It’s worth thinking about why Voyager 1 and 2, now coming up on their 40th year of operation, are still sending back data. After all, mission longevity becomes increasingly important as we anticipate missions well outside the Solar System, and the Voyagers are giving us a glimpse of what can be done even with 1970’s technology. We owe much of their staying power to their encounters with Jupiter, which demanded substantial protection against the giant planet’s harsh radiation, a design margin still used in space missions today.
The Voyagers were the first spacecraft to be protected against external electrostatic charges and the first with autonomous fault protection, meaning each spacecraft had the ability to detect problems onboard and correct them. We still use the Reed-Solomon code for spacecraft data to reduce data transmission errors, and we all benefited from Voyager’s programmable attitude and pointing capabilities during its planetary encounters.
Pioneer 6 was a doughty vehicle, but Voyager 2 (launched before Voyager 1) passed its record as longest continuously operating spacecraft back in August of 2012, while Voyager 1 eclipsed Pioneer 10’s distance mark in 1998 and is now traveling some 21 billion kilometers out. Voyager 1 is our sole spacecraft to leave the heliosphere, though Voyager 2 is expected to follow it in a few years, and we’ve already acquired important information, such as the fact that cosmic rays are four times more abundant in interstellar space than near the Earth.
You can see how all this begins to build the foundation for a ‘true’ interstellar mission, by which I mean one designed solely for the purpose of penetrating the local interstellar medium and reporting data from it. The heliosphere, Voyager has shown us, wraps around our Solar System and helps to provide a radiation shield for the planets. Missions both robotic and manned will need to be designed around the cosmic ray issues Voyager has uncovered.
Image: Voyager 1 image of Io showing active plume of Loki on limb. Heart-shaped feature southeast of Loki consists of fallout deposits from active plume Pele. The images that make up this mosaic were taken from an average distance of approximately 490,000 kilometers. Credit: NASA/JPL/USGS.
Still thinking interstellar, the Voyagers are telling us about the solar wind’s termination shock, that region where charged particles from the Sun slow to below the speed of sound as they push out into the interstellar medium — these are Voyager 2 measurements. Voyager 1 has measured the density of the interstellar medium as well as magnetic fields outside the heliosphere. The final benefit: We’ll have Voyager 2 outside the heliosphere while still in communication, so we can sample the interstellar medium from two different locations.
I always think of long spacecraft missions in terms of the people who work on them. Voyager is pushing on the ‘lifetime of a researcher’ rubric that some consider essential (though I disagree), the notion that missions have to be flown so that those who worked on them can see them through to destination. But of course the Voyagers have no destination as such; they’ll press on in a galactic orbit that takes fully 225 million years to complete. And as our spacecraft get even more rugged and capable of autonomy, we’ll soon take it as a given that multiple generations will be involved in seeing any complex mission through to completion. (See Voyager to a Star for my riff on a symbolic ‘extension’ to the Voyager mission).
Image: These two pictures of Uranus — one in true color (left) and the other in false color — were compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. The spacecraft was 9.1 million kilometers from the planet, several days from closest approach. The picture at left has been processed to show Uranus as human eyes would see it from the vantage point of the spacecraft. Credit: NASA/JPL.
We have, according to the Jet Propulsion Laboratory, perhaps until 2030 before data from the Voyagers ceases. Each spacecraft contains three radioisotope thermoelectric generators (RTGs) running off the decay of plutonium-238. And as this JPL news release reminds us, with the spacecraft power decreasing by four watts per year, engineers have to be creative at figuring out how best to squeeze out data results under extreme power constraints.
For a mission this long, that means consulting documents written decades ago and at a completely different stage of technological development.
“The technology is many generations old, and it takes someone with 1970s design experience to understand how the spacecraft operate and what updates can be made to permit them to continue operating today and into the future,” said Suzanne Dodd, Voyager project manager based at NASA’s Jet Propulsion Laboratory in Pasadena.
Image: Global color mosaic of Triton, taken in 1989 by Voyager 2 during its flyby of the Neptune system. Triton is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn’s giant moon, Titan). The greenish areas include what is called the cantaloupe terrain, whose origin is unknown, and a set of “cryovolcanic” landscapes apparently produced by icy-cold liquids (now frozen) erupting from Triton’s interior. Credit: NASA/JPL/USGS.
It’s been quite a ride. Voyager discovered Io’s volcanoes and imaged rings around Jupiter, Uranus and Neptune, while finding hints of the apparent ocean within Europa that carries so much astrobiological interest. Between them, the Voyagers found a total of 24 new moons amongst the four planets they visited, detecting lightning on Jupiter and a nitrogen-rich atmosphere at Titan, the first to be found outside the Earth itself. And who can forget that bizarre terrain on Triton, or the tortured surface of Uranus’ moon Miranda?
“None of us knew, when we launched 40 years ago, that anything would still be working, and continuing on this pioneering journey,” said Ed Stone, Voyager project scientist based at Caltech in Pasadena, California. “The most exciting thing they find in the next five years is likely to be something that we didn’t know was out there to be discovered.”
Image: The Voyagers outbound. A representation of the heliosphere, including the termination shock (TS), the heliopause and the interstellar medium where the heliosphere ends. Credit: Science, NASA/JPL-California Institute of Technology. Note: In this image, the locations of the Voyagers are updated only to September 2011, by Brad Baxley, JILA.
Who knew that Voyager’s measurements of solar wind plasma, low-frequency radio waves, charged particles and magnetic fields would still be informing us fully forty years on? The next spacecraft to cross the heliosphere after Voyager, this time designed for just that purpose, will surely live even longer, challenging our conceptions of human achievement across generations and our willingness to tackle projects involving not just deep space but deep time.
This mission isn’t over. Go Voyager.