A probe launched nearly half a century ago is about to reach a distance that no human-made object has ever crossed.
Voyager 1 has been traveling since September 1977. Originally, its mission included exploring Jupiter and Saturn. It passed those planets long ago and continues to move away from the Sun. On November 18, 2026, according to NASA's calculations, it should reach a very particular distance from Earth.
Artistic depiction of Voyager 1, NASA's legendary space probe.
Credit: NASA/JPL-Caltech
This distance corresponds to one light-day, or approximately 25.9 billion kilometers. It represents the distance light travels in 24 hours. Even at nearly 300,000 kilometers per second, light now takes almost a day to reach the probe or to get to us from Voyager 1.
And this changes a lot for the teams operating it. To send an instruction, engineers transmit a radio signal from Earth. This signal travels at the speed of light. Beyond this new limit, it will take more than 24 hours to reach the probe.
If Voyager 1 responds immediately, its message will have to travel the same distance in the opposite direction. A simple exchange will therefore take at least two days. Under these conditions, it is impossible to control it directly like a remote-controlled robot. The teams must prepare their instructions in advance and wait patiently for the results.
Yet the probe has not stopped. It is traveling at about 61,000 kilometers per hour relative to the Sun. This considerable speed on our scale remains tiny compared with the speed of light. That is why it has needed almost fifty years to cover this distance.
Voyager 1 has already been in interstellar space since 2012. It crossed the region where the stream of particles emitted by the Sun dominates its surroundings. However, it has not left the Sun's entire gravitational influence. It is simply continuing its journey through a region very different from that of the planets.
Crossing one light-day will not cause any change on board. It is a reference point chosen to measure its distance. NASA expects this milestone on November 18 and is organizing a celebration as early as November 17.
Another probe is continuing its journey farther behind it: Voyager 2. Also launched in 1977, it explored Uranus and Neptune. According to NASA's forecasts, it will not reach a distance of one light-day from Earth until November 2035.