Thirty-five years ago today, Voyager 1 launched on its epic grand tour of the outer solar system. Now, it's about to start another phase of its journey: it's about to cross the boundary that marks the end of our solar system. It will be the first man made object to enter this completely unknown and unchartered realm of interstellar space.
Our solar system, that is the eight planets (or nine if you're a Pluto hugger) and all the small bodies including Kuiper belt and Oort cloud objects, are contained within a giant plasma bubble. This bubble is the hot and turbulent area created by the stream of charged particles coming from the sun. They stop right around where Voyager 1 is, and outside the bubble is a new frontier in the Milky Way. Scientists except this realm of interstellar space will be a relatively calm environment, especially compared to the solar system's boundary. When exactly the spacecraft will cross over to the other side isn't totally clear; it could take days, weeks, or years. All we really know is that the boundary is near.
Both spacecrafts' journeys started in 1966 when Gary Flandro, a graduate student working at NASA's Jet Propulsion Laboratory, realized that the planets were literally about to align. If launched in the late 1970s, a spacecraft could use gravity assists to visit the outer planets: Jupiter, Saturn, Uranus, and Neptune. It would be like a giant game of pinball with the spacecraft slingshotting from planet to planet.
It was too good an opportunity for NASA to pass up, and too rare occurring every 177 years. So the twin Voyager spacecraft were born. Armed with a suite of science instruments, information on where to find Earth, and a gold record carrying the sounds and languages of the planets, they were sent on their cosmic journey. Voyager 2 actually launched first on August 20, 1977; Voyager 1 followed on September 5 on a shorter trajectory that would take it Jupiter first.
Voyagers 1 and 2 launched from the same site in Florida, visited Jupiter and Saturn before parting ways. Voyager 2's launch date actually gave it many more options for interesting trajectories than Voyager 1; it visited Uranus and Neptune. Voyager 1, on the other hand, reached Saturn at such an orientation that is slingshotted out of the plane of the ecliptic, the plane where all the planets orbit the sun.
On November 12, 1980, Voyager 1 left Saturn and began its trip out of the solar system and kept on going. NASA had no real plans to keep in touch with either spacecraft after leaving Saturn, but it did.
Currently, Voyager 1 is more than 11 billion miles from the sun or 107 times as far as the Earth is from the sun. At that distance, it takes a signal fifteen hours to travel back to Earth. Voyager 2 trails behind at just 9 billion miles from the sun. The data that is coming back suggests that the spacecraft is in a rapidly changing environment. The protective sphere of the sun's magnetic field – the heliosphere – is starting to fail and untempered cosmic rays are striking the spacecraft.
What's really impressive is that both Voyagers are still kicking and talking to Earth with decades old technology. Each one only has 68 kilobytes of memory and an eight-track tape recorder; Curiosity, the rover that landed on Mars last month, has 2 gigabytes of digital memory.
And it doesn't look like Voyager 1 will go silent anytime soon, even after it break through the solar system's barrier. There's nothing stopping the signal returning to Earth. Both Voyagers are powered by radioisotope thermoelectric generators (RTGs), the same system that Curiosity has for 12 years of possible power on Mars. RTGs use the electricity generated from decaying plutonium 238 to make heat to keep the spacecraft working. Mission scientists don't anticipate shutting down any of Voyager 1's instruments and systems, a measure to conserve power, before 2020. By then, it will likely be well beyond the solar system and have exceeded its primary mission by more than 30 years. It's unrealistic to expect every NASA mission to be such an over-the-top success, but talk about bang for your buck. Here's to hoping Voyager still has a few good years left.