A space travel time calculator for real speeds and relativistic trips
How long would it take to drive to the Moon, fly to Mars in an airliner or reach the nearest star aboard Voyager 1? This space travel time calculator answers it for the Moon, the Sun, every planet, a set of nearby stars, the centre of the Milky Way, the Andromeda Galaxy or any distance you type. Choose a speed from walking pace to 99% of the speed of light, or switch to the relativistic mode in which the ship accelerates at a steady 1 g, turns round half-way and brakes to a stop.
In both modes you get the time that passes on Earth and the time that passes on the ship’s own clock, so the page doubles as an interstellar travel time calculator and a time dilation calculator for space trips. A table repeats the answer for every destination at the same setting, and the working is written out underneath. Everything is computed in your browser.
How to use the Space Travel Time Calculator
- Pick a Destination. For planets choose the Planet distance: closest approach, farthest, or today’s distance computed from the planet’s orbit. Or type any Distance and unit.
- Choose Constant speed and a preset speed (car, airliner, Voyager 1, a percentage of light speed) or type your own in km/h, km/s, mph or % of c.
- Or choose Accelerate, then decelerate, enter the acceleration in g, and optionally a top speed at which the ship coasts.
- Read the Earth time, ship time, top speed and light travel time, then the working and the table of all destinations.
- Press Copy result to paste the answer into notes or homework.
Space Travel Time Calculator features
Real reference speeds
Walking, a motorway car, an airliner, the space station, Apollo 10, New Horizons, Voyager 1 and the Parker Solar Probe’s record.
Ship time and Earth time
Special relativity is applied in both modes, so 99% of c shows the seven-fold slowing of the ship’s clock.
Constant-acceleration trips
The relativistic rocket formulas give the classic 1 g answers: about 3.5 years on board to reach Proxima Centauri.
Planet distances that change
NASA closest and farthest distances, or today’s distance from the same orbital elements as the planets-tonight page.
Fuel reality check
The fuel a perfect matter–antimatter photon rocket would burn per kilogram of payload.
The formulas behind the answers
Constant speed. Earth time is distance ÷ speed. The traveller’s clock runs slow by the Lorentz factor γ = 1 ÷ √(1 − v²/c²), so ship time = Earth time ÷ γ. At everyday speeds γ is so close to 1 that a 160-day drive to the Moon at 100 km/h leaves the driver younger by only about 60 nanoseconds. At 99% of light speed γ = 7.09, so the 4.29-year trip to Proxima Centauri lasts about seven months on board.
Constant acceleration with a flip. The ship accelerates at a steady proper acceleration a (what the crew feel) for half the distance D, turns round and decelerates for the other half. For each half the Lorentz factor at the turnaround is γ = 1 + aD/c², ship time is (c/a)·arcosh γ and Earth time is √((D/c)² + 2D/a). These are the equations of the relativistic rocket from the physics FAQ by Philip Gibbs and Don Koks. At 1 g, c/a is 0.969 years, which is why 1 g trips across a few light-years take only a few years on board, and the Andromeda Galaxy, 2.5 million light-years away, is about 28.6 years by the ship’s clock while 2.5 million years pass on Earth.
With a top speed limit the ship accelerates to that speed, coasts and then brakes. The fuel figure assumes a perfect photon rocket that turns matter into light: the fuel-to-payload ratio is e2φ − 1, where φ is the rapidity reached. About 38 kg of fuel per kilogram of payload are needed for the 1 g trip to Proxima Centauri, and the number explodes for longer trips.
Typical answers
| Trip | Setting | Earth time | Ship time |
|---|---|---|---|
| Moon (384,400 km) | Car, 100 km/h | 160 days | 160 days |
| Mars (closest, 54.6 million km) | Voyager 1, 61,200 km/h | 37 days | 37 days |
| Proxima Centauri (4.25 ly) | Voyager 1 | about 74,900 years | about 74,900 years |
| Proxima Centauri | 1 g, flip half-way | 5.87 years | 3.54 years |
| Andromeda (2.5 million ly) | 1 g, flip half-way | about 2.5 million years | about 28.6 years |
What the calculator leaves out
For distance units on their own see the light year converter, for the twin paradox in detail the time dilation calculator, and for real chemical rockets the rocket equation calculator.
- Orbits: spacecraft travel on curved transfer orbits timed to planetary positions, so real missions take longer than the straight-line figure (Mars about seven to nine months, Pluto nine and a half years for New Horizons).
- Gravity: the pull of the Sun, planets and target star is ignored.
- Engineering: no known propulsion can reach a few per cent of light speed, let alone sustain 1 g for years.
- Hazards: interstellar dust and gas become deadly radiation at relativistic speeds.
Space Travel Time Calculator FAQ
How long would it take to get to Mars?
In a straight line at Voyager 1’s speed it would be 37 days at closest approach, but real missions follow a transfer orbit and take about seven to nine months. Pick Mars and “Today (computed)” to see the current distance.
How long would it take to travel one light-year?
At the speed of light, one year by Earth’s clocks. At Voyager 1’s 61,200 km/h about 17,600 years, and at 1 g with a flip half-way about 1.9 years on board while 2.2 years pass on Earth.
What is the difference between ship time and Earth time?
Moving clocks run slow. Earth time is what calendars at home show when the ship arrives; ship time is what the crew’s clocks and bodies record. The difference is negligible below a few per cent of light speed.
Why does a 1 g ship reach far stars in a human lifetime?
Steady 1 g acceleration pushes the ship ever closer to light speed and the time dilation grows exponentially with ship time, so distances of thousands of light-years shrink to a couple of decades on board, even though thousands of years pass on Earth.
Is this interstellar travel time calculator accurate?
The arithmetic follows special relativity exactly. The inputs are idealised: constant speed or constant acceleration in a straight line, with no orbits, gravity or engine limits.
Can anything travel faster than light?
No. The calculator refuses 100% of c because a ship with mass would need infinite energy to reach it.