Reference
Light travel time
See how long light takes to reach each planet from the Sun, then compare that with the 6 to 44 minute round-trip delay for Mars communication.
Right now, a signal to Mars takes
One-way at current Earth-Mars distance: updating
Updating
Round-trip is double that: Updating.
A signal arriving on Mars right now
...was transmitted from Earth at this clock time
Updating
Waiting for the current Earth-Mars distance.
| Body | Distance (AU) | From Sun | From Earth (mean) | Notes |
|---|---|---|---|---|
Mercury Planet | 0.387 | 3m 13s | 5m 7s range 4m 20s 12m 25s | Closest planet to the Sun, so its one-way signal from the Sun is the shortest. |
Venus Planet | 0.723 | 6m 1s | 2m 19s range 2m 18s 14m 18s | Nearest planet to Earth, at inferior conjunction the round-trip ping is under 5 minutes. |
Earth Planet | 1.000 | 8m 19s | - | We see sunlight as it was 8 minutes 19 seconds ago, fire the Sun at noon and you'd have until 12:08 to notice. |
Moon Natural satellite | 1.000 | 8m 19s | 1.28s | From Earth, the Moon is only 1.28 light-seconds away, Apollo crews had near-immediate radio with Houston. |
Mars Planet | 1.524 | 12m 40s | 12m 39s range 3m 2s 22m 16s | Mars rover commands and downlinks face a one-way delay of 3 to 22 minutes depending on Earth-Mars geometry. |
Jupiter Planet | 5.204 | 43m 17s | 43m 17s range 32m 53s 53m 41s | Juno's commands take a ~35 to 53 minute one-way trip, it has to fly autonomously through perijove passes. |
Saturn Planet | 9.582 | 1h 19m | 1h 19m range 1h 11m 1h 27m | Cassini's mission engineers worked with round-trip delays of more than two and a half hours. |
Uranus Planet | 19.20 | 2h 40m | 2h 40m range 2h 31m 2h 48m | Voyager 2's 1986 Uranus flyby commands were prepared days in advance, there was no time to react. |
Neptune Planet | 30.07 | 4h 10m | 4h 10m range 4h 01m 4h 18m | Last big planet before the Kuiper belt, round-trip is more than 8 hours, so a single command-and-confirm exhausts a working day. |
Pluto Dwarf planet | 39.48 | 5h 28m | 5h 28m range 5h 19m 5h 37m | New Horizons sent its 2015 flyby data home over 15 months at ~5.5 hour one-way delay. |
Voyager 1 Spacecraft (interstellar) | 165.50 | 22h 56m | 22h 56m | Approximate distance as of 2026, the probe drifts ~3.6 AU per year, so this value keeps growing. |
Proxima Centauri Star (nearest neighbour) | 268,770 | 4y 3m | 4y 3m | The closest star to the Sun, 4.2465 light-years away, a hello-back radio exchange would take 8 and a half years. |
ISS (Earth orbit) Space station | 2.8e-6 | 8m 19s | 0.0014s | Roughly 408 km up, light makes the trip in well under a millisecond, so astronaut-Houston comms feel real-time. |
The speed of light is exact, by definition
Since 1983, the speed of light in vacuum has been an exact value: 299,792,458 metres per second. That year the Conférence Générale des Poids et Mesures redefined the metre as the distance light travels in 1/299,792,458 of a second, which makes c a defined constant rather than something we measure. The astronomical unit was fixed similarly by IAU Resolution B2 in 2012 at exactly 149,597,870,700 metres. Divide those two and you get the 8 minute 19 second light-time from the Sun to Earth that the table above starts from.
Why "we see the Sun as it was 8 minutes ago"
Light is fast but finite. Every photon you see from the Sun left it around 499 seconds before reaching your eye. If the Sun blinked out right now, you would notice at 12:08 by your local clock. Looking outward the delay piles up: Jupiter you see as it was 35 to 53 minutes earlier, Saturn 71 to 87 minutes earlier, Neptune more than four hours behind. The further out, the older the picture. Looking at Proxima Centauri tonight, you are watching it as it was 4 years 3 months ago.
Why interplanetary missions plan around the delay
A Mars rover command sent from Earth takes between 3 and 22 minutes to arrive depending on where Mars is in its orbit, and the same again to acknowledge, a 6 to 44 minute round-trip in which the rover has to handle anything that goes wrong on its own. That is why the rovers use onboard autonomy for driving and for entry, descent and landing: you cannot joystick anything across a quarter- hour delay. The further out the mission, the wider the gap. Juno's commands to Jupiter face a one-way trip of ~35 to 53 minutes; New Horizons during its 2015 Pluto flyby was operating with a one-way delay of about four and a half hours and sent its full dataset home over the following fifteen months.
Voyager 1 takes nearly a day
Voyager 1, launched in 1977 and in interstellar space since it crossed the heliopause in 2012, was about 172 AU from the Sun in September 2026. A radio signal from the Deep Space Network now takes almost 24 hours to reach it, and the same again to hear anything back. That delay grows by a few seconds every day as the probe continues outward at about 17 km/s, so in late 2026 it becomes the first spacecraft a full light-day from Earth. For scale, a signal to the nearest other star, Proxima Centauri, would take 4 years 3 months one-way.
Related tools
For a live Mars clock and the Curiosity / Perseverance mission sol, see time on Mars. Curious how long a year on each planet is? Compute your age on other planets. And for the orbital mechanics behind these distances, the orbital periods table compares sidereal and synodic cycles.