A dozen of the fifty-four dispatches in this archive are about spacecraft: Cassini’s last orbits around Saturn and its new view of Mimas, Dawn’s images of Ceres, the Trace Gas Orbiter’s first look at Mars, the Galileo navigation satellites, plans for a lunar base. All of them quietly assume something the reports themselves never spell out — nobody drives a probe. You send an instruction, and then you wait.
This works out how long the wait is. It is the simplest sum in spaceflight: distance divided by the speed of light, which is 299,792.458 kilometres per second. Radio travels at exactly that speed, so the answer is the same whether you are sending a command or receiving a photograph.
Signal delay
One way—
There and back—
Why a range and not a number
Because the planets keep moving. Earth and Mars ride different orbits at different speeds, so the gap between them is about 54.6 million kilometres when they line up on the same side of the Sun and about 401 million when the Sun is between them. A single figure for “the distance to Mars” would be wrong almost all of the time, so this gives you both ends.
What the round trip means
The round trip is the honest number for anyone operating a spacecraft: send an instruction, wait for it to arrive, wait again for the acknowledgement to come back. With Saturn that is over two hours before you learn whether anything happened. This is why probes are not flown by hand but loaded with sequences they execute alone, and why a landing cannot be steered from Earth — by the time you saw a problem, it would be long over.
The small print
These are straight-line distances between the two bodies, and light in a vacuum. Real missions have it slightly worse: the signal is routed through ground stations, queued behind other spacecraft on the same antenna, and slowed a fraction by the atmosphere at each end. The figures here are the floor, not the schedule.
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