Free hard sci-fi comms delay calculator. Pick origin and destination (Earth, Moon, Mars, asteroid belt) and conjunction or opposition; outputs one-way light-speed delay, conversation lag, and an FTL multiplier override.
Built and maintained by Paul Clark, Redmoon Software
Use when writing hard sci-fi where realistic comms delays drive plot tension. A character on Mars waiting 26 minutes for a reply to "we have a problem" is a great scene beat.
NASA-grade tools require ephemeris data. This is a writer's shorthand — accurate enough to ground your scene without slowing you down.
Your characters on Earth face a 4m 19s delay waiting for the mars outpost to acknowledge their broadcast.
Each destination uses nominal closest-approach and farthest-opposition distances in astronomical units (AU).
One-way delay = distance / (light speed × FTL multiplier). Conversation lag is twice that — you send a message, the receiver replies.
For dramatic effect, set FTL > 1 to model an Alcubierre-style fictional drive.
Nominal closest/farthest orbital approaches in astronomical units (AU). Real opposition / conjunction varies year to year.
Yes for radio-speed delays. FTL is fictional — use the multiplier as a story convenience.
The planets orbit at different speeds, so their separation swings from about 3 light-minutes at closest approach (opposition) to over 20 light-minutes when on opposite sides of the Sun (conjunction). That is why the one-way delay is given as a range rather than a fixed number.
A reply must travel back as well as the original message travel out, so a full round trip takes twice the one-way light-time. At Mars opposition that means waiting roughly six minutes before you could hear an answer.
Input
Earth → Mars, closest approach, light speed.
Output
One-way: 4:20 minutes. Conversation lag: 8:40.
Earth–Mars closest approach is ~0.52 AU = 78 million km. At light speed (~300,000 km/s), one-way takes ~260 seconds — about 4 min 20 s.
Signal delay is distance divided by the speed of light, and the distances are the real orbital ones: the Moon at about 0.00256 AU, Mars ranging from 0.52 AU at closest approach to 2.52 at conjunction, Jupiter around 5.2, Saturn near 9.5.
The Mars range is the number worth internalising. One-way delay runs from roughly four minutes to over twenty depending on where the planets are, so a round trip is anywhere from eight to forty-plus minutes. Nothing engineering can do reduces it — it is the speed of light, not a bandwidth limit.
That is why real deep-space operations are built on autonomy and scripted sequences rather than remote control. A rover cannot be driven with a joystick when the delay is longer than the time it takes to hit a rock.
The FTL factor divides the light-lag figure, which lets you keep a consistent communication delay in a setting that has faster-than-light signalling. Setting it to 1 gives you real physics.
The value of computing it either way is dramatic rather than technical: conversation is impossible at Mars distances, so scenes have to be written as exchanges of messages, and knowing whether your setting allows real-time contact decides how plots involving distant help can work at all. Deciding the factor once and applying it consistently is what stops a story quietly having it both ways.
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