Interactive
How long does a signal take?
Nothing in the universe outruns light, so every message to deep space runs late. A hello sent right now reaches the Moon in about 1.3 seconds, Mars in a few minutes, Neptune in about 4 hours, and Voyager 1, the farthest thing humans have ever made, in just under 24 hours. Every number on this page is computed live from real positions, so it is true at the second you read it.
Computing live distances…
Planning the actual trip instead? The Space Travel Calculator turns these distances into travel times for real and imagined engines. Or check Sky Tonight to see which of these worlds are up in your sky right now.
Under the hood
How we calculate it
Planets, the Sun, and the Moon use the same astronomy engine that powers Sky Tonight: JPL Keplerian orbital elements for the planets, computed for the exact second you load the page, and a precision lunar model for the Moon. The one-way delay is then just distance divided by the speed of light, 299,792.458 km/s exactly. Because the planets genuinely move, the table changes while you watch: that is the point.
The Voyagers are too far out for the planet engine, so we track them the honest way: a straight-line model. Each probe gets a fixed direction and a constant outward speed, anchored to a JPL Horizons position published in November 2026: Voyager 1 at 172.3 AU from the Sun receding at 16.99 km/s, Voyager 2 at 144.5 AU receding at about 15 km/s. Fifty years out, nothing out there pushes on them, so their paths are ruler straight and the model stays within a few light-seconds of JPL for months at a time.
One subtlety we show rather than hide: the light-day milestone is measured from Earth, and Earth moves. Voyager 1’s distance from us breathes by millions of kilometres across the year as our planet swings around the Sun, even though the probe itself never slows down.
Sources
- NASA, “NASA Invites Media, Public to Join Voyager 1 One-Light Day Activities” (November 10, 2026): the light-day milestone and its instant, 10:16 UTC on November 18, 2026.
- NASA Jet Propulsion Laboratory, Horizons ephemeris system: the November 2026 Voyager positions used as this page’s anchors (Voyager 1 at 172.3 AU from the Sun; Voyager 2 at 144.5 AU), via TheSkyLive’s published Horizons ephemerides.
- Standish, E. M., JPL: “Keplerian Elements for Approximate Positions of the Major Planets”: the orbital elements the planet engine runs on.
- Bureau International des Poids et Mesures, SI Brochure: the speed of light, exactly 299,792.458 km/s by definition.
Common questions
How long does a radio signal take to reach Mars?
It depends on where Mars is in its orbit. At its closest, Mars is about 3 light-minutes away; when it sits on the far side of the Sun, the same signal takes about 22 minutes. There is no single answer, which is why this page computes the figure live from Mars’s real position. This delay, changing day by day, is why Mars rovers cannot be driven with a joystick and have to navigate on their own.
How long does light take to reach the Moon?
About 1.3 seconds on average. The Moon sits roughly 384,400 km away, and because its orbit is an ellipse, the true delay swings between about 1.2 and 1.4 seconds across the month. You can hear that pause in recordings of Apollo conversations: Houston speaks, and the answer always comes a beat and a half late.
How long does sunlight take to reach Earth?
About 8 minutes 20 seconds on average. Earth’s orbit is slightly elliptical, so the honest figure swings between about 8 minutes 11 seconds in early January, when we are nearest the Sun, and about 8 minutes 27 seconds in early July. The sunlight warming you right now left the Sun’s surface before you started reading this page.
When does Voyager 1 reach one light-day from Earth?
NASA puts the crossing at 10:16 UTC on Wednesday, November 18, 2026, when Voyager 1 is about 25.9 billion km (16.1 billion miles) from Earth: the distance light travels in exactly 24 hours. From that moment, a command takes a full day to reach the probe and its reply takes another day to return, so every conversation with Voyager 1 becomes a two-day exchange. Voyager 1 launched on September 5, 1977; the milestone arrives in its 50th year of flight.
How far away is Voyager 2?
Voyager 2 is the second most distant human-made object. In November 2026 it is about 21.7 billion km from Earth, so a one-way signal takes about 20 hours. It crossed into interstellar space in November 2018, six years after Voyager 1, and it is heading south of the plane of the planets, roughly toward Sirius, which it will pass at a distance of about 1.7 light-years some 300,000 years from now.
Why can’t we just phone Mars and talk in real time?
Because nothing known to physics travels faster than light, and radio waves and laser pulses are light: they all move at exactly 299,792.458 km/s. Even on the best day of the decade, Mars is three light-minutes away, so the fastest possible exchange is question, three-minute wait, answer, three-minute wait. Real missions plan around this: commands are bundled, spacecraft are given autonomy, and landings run entirely on pre-loaded instructions, which is why engineers call the descent “seven minutes of terror”: by the time the signal saying “I have landed” reaches Earth, the landing is already over.
How accurate are the numbers on this page?
Planet, Sun, and Moon distances are computed from the same live engine behind our Sky Tonight page: JPL Keplerian orbital elements for the planets and a precision lunar model for the Moon, good to well under a light-minute of travel time for every world listed. The two Voyagers use a straight-line model anchored to JPL Horizons positions published in November 2026 (Voyager 1 at 172.3 AU from the Sun, Voyager 2 at 144.5 AU), with their published outward speeds. Their flight is so steady that the model stays within a few light-seconds of JPL for months. NASA’s own live counters may differ by a few seconds for the probes; treat these figures as exact for the planets and honest to a few seconds for the Voyagers.