Signal / Light Travel Time Calculator

Light and all electromagnetic signals (radio, X-ray, etc.) travel through space at the same speed:

c = 299,792.458 km/s

Travel time formula:

t = d / c

This means there is an unavoidable delay in any communication or observation across space. When you look at the Sun, you see it as it was 8.3 minutes ago. When scientists on Earth send a command to a Mars rover, it takes 3 to 22 minutes one way, depending on where Mars is in its orbit.

Key distances and light travel times:

• Earth to Moon (384,400 km): 1.28 seconds
• Earth to Sun (149.6 million km / 1 AU): 8.32 minutes
• Earth to Mars at closest (54.6 million km): 3.03 minutes
• Earth to Mars at average (225 million km): 12.5 minutes
• Earth to Mars at farthest (401 million km): 22.3 minutes
• Earth to Jupiter at opposition, its closest (628 million km): 34.9 minutes. Averaged over Jupiter’s orbit the distance is nearer 778 million km, which is 43.3 minutes
• Earth to Neptune at average (4.4 billion km): 4.1 hours
• Earth to Pluto at average (5.9 billion km): 5.5 hours
• Earth to Proxima Centauri (4.24 light-years): 4.24 years
• Earth to Andromeda Galaxy (2.537 million ly): 2.537 million years

Why the planet numbers come as ranges. Earth and every other planet are on their own orbits, so the gap between any two of them changes constantly. Mars swings between 54.6 and 401 million km, a factor of seven, and that is the difference between a 3-minute command delay and a 22-minute one. Mission planners do not average it, they plan for the worst case, which is why rovers are written to drive themselves rather than wait for instructions. The presets on this page are labelled with which figure they use.

The light-year defined: One light-year is the distance light travels in exactly one Julian year (365.25 days). It equals 9.461 × 10¹² km, or about 63,241 AU.