Time delay between Mars and Earth

Spacecraft event time vs. Earth receive time

Mars Express Light Time Delay Display

A photo of the Mars Express delay display on the control system, showing us the critical numbers of one-way light time, two-way light time and the distance from Earth.

One of the most difficult things about operating a spacecraft around Mars (not to mention the different time zones), compared with the Earth, is that it's so far away!

Mars is so far away in fact that it takes radio signals quite a long time to get from the spacecraft back to Earth. During Curiosity EDL, this delay will be 13 minutes, 48 seconds, about mid-way between the minimum delay of around 4 minutes and the maximum of around 24 minutes.

This makes it a challenge to operate Mars Express because it's hard to have a conversation with the spacecraft, or react if anything happens on board. If there is a problem and the spacecraft tells us, we won't know for 13 minutes, and then even if we react straight away it'll be another 13 minutes before our instructions get back to Mars – there's a lot that can happen in half an hour at Mars (for example a whole Curiosity landing)!

To keep Mars Express flying safely, we load all the commands for the mission in advance and built in lots of autonomy to let the spacecraft take care of itself – you could say that for the Curiosity landing we're running completely on autopilot!

The delay is nothing to do with the spacecraft or the hardware on the ground – it can't be improved by a faster computer or a more powerful radio. In fact it is obeying the fundamental speed limit of the universe – the speed of light.

At 1,079,000,000 km/hour, light is pretty quick; you could get from here to the Moon in a little over a second! But that just underlines how far away Mars is.

All light (or electromagnetic radiation, which includes radio signals) travels up to this speed, and radio waves from Earth to Mars Express and back are no exception. Take a look at the Wikipedia article on the speed of light and you'll see how, in 1905, Einstein came upon the concept of this cosmic speed limit.

Above all, for tomorrow's coverage of the Curiosity landing it makes it challenging for us to work out when to tell you what's happening (as you've seen in our three column timeline)!

At ESOC, we talk about two different times – Spacecraft Event Time (SCET) and Earth Received Time (ERT). The former is what's actually happening at Mars right now, although we won't hear about it until over 13 minutes later, a time we call ERT.

The delay between the two is usually called the One-Way Light Time (OWLT) and the time for a message to go to Mars and come back is the Two-Way Light Time (TWLT), or round-trip time.

During all our coverage we'll follow NASA's lead and generally communicate events here and on Twitter to you in ERT because that's when we'll actually know what's happened. If we do communicate something in SCET we'll let you know so you (and us too) don't get confused – it's all part of the fun of exploring the Solar System!

14 thoughts on “Time delay between Mars and Earth

  1. Pingback: Minecraft + Marte = Voxel Mars | robologs

  2. The distance from earth to sun is higher than that to Mars. It takes 8 minutes to suns light to reach earth. Why then signal takes 13 minutes to reach earth?

    • That's because the light from the sun is photonic whereas the radio signals are electromagnetic radiation (sort of light), they don't travel at the same speed as light.

      • No, all waves travel at the same speed. It takes light eight minutes on a one way trip, 16 minutes on a two way trip. Please learn to into physics

      • Incorrect. Allow me to quote directly from the article you just read since you seem to have missed it.

        "At 1,079,000,000 km/hour, light is pretty quick; you could get from here to the Moon in a little over a second! But that just underlines how far away Mars is.

        All light (or electromagnetic radiation, which includes radio signals) travels up to this speed, and radio waves from Earth to Mars Express and back are no exception."

        The time from Earth to Mars varies between 4 and 24 minutes because earth (and mars) are both orbiting the sun, not each other. The distance between them therefore can change quite dramatically depending on where we are in our respective orbits.

        When Mars is directly behind Earth at the closest possible point, it is only four minutes away. When it is at the farthes point opposite us behind the Sun, it is 24 minutes away. The Sun meanwhile, which we orbit, remains at roughly 8 minutes away since our orbit around the Sun is very nearly circular.

        • One more note, assuming Mars is also a circular orbit (which it is not) you could assume that the distance from mars to the sun is roughly 12 minutes and implying that the maximum distance would be 8 + 12 which would only be 20 minutes.

          However this is not the case since Mars' orbit is somewhat eliptical and in a slightly different plane from Earths orbit as well making it possible for them to be as distant as 24 minutes.

    • The article gives you the clue doesn't it? Since the orbit of Mars is elliptical (not circular!), the epihelion and perihelion (i.e. shortest distance and the furthest Mars is from Earth) differs periodically.

    • The distance between the Earth and the Sun is based on our orbit and is relatively constant (about 92,960,00 miles). Since both the Earth and Mars orbit the sun at different distances and speed, the distance between us constantly changes. At times Mars is on the other side of the Sun from us and other times it is on the same side. When Mars is closest to Earth it can be as close as 34,000,000 miles, but when they are the furthest apart they can be almost 250,000,000 miles apart.

      • Hi,
        We gotta come to a common term for distance. Metric is proven to be easy and base of all science. When please switch to metric and use kilo meter.
        thanks

  3. Varges, we are not necessarily the same side of the sun at the same time - so the longest time is for when Earth and Mars are 180 degrees opposite each other and both at aphelion, and the shortest would be when we are both the same side, with Mars at perihelion and Earth at aphelion.:

    o ---------------O-------------------o o-------o------------O

  4. This is extremely important and could be cause fundament change in physics since Maxwell's theory is justified by the relation between the velocity of light and the radio induction effect.

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