Solar System Simulator · What if? · lightspeed
How long does a message to Mars take?
Between about 3 and 22 minutes one way — the gap between Earth and Mars swings from 55 to 400 million km. Send a pulse across today's actual distance and time it.
Light is fast; space is bigger
Radio travels at the speed of light, about 300,000 km a second — and it still takes 8 min 20 s to cover the one astronomical unit from the Sun to Earth. Mars orbits half again as far out, and both planets keep moving, so the gap between them is never the same twice: from about 55 million km when they line up on the same side of the Sun to about 400 million km when Mars is round the back. A message takes 3 minutes at one extreme and 22 minutes at the other, and the same again for the reply.
Why Mars rovers drive themselves
A round trip of six to forty-four minutes rules out a joystick. Rover drivers on Earth plan a day's route, send it, and find out the next morning how it went; the rover watches for rocks on its own. The most famous example is a landing: Perseverance's "seven minutes of terror" on 18 February 2021 were already over — it was sitting on the surface, or it wasn't — 11 min 22 s before the first signal reached Earth. Every landing is history by the time we hear it.
Other destinations
| From Earth to… | One-way light time |
|---|---|
| the Moon | 1.3 s |
| the Sun | 8 min 20 s |
| Mars | 3 – 22 min |
| Jupiter | 33 – 54 min |
| Saturn | 1 h 6 min – 1 h 32 min |
| Neptune | 3 h 59 min – 4 h 21 min |
| Voyager 1 (September 2026) | about 24 hours |
Ranges run from the two orbits' nearest to farthest configurations, using the same orbital elements the simulator uses; the planets' actual positions on a given day fall somewhere inside them. In the simulator you can also send the pulse to Jupiter.
Why the simulator freezes the planets
At any playback speed where you can see the planets move, light would cross the gap in a fraction of a frame — instantaneous, and useless as a demonstration. So the lesson stops the clock: the planets hold still at today's real gap while the pulse advances on the wall clock, one second for every three light-minutes, and a ring marks its arrival with the time to the second. Start time again mid-flight and the transmission is lost, which is the honest outcome.
What the simulator does
- The card shows the live gap between Earth and Mars in millions of kilometres, as the planets sit right now.
- The planets freeze and a pulse crosses at one second per three light-minutes, drawn along the straight world-space line between the two bodies.
- The arrival ring fires and the card gives the crossing time to the second.
Questions people ask
Can you have a live conversation with someone on Mars?
No. Even at the closest approach a reply takes six minutes to come back, and at the far side of the Sun three-quarters of an hour. Conversation with Mars will always be more like email than a phone call.
How long does a message to the Moon take?
About 1.3 seconds each way — which is why the Apollo radio exchanges have that slight hesitation in them.
How long does light take to reach Earth from the Sun?
About 8 minutes 20 seconds (499 seconds) for the 149.6 million km. The sunlight you see left the Sun before you started reading this page.
Does the delay to Mars change?
Constantly. It cycles between about 3 and 22 minutes as the two planets orbit at different speeds, repeating roughly every 26 months — the same cycle that sets Mars launch windows.
Keep going
Every number on this page comes from the simulator's own physics or the real mission record it is checked against. Where the simulation and reality differ, the page says so.