In 2024, engineers rescued Voyager 1 after it spent months transmitting gibberish, remotely rewriting and relocating pieces of 46-year-old code around a failed memory chip from more than 24 billion kilometres away.

In late 2023, one of the most distant machines humanity has ever built started talking nonsense. Voyager 1, more than 24 billion kilometres from Earth, began sending back a meaningless, repeating pattern instead of data, and it kept doing so for about five months. In 2024, engineers brought it back, not by sending anyone to fix it, which is impossible, but by remotely rewriting and relocating pieces of its 46-year-old software around a dead memory chip, with every command they sent taking the better part of a day to arrive.

When the oldest working spacecraft went quiet

Voyager 1 launched in 1977 and is now the most distant human-made object in existence, travelling through interstellar space beyond the edge of the Sun’s influence. It still communicates with Earth through NASA’s Deep Space Network, an achievement in itself for hardware approaching half a century old.

On 14 November 2023, that communication broke down in an unusual way. The spacecraft was still there, still receiving commands and, by all signs, still carrying them out. But the data it sent home had turned into a single repeating pattern that carried no information. Voyager was alive and listening. It had simply stopped being able to say anything intelligible.

Finding the fault

It took months to work out why. By early 2024, engineers had traced the trouble to the Flight Data Subsystem, one of the spacecraft’s three onboard computers, and the one that gathers and packages information before it is transmitted.

A single chip in that computer’s memory had failed. A stuck bit had knocked out roughly three per cent of the subsystem’s memory, and, crucially, that memory held part of the software’s own code. Around 256 words of storage were simply gone.

On Earth you would replace the chip. At 24 billion kilometres, there is no replacing anything.

The fix was to move the code

If the failed memory could not be repaired or swapped, the code that had lived there had to go somewhere else. That was harder than it sounds, because no single unused stretch of the subsystem’s small memory was large enough to hold it all.

So the team took the affected code apart. They broke it into sections, found room for each section in a different corner of the memory, and rewrote the internal cross-references so that the scattered pieces would still call on one another and run as a single working program. It is the sort of delicate surgery engineers usually perform on a machine sitting in front of them. This one was done blind, on a computer they had last physically touched in the 1970s.

The wait of nearly two days

Distance made every step slow. A command sent to Voyager 1 has to cross about 24 billion kilometres, which takes light, and therefore radio, roughly 22.5 hours. The reply takes just as long, so a single question and answer is close to a two-day exchange.

On 18 April 2024, the engineers sent the command to move the key section of code. Nearly a full day later it reached the spacecraft. The day after that, the answer came back: Voyager 1 was returning readable data again. Engineering information about the craft’s own health came first, in April, and by June the four still-operating science instruments were sending usable readings once more.

Why it was hard beyond the distance

The light-time delay was only part of the difficulty. Voyager’s computers are designs from the early 1970s, with a total memory a tiny fraction of what sits in an ordinary phone today. Most of the engineers who built them have long since retired, so the team fixing the fault had to work from decades-old documents and manuals, reading and re-reading code written half a century ago until they understood it well enough to rearrange it safely.

There was also very little room for error. A command that was wrong in the wrong way, sent to a fragile, irreplaceable spacecraft at the far edge of the solar system, could have ended the mission outright.

The repair worked on the first serious attempt.

What to watch

The fix bought time, not permanence. Voyager 1’s real limit is not its software but its power: the plutonium generator that runs it loses a few watts every year, and instruments are being switched off one by one to stretch what remains. The spacecraft will fall silent within a matter of years no matter how cleverly it is nursed along.

Until then, the thing to watch is simply how long it lasts, and whether the ageing craft throws up further faults that its small, improvising team has to solve from more than a day’s radio-flight away. For now, a computer built before the personal computer existed is still working, kept alive by people reaching across interstellar space to rewrite code older than most of them.