One light-day from home
On 18 November 2026, a radio signal from Earth needs a full day to reach Voyager 1. The arithmetic of that distance explains why the mission still works, and why it cannot work for much longer.
NASA has put a time on it: 2:16 in the morning, Pacific time, on 18 November 2026. At that moment Voyager 1, launched on 5 September 1977, is one light-day from Earth, about 25.9 billion kilometers. No human-made object has been that far away before. Nothing on the spacecraft changes when it crosses the line. It keeps coasting outward at about 17 kilometers per second, through a thin plasma the Sun’s wind has never touched.
For the small team at NASA’s Jet Propulsion Laboratory who still fly it, the number has a practical meaning. Every exchange now takes two days: one for the question, one for the answer. If a command does something unexpected, nobody finds out until 48 hours after it was sent.
Five hundred photons per bit
It is worth working out how faint Voyager has become, because the result is hard to believe. The spacecraft’s transmitter puts out about 23 watts, roughly a refrigerator bulb. Its 3.7-meter dish concentrates that power by a factor of about 60,000, which sounds like a lot until you follow the beam outward. By one light-day it has spread to something like 370 million kilometers across, tens of thousands of Earths wide.
Apply the inverse-square law and put a 70-meter antenna in the way, and the power collected comes to about 5 × 10-19 watts. At Voyager’s X-band radio frequency, that is on the order of 80,000 photons arriving each second. The spacecraft sends about 160 bits per second, so each bit of science data reaches Earth carried by roughly 500 photons.
These are our own back-of-the-envelope figures, and they ignore losses in the receiver chain. But they show why the mission depends on the largest dishes of NASA’s Deep Space Network, on receivers cooled to a few degrees above absolute zero, and on error-correcting codes designed to rebuild a message from a signal barely above the noise.
A distance that breathes
One detail rarely makes it into the anniversary coverage: Voyager 1 does not get steadily further from Earth. The spacecraft moves away from the Sun at about 3.6 astronomical units per year. Earth, circling the Sun, can move toward or away from the spacecraft at up to about 5 astronomical units per year along that line of sight. For part of each year, Earth’s motion wins, and the distance between the two shrinks even as Voyager heads out of the solar system. The one-light-day figure is a milestone on a line that wobbles upward, a small reminder that the deep-space numbers we quote are measured from a moving platform.
Turning things off
Voyager’s power comes from radioisotope thermoelectric generators, which turn the heat of decaying plutonium-238 into electricity. The output falls by about four watts every year, and the mission has spent the past decade deciding what to sacrifice next: heaters first, then instruments. Voyager 1 launched with ten science instruments. After its cosmic ray subsystem was switched off in 2025 and its low-energy charged particle instrument in April 2026, two remain: the magnetometer and the plasma wave instrument.
Those two are a fitting pair to keep. It was the plasma wave instrument that told scientists Voyager 1 had left the Sun’s domain. In 2012 the spacecraft crossed the heliopause, the boundary where the solar wind gives way to interstellar space, and there was no working instrument aboard to sample that plasma directly. But when a burst of solar activity later set the surrounding gas ringing, the frequency of the oscillations revealed how dense the plasma was: about 40 times denser than the material just inside the boundary. Voyager 1 was somewhere new, and it had taken a solar storm to prove it. Voyager 2, on a slower route past Uranus and Neptune, followed in November 2018.
Repairs at a two-day remove
In November 2023 Voyager 1 began sending a repeating pattern of meaningless ones and zeros. After months of work, engineers traced the fault to a failed chip in the flight data subsystem, the computer that packages science and engineering data. The chip could not be fixed, so they rewrote the software around it, splitting the affected code into pieces and moving them to other parts of a memory that holds less than 70 kilobytes. Engineering data returned in April 2024. Science data followed in June.
The chip could not be fixed, so they rewrote the software around it.
The thrusters that keep the antenna pointed at Earth have a problem of their own: a residue has built up in their narrow fuel lines over the decades, and the team has rotated between sets of thrusters to stay ahead of the clogging. In 2025 they brought back a set of roll thrusters that had been considered dead since 2004, an attempt made more urgent by planned maintenance on the big Canberra dish.
A photograph from the far side
On 14 February 1990, at the suggestion of Carl Sagan, Voyager 1 turned its camera back toward the Sun and took a series of images of the planets. In one frame, Earth appears as a speck less than a pixel wide, caught in a ray of scattered sunlight. The cameras were switched off soon afterward to save power and memory, and they have never been used again. We return to that picture, and to why people keep looking back, in our essay on Earthrise.
The spacecraft is now more than four times further from Earth than when it took that picture. If it could still look back, the Sun would be the brightest star in its sky, and Earth would be lost in the glare.
What one light-day means
A light-day is a very small distance on the scale of the galaxy. The nearest star, Proxima Centauri, is more than four light-years away; at Voyager’s speed that distance takes about 75,000 years, and Voyager 1 is not heading there anyway. In some forty thousand years it will pass within about 1.6 light-years of a faint star in the constellation Camelopardalis. Measured against that journey, the spacecraft has barely left the driveway.
It carries a message for that journey all the same. Bolted to its side is a gold-plated copper record holding greetings in 55 languages, music, natural sounds and encoded images, with instructions on the cover and a map fixing the Sun’s position against fourteen pulsars. The record will outlast the machine by a very long way. The engineers’ ambition is more modest: to keep at least one instrument running into the 2030s. Until then the team keeps doing what it has always done, sending careful commands into the dark, waiting, and listening for a reply that now takes a day to arrive.
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