—end up accomplishing so much? Along with careful planning and ongoing improvements, Voyager benefitted from a critical but unreliable element.
“Luck!” said Edward Stone, California Institute of Technology (Caltech) Voyager project scientist, at the recent JPL-sponsored 35th anniversary celebration of the Voyager mission. “There’s a lot of luck in this business.”
The program’s luck began in 1965 when Gary Flandro, a Caltech grad student working at NASA for the summer, figured out that a spacecraft launched in 1977 could capitalize on a rare planetary alignment that occurs once roughly every 175 years. This alignment would enable the spacecraft to fly by all four outer planets—Jupiter, Saturn, Uranus, and Neptune—using a gravity-assist technique to swing from planet to planet, picking up speed to continue forward. This approach would reduce the flight time to Neptune from the 30 years it would normally take to 12 years, and obviate the need for large onboard propellant systems.
The Voyager program was determined to take advantage of this rare planetary arrangement. But choosing the best path to follow wasn’t easy. More than 10,000 possible trajectories were considered. The key problem was timing: the selected trajectory had to aim Voyager at a spot where Jupiter was expected to be in the future. Reaching Jupiter at the right moment would allow the spacecraft to use a gravitational slingshot—leveraging Jupiter’s powerful gravitational field—to power its journey on to Saturn.
Stone explained, “If we had not flown by Jupiter, we would not have been able to get to Saturn. It was only by flying by Jupiter and getting Jupiter’s slingshot that we managed to make it to Saturn. And Saturn gave us another slingshot. The amount of energy you get from a slingshot has to do with how you fly by a planet and the closer you fly by. Basically, Voyager was able to fly very close to these planets and get really big boosts along the way.”
Flying close to the large planets was not without risk. So while Voyager 1 was directed to pass very near to Jupiter and then on to Saturn, Voyager 2 was given a different trajectory. It would also approach Jupiter, but at a greater distance from the planet’s gravitational field and radiation belts. This way, if Voyager 1 were harmed by its close proximity to the planet, Voyager 2 could continue the joint mission on toward Saturn.
But Voyager 1 was successful. So successful, in fact, that Voyager 2 was able to extend its mission in another direction, using gravity-assist aiming points to explore Uranus and Neptune as well.
Voyager 1 launched from Cape Canaveral, Florida, on September 5, 1977. It reached Jupiter a year and a half later, on March 5, 1979, and then Saturn on November 12, 1980. After completing its mission to explore the two planets, their larger moons, and Saturn’s rings, Voyager 1’s trajectory sent it northward beyond the planetary orbital plane. The spacecraft began returning information about interplanetary space.
Voyager 2, which launched earlier than Voyager 1—on August 20, 1977—followed a longer trajectory. The spacecraft encountered Jupiter on July 9, 1979, and Saturn on August 25, 1981. It went on to explore the remaining gas giants, reaching Uranus on January 24, 1986 and Neptune on August 25, 1989. Voyager 2 then began following a path southward into interplanetary space.
A new mission was born: to penetrate interstellar space so scientists could learn more about interstellar fields, particles, and waves. This mission was made possible by technological improvements both on the ground and on the Voyagers themselves. The antennae were extended and software improved. Non-essential functions were shut down to preserve energy and computer capacity. This wasn’t only critical for the interstellar mission; it enabled the extended Voyager 2 planetary mission as well. “If we had done nothing to the spacecraft and nothing on the ground, we could not have returned a single image from Neptune,” said Stone.
Because their original mission took them far from the sun, the spacecraft are not solar powered. Instead, they possess radioisotope thermoelectric generators (RTGs) that power onboard systems and instruments. As power decays, systems are turned off. The Voyagers return information to Earth through the Deep Space Network (DSN).
Both Voyagers are now on trajectories that will eventually take them out of the solar atmosphere—called the heliosphere—into interstellar space. The heliosphere, governed by the sun’s magnetic field, is filled with particles (mainly ionized hydrogen) that flow from the sun. This high-speed flow of particles is known as solar wind. Interstellar space is a different realm entirely, containing particles from exploded supernovae as well as wind and cosmic rays generated by the explosions.
In 2004, Voyager 1—which travels at a rate of a billion miles every three years—crossed the “termination shock”: the point at which solar wind slows abruptly from supersonic to subsonic speeds. Voyager 2 crossed over in 2007. Both are now in the heliosheath. While still controlled by the sun’s magnetic field, in this area solar winds begin to mix with interstellar gases. Voyager 1 is now 11.3 billion miles from Earth. Voyager 2 is 9.3 billion miles away. The spacecraft are expected to leave the heliosphere within 10 to 20 years of crossing the termination shock.
Timing is critical. The spacecraft have enough power to operate systems until 2020. At that point, instruments will be turned off one by one until the last is shut down in 2025. After that, scientists will be unable to learn anything more about interstellar space from Voyager. But the spacecraft themselves will continue to travel out into the galaxy.