Voyager 1, launched in 1977, is now so far from Earth that a radio signal travelling at the speed of light takes more than 22 hours to reach it. And it is still working, still sending data back from interstellar space, running on a nuclear power source that today produces less electricity than a couple of household light bulbs.
Nearly half a century after it left, the most distant object humans have ever made is still, faintly, talking to us.
How far is “over 22 hours”?
Voyager 1 sits about 25 billion kilometres from Earth, more than 170 times the distance between the Earth and the Sun, and it is still moving outward. At that range even light, the fastest thing there is, needs around 23 hours to cross the gap, a figure that keeps creeping up as the spacecraft recedes.
That delay makes ordinary conversation impossible. A command sent from Earth takes the better part of a day to arrive, and any reply takes just as long to come back, so a single exchange is close to a two-day round trip. Controllers cannot react to anything in real time. They send instructions into the dark and wait almost two days to learn what happened.
Still talking from interstellar space
What makes this more than a distance record is where Voyager 1 is. In 2012 it crossed the heliopause, the boundary where the Sun’s outward flow of particles gives way to the thin gas between the stars, and became the first spacecraft to enter interstellar space. It has been sending back measurements from that region ever since.
Two of its instruments are still switched on, studying the plasma and magnetic fields around it, and their readings travel all the way back to Earth through NASA’s Deep Space Network of giant antennas. By the time that signal arrives it is almost unimaginably faint, a whisper of a transmitter fainter than a fridge bulb, picked out of the noise after a day in flight.
The nuclear battery
None of it would be possible without an unusual power supply. Out where Voyager flies, sunlight is far too weak for solar panels, so the spacecraft carries its own source: three generators that turn the heat from slowly decaying plutonium into electricity. They are, in effect, nuclear batteries, and they need no moving parts and no sunlight to work.
The comparison to a small battery is a fair way to picture how little power is involved, though the hardware itself is a set of plutonium-fuelled generators rather than anything you would recognise from a car. What matters is the output. At launch the system produced about 470 watts. Today it delivers roughly 230 watts and falling, losing around four watts every year as the plutonium decays and the generators wear. A spacecraft billions of kilometres away is running the whole of its remaining life on less power than many kitchen appliances draw.
Fighting for every watt
That steady decline, not any breakdown, is what will eventually end the mission. To stretch the dwindling supply, NASA has been switching off systems one at a time, shutting down heaters and retiring instruments to keep the essentials alive. In 2026 engineers powered down another of its detectors for exactly this reason.
Managed carefully, the trickle of power may be enough to keep at least basic engineering data flowing into the 2030s. But the trend only runs one way. Each year there is a little less to work with, and one by one the remaining instruments will have to be turned off until nothing is left to run.
What to watch
There is a milestone almost within reach. Voyager 1 is closing in on a distance of one light-day from Earth, the point at which its signal takes a full 24 hours to arrive, expected to fall around November 2026. No human-made object has ever been that far away.
Beyond that, the story is simply one of endurance against a fading power source. The question is no longer whether the spacecraft works, but how long a 1970s machine can keep whispering on a shrinking trickle of nuclear electricity before the last watt is gone. For now, a probe older than most of the people tracking it remains the farthest-flung voice humanity has, reporting back from beyond the edge of the Sun’s reach on a few hundred watts and a very long memory.
