Light-Years to Travel Time

Travel time across light-years

A light-year is how far light travels through vacuum in one Julian year, about 9.461 × 10¹⁵ m. Suppose a probe cruises at some fraction v of the speed of light c. The travel time in years, measured in the observer's frame, comes out to t = d / (v/c), where d is the distance in light-years. Take Proxima Centauri: 4.2 ly at 10% c gives you 42 years. A few markers help put distances in perspective. The Moon is 1.28 light-seconds out, the Sun 8.3 light-minutes, Pluto roughly 5 light-hours, and Voyager 1 about 22 light-hours from Earth. That probe is actually moving at ~0.0057% c, so at its real pace Proxima would be ~74,000 years away.

Applications

Science communication and astronomy teaching, rough feasibility checks on interstellar mission concepts (Breakthrough Starshot aims for ~20% c), working out Deep Space Network round-trip latency for probes, and quick back-of-the-envelope physics when you're building out a sci-fi world.

FAQ

Does this account for relativity? It doesn't. The formula gives the time an Earth observer would measure. Proper time onboard runs shorter by the Lorentz factor √(1 − v²/c²), which only matters much above ~30% c.

What about acceleration? The model assumes a constant cruise speed. Real missions burn time speeding up and slowing down, which tacks on years and chews through enormous propellant budgets.

How fast can we actually go today? The fastest human-made object, the Parker Solar Probe, tops out near 0.064% c at perihelion, and that's still nowhere near any meaningful fraction of light speed.