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.
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.
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.
קלט
Earth → Mars, closest approach, light speed.
פלט
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.
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