Posted by In November 2023 Voyager 1 started talking nonsense. Instead of science data, the spacecraft was sending back a repeating pattern of ones and zeros, and it took engineers at NASA’s Jet Propulsion Laboratory months to trace the problem to a single failed memory chip in a computer designed in the early 1970s. Their fix was to break the affected code into pieces and tuck it into other corners of the memory, then send the patch on a journey of nearly a full day each way. By the spring of 2024 the probe was making sense again. Stories like that are why people keep asking about Voyager 3. If two spacecraft built with 1970s electronics can still do science from interstellar space, what could a new one do?
There was never a third
Start with the plain fact: no spacecraft called Voyager 3 was ever launched, and NASA has no mission by that name. The program had two probes. Voyager 2 left Cape Canaveral on August 20, 1977, and Voyager 1 followed on September 5 on a faster path, which is why the numbering looks backwards.
Both were made possible by a lucky arrangement of the outer planets. In the mid-1960s a JPL summer intern named Gary Flandro worked out that Jupiter, Saturn, Uranus and Neptune would line up in the late 1970s in a way that happens only about once every 175 years, allowing one spacecraft to swing from planet to planet using gravity assists. An ambitious “Grand Tour” with several spacecraft was proposed and then cancelled for cost. What survived was a cheaper pair of probes, officially aimed only at Jupiter and Saturn. Voyager 2’s onward trip to Uranus in 1986 and Neptune in 1989 was, in effect, a bonus the engineers had quietly made possible.
That alignment will not come round again for a long time. Any Voyager 3 would be a different kind of mission.
What the first two found
It is hard to overstate how little was known before 1979. Jupiter’s moon Io was a coloured dot in a telescope. In March of that year a JPL navigation engineer, Linda Morabito, looked at a Voyager 1 image and noticed a strange crescent over its edge. It was a volcanic plume. Io turned out to be the most volcanically active body in the Solar System.
Then came the rings of Saturn resolved into thousands of ringlets, the haze of Titan, the tilted magnetic field of Uranus, and on Neptune’s moon Triton, dark plumes rising from a surface colder than almost anywhere else ever measured. No spacecraft has been back to Uranus or Neptune since. Everything we have seen up close of those two planets still comes from Voyager 2.
Out past the wind
After the planets, the Voyagers kept going, and their job changed. The Sun blows a wind of charged particles that inflates a vast bubble called the heliosphere. Voyager 1 crossed its edge, the heliopause, in August 2012, about 121 times the Earth-Sun distance from the Sun. Voyager 2 followed in November 2018. They are the only working human-made objects to have sampled the space between the stars.
That is a remarkable result from instruments never designed for it. The plasma instrument on Voyager 1 had already failed by the time it crossed the boundary, so scientists inferred the change from other measurements. Imagine what a probe built for that region from the start could do.
Why a successor makes sense
That is the real argument for a successor, whatever it ends up being called. Researchers at the Johns Hopkins University Applied Physics Laboratory have spent years studying a concept usually called Interstellar Probe, a spacecraft designed to leave the heliosphere much faster than the Voyagers and travel several hundred times the Earth-Sun distance within roughly fifty years. Its purpose would be to measure the heliosphere from outside, sample interstellar dust and gas directly, and perhaps look back at the shape of the Sun’s bubble, which nobody has seen. It is a study, not an approved mission. It would have to compete for money like everything else.
Most of what would make it better is not exotic. Modern detectors, electronics and data handling would fit far more capability into the same mass. Onboard autonomy would let a spacecraft handle routine faults without waiting nearly two days for a round trip of commands. And it could be pointed at targets chosen from what the Voyagers taught us, rather than discovered by accident.
The physics problems that don’t go away
Speed is the first. Voyager 1 is moving at about 17 kilometres per second relative to the Sun. That sounds fast until you work out that it would need more than seventy thousand years to cover the distance to Proxima Centauri, even if it were heading that way, which it is not. New Horizons, launched in 2006, left Earth faster than any spacecraft before it, but after its Jupiter flyby and the long climb away from the Sun it has ended up a little slower than Voyager 1. Getting meaningfully faster usually means a very heavy launch vehicle, a close gravity assist from Jupiter, possibly a burn deep in the Sun’s gravity well, or new propulsion.
Power is the second. Beyond Jupiter, sunlight is too weak for practical solar panels, so the Voyagers run on radioisotope generators fuelled by plutonium-238. Their output drops by a few watts every year, and the team has been switching instruments off one by one to keep the rest alive. Plutonium-238 is scarce and expensive to produce. A successor would need it too.
The third is simply hearing the thing. Voyager 2’s southern trajectory means only one Deep Space Network dish, in Canberra, can send it commands, and when that antenna was taken offline for upgrades in 2020, the spacecraft spent months unable to receive instructions. A new probe would rely on the same network, and its signal would be faint in the same way.
Sails, lasers and other long shots
Solar sails are no longer purely theoretical. Japan’s IKAROS unfurled one on the way to Venus in 2010, and The Planetary Society’s LightSail 2 raised its own orbit using sunlight in 2019. Electric propulsion, proven on missions such as Dawn, is efficient but slow to build up speed. Nuclear propulsion has been studied for decades and remains largely on paper.
At the far end is Breakthrough Starshot, announced in 2016, which proposes using a huge ground-based laser array to push gram-scale probes on ultra-thin sails to a fifth of the speed of light, reaching the Alpha Centauri system in about twenty years. Every part of that plan, from the laser to the sail material to sending data home, involves problems nobody has solved. It is worth taking seriously as research. It is not a mission yet.
A name that may never be used
Missions get names from programs and committees, not from popular demand, so the next deep-space explorer will very likely be called something else. That probably does not matter. Voyager 3 has become shorthand for an idea: a probe built on purpose to go where the Voyagers wandered by accident, and to send back more than a trickle of data when it gets there.
Voyager 1 and 2 will fall silent at some point in the coming years as their power runs down. Each carries a golden record of sounds and images from Earth, chosen by a committee chaired by Carl Sagan, for a reader that may never exist. A Voyager 3, if anyone builds one, would inherit that strange mix of engineering and hope.
For more on the Voyagers, interstellar probes and the rest of the outer Solar System, keep exploring SETIworld, where new deep-space results and the arguments around them are followed closely.
