Voyager 1, the spacecraft that launched on a tour of the solar on Sept. 5, 1977, is getting ready to enter interplanetary space.
The spacecraft’s journey started in 1966 when Gary Flandro, then a graduate student working at NASA’s Jet Propulsion Laboratory, discovered that the planets were about to align. Not just for him, but for the whole solar system.
He found that in the late 1970s and early 1980s, Jupiter, Saturn, Uranus, Neptune, and Pluto would be at the right relative positions in their orbits that a spacecraft could visit each of them on a grand tour. The secret was gravity assists, using the planets’ gravity to slingshot the spacecraft from one to the next.
To take advantage of the opportunity, NASA developed the twin Voyager spacecraft. Both would fly by Jupiter and Saturn, adding valuable scientific data to what was then a very scant knowledge of the outer solar system. Ground-based studies revealed the planets’ most basic properties, and hinted that there were fascinating discoveries still to make.
In the early 1970s, Pioneer 10 and Pioneer 11 made the first flybys of the gas giants and their instruments began revealing just how complex those worlds are.
With this as the background, Voyager wasn’t just a great opportunity, it was the logical next step is understanding our solar system.
Voyager 2, which was actually the first to launch on August 20, 1977, ended up with more opportunities on its trajectory. After flying by Jupiter and Saturn, it would be in the right place to continue its mission and fly by Uranus.
Voyager 1 was a little more limited. Its orientation after flying by Saturn would send it out of the ecliptic, the plane where all the planets orbit the sun.
But even without another planetary target, Voyager 1 would keep on going. It would, as the 1977 pre-launch press kit described, continue “outward from the solar system and across the boundary of the wind of charged particles (solar wind) that streams outward from the Sun, thus penetrating into interstellar space.” At the time, there was no concrete plan to keep in touch with the spacecraft after it left Saturn.
On Nov. 12, 1980, Voyager 1 began its trip out of the solar system. The spacecraft was on its path “searching for the outer limit of the solar wind –- that presumed boundary… where the influence of the sun gives way,” as NASA described it in a 1980 press release. It kept on going, and NASA hasn’t lost touch despite arguments that its continuing budget spent on the mission every year could be put to “better use.”
Currently, Voyager 1 is at the furthest reaches of our solar system more than 107 times as far as the Earth is from the sun. Data suggests that the spacecraft is in a region where the environment is changing rapidly and that the protective sphere of the sun’s magnetic field — the heliosphere — is failing. More cosmic rays seem to be hitting the spacecraft without the cushioning effect of the heliosphere.
From the edges of the solar system, it takes a signal traveling from the spacecraft fifteen hours to reach Earth. Voyager 2 isn’t too far behind. It flew by Uranus and Neptune in the 1980s before slipping below the ecliptic. Both have since returned stunning portraits of the whole solar system.
Nothing is stopping Voyager 1 from talking to Earth after it passes through the heliosphere save its steadily dwindling power supply. Both Voyager spacecraft are powered by radioisotope thermoelectric generators (RTGs) that use the electricity generated from decaying plutonium 238 to make heat to keep the spacecraft working. At launch, Voyager 1′s RTG was releasing 470 watts; by 2010 its output had fallen to about 285 watts.
For now, it looks like Voyager 1 will last until about 2020 before mission scientists start powering down instruments and systems to conserve what remains of its power. By then it will be through the heliosphere having exceeded its primary mission by over thirty years.
Image: Artist’s rendering of Voyager 2 in the outer regions of the heliosphere, the magnetic bubble around the solar system generated by the solar wind. Credit: NASA