Hail Mary Science
Spaceflight and relativity

Space communication delay

No signal travels faster than light. Pick a destination and the tool shows how long a message takes to get there and how long the reply takes to come back, from the Moon to other galaxies.

Key formula

t = d / c · c ≈ 299,792 km/s

Try it

How to use the simulator

No signal travels faster than light, at about 300,000 km per second. On human scales that feels instant, but in space it isn't. The tool shows how long a message takes to reach different destinations and how long you wait for a reply, on a logarithmic scale that runs from the Moon to other galaxies.

Controls

  • Destination buttons: the Moon, Mars, Jupiter, the Voyager 1 probe, Proxima Centauri, Tau Ceti and Andromeda.

Things to try

  1. Pick the Moon: just over a second each way, so talking to astronauts there already has noticeable pauses.
  2. Pick Mars: about 12 minutes each way on average. That's why rovers have to drive themselves.
  3. Pick Tau Ceti: almost 12 years there and as many back. On a mission like that, asking a question and getting the answer takes half a lifetime.

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Light speed as the limit

Radio waves, lasers and every other electromagnetic signal travel at light speed, about 300,000 km per second. On Earth that feels instant: light could circle the planet seven and a half times in one second. But distances in space are so large that the delay becomes a practical problem.

The round-trip delay is twice the one-way delay. That is what really limits a conversation: the message has to arrive, and then you have to wait for the answer.

Real consequences

With the Moon, the round trip is about 2.6 seconds, enough to notice pauses in a conversation. With Mars it ranges from about 6 to 44 minutes depending on where the planets are, so rovers can't be driven in real time: they get the day's instructions and work out by themselves how to avoid obstacles.

Voyager 1, the most distant human-made probe, is around one light-day away in 2026. Every command sent to it takes almost a day to arrive and the confirmation another day to come back.

Worked example

Example: talking to Mars

  1. At its closest, Mars is about 55 million km away. 55,000,000 / 300,000 ≈ 183 s: about 3 minutes one way.
  2. At its farthest it is about 400 million km away: 400,000,000 / 300,000 ≈ 1,333 s, about 22 minutes one way.
  3. A question and its answer can therefore take anywhere from 6 to 44 minutes. The tool uses an average distance of about 12 minutes each way.

In Project Hail Mary

The novel's mission is almost 12 light-years from home. Any message to Earth would take nearly 12 years to arrive, so the crew can't ask for help or wait for instructions: they have to work everything out alone.

This tool appears in:Chapter 20The Sacrifice and the Silence

Frequently asked questions

Does quantum entanglement allow faster-than-light communication?

No. Entanglement correlates the outcomes of measurements in two places, but it can't carry information: each side sees random results until it compares notes with the other through a normal channel.

What is a light-year?

The distance light travels in a year: about 9.46 trillion kilometers. It is a unit of distance, not time.

How does NASA talk to such distant probes?

Through the Deep Space Network, three complexes of antennas up to 70 meters across in California, Madrid and Canberra, spaced about 120° apart so one always faces each probe.

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