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!

Mars Express – timeline for MSL support

Early on 6 August, Mars Express will receive crucial signals from NASA’s Mars Science Laboratory mission as it delivers the car-sized Curiosity rover onto the Red Planet. The ESA spacecraft will begin tracking the NASA mission 45 minutes before it enters the martian atmosphere; an ESA ground station will also record vital signals.

The highlight of ESA’s support for NASA’s Curiosity landing happens at 06:29 on Monday, 6 August, when the Mars Express Lander Communication (MELACOM) system is switched on. Recording of the radio signals transmitted by the Mars Science Laboratory (MSL) is planned to begin at 07:09 and end at 07:37 (all times shown as ground event time in CEST).

ESA’s ground tracking station in New Norcia, Australia, will also listen and record signals from the NASA mission at the same time.

CEST = UTC + 2 hours
Earth time = Mars time + 13min:48sec
MEX: Mars Express
MSL: Mars Science Laboratory
NNO: ESA New Norcia station
AOS: Acquisition of signal
S/C: Spacecraft
All times subject to change

Event Earth CEST Earth UTC S/C UTC Notes
DSS-15 (G/S) AOS 4:03:00 2:03:00
NNO AOS MEX 4:03:00 2:03:00
NNO AOS MSL 4:05:00 2:05:00
DSS-15 LOS MEX 6:05:00 4:05:00
NNO LOS MEX 6:05:00 4:05:00
NNO start recording MSL signals 6:25:00 4:25:00
MEX starts slew to point at MSL 6:06:30 4:06:30 3:52:42
MEX MELACOM reciever ON 6:28:48 4:28:48 4:15:00
MEX ends slew to point at MSL 6:36:38 4:36:38 4:22:50 Now pointing at MSL
MEX starts recording MSL signals 7:08:48 5:08:48 4:55:00
MSL Cruise Stage separation 7:14:34 5:14:34 5:00:46 MSL starts transmitting
MRO UHF TM capture starts 7:21:34 5:21:34 5:07:46
MSL Atmosphere entry 7:24:34 5:24:34 5:10:46
MSL Start plasma attenuation 7:26:13 5:26:13 5:12:25
ODY UHF bent pipe relay start 7:26:34 5:26:34 5:12:46
MSL End plasma attenuation 7:27:13 5:27:13 5:13:25
MSL Parachute deploys 7:28:46 5:28:46 5:14:58
MSL Heat shield separation 7:29:07 5:29:07 5:15:19
MSL Backshell separation 7:30:40 5:30:40 5:16:52
MSL Curiosity separation 7:31:17 5:31:17 5:17:29
Curiosity touchdown 7:31:37 5:31:37 5:17:49 Planned
MEX MELACOM reciever OFF 7:36:48 5:36:48 5:23:00
ODY UHF bent pipe relay end 7:37:37 5:37:37 5:23:49
MEX starts slew to point at Earth 7:38:58 5:38:58 5:25:10
NNO stop recording MSL signals 7:40:00 5:40:00
MEX ends slew to point at Earth 8:09:46 6:09:46 5:55:58 Now pointing at Earth
MEX transmitter ON 8:09:48 6:09:48 5:56:00
MEX start sending TM 8:15:00 6:15:00 6:01:12
MEX start recording download 8:15:31 6:15:31 6:01:43
MEX stop recording download 8:40:31 6:40:31 6:26:43
ESOC team passes data to NASA 8:42:00 6:42:00