Voyager 1 nears a one-light-day milestone
Voyager 1 is expected to be one light-day from Earth on November 18. At that distance, a radio signal would take about 24 hours to reach the spacecraft.
What is a light-day?
A light-day is the distance light travels in 24 hours. Radio waves travel at the speed of light, so when Voyager 1 reaches that distance from Earth, a message sent to it will need about a day to arrive. A reply would need about another day to travel back.
NASA expects Voyager 1 to cross this mark on November 18, 2026. The milestone describes its distance from Earth. It is still ahead as of this story’s publication; the spacecraft has not already reached it.
Why it is remarkable
Voyager 1 launched in 1977 and has kept traveling far beyond the planets it visited. Its coming light-day milestone makes the scale of that journey easier to picture: even the fastest possible signal needs a full day to catch up.
The Voyager and Earth graphic above is an original explanatory illustration, not a spacecraft photograph.