A light-day is the distance light travels in 24 hours: about 25.9 billion kilometres, roughly 173 times the distance between Earth and the Sun. Voyager 1 becomes the first spacecraft one light-day from Earth on 18 November 2026, at about 10:16 UTC, so its radio signals take a full day to reach us.
Understanding Light-Day
From light-seconds to light-years
Units based on the speed of light turn a distance into a travel time, which makes them natural for space: the number says at once how long a signal or an image has been on its way. Each is the speed of light multiplied by a fixed span of time, so each is an exact length. The light-day sits between the light-hour, a handy scale for the outer planets, and the light-year, the scale of the gaps between stars.
| Unit | Distance | For example |
|---|---|---|
| Light-second | About 299,792 km | The Moon is about 1.3 light-seconds from Earth |
| Light-minute | About 18 million km | Sunlight takes about 8 minutes 19 seconds to reach Earth |
| Light-hour | About 1.08 billion km | Neptune is about 4 light-hours from the Sun |
| Light-day | About 25.9 billion km | Voyager 1 is a light-day from Earth from 18 November 2026 |
| Light-year | About 9.46 trillion km | Proxima Centauri, the nearest star, is 4.24 light-years away |
What changes when Voyager 1 passes one light-day
Nothing changes on board. Voyager 1 keeps coasting outwards at about 17 km/s, covering roughly 3.6 times the Earth-Sun distance every year, as it has since its 1980 flyby of Saturn and the moon Titan bent its path up and out of the plane of the planets. What changes is the conversation. From then on, a command radioed from Earth takes more than a day to arrive and the reply more than a day to come back, so a question and its answer take about two days. Every message passes through the giant dishes of the Deep Space Network, which still pick out the spacecraft's faint signal at about 160 bits per second.
Measured from Earth or from the Sun
Voyager 1's distance from Earth does not grow perfectly smoothly. As Earth circles the Sun it spends part of each year moving towards the spacecraft and part moving away, so the distance rises and falls by several light-minutes around its steady outward climb. The crossing on 18 November 2026 is for good: none of the later yearly dips brings Voyager 1 back below one light-day. Measured from the Sun instead, it reaches the same distance early in 2027, because in November Earth lies on the far side of the Sun from the spacecraft.