Spacecraft event time vs. Earth receive time
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!
Discussion: 68 comments
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.
Em, don’t radio waves travel at the speed of light in vacuum？
Yes radio signal is fast but light travels faster
Orbital distance is lower. Orbits are different. Therefore, whilst light speed = radio speed. distance travelled depends on orbital location distance!
Both travel at the speed of light, it takes 13 minutes for us to send a signal and get a signal back, it goes both ways so it takes a while. To just send a signal there, it takes 6 minutes and 30 seconds.
No, all parts of the electromagnetic spectrum travel at the speed of light in a vacuum; they simply have different wavelengths/frequencies.
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
A “photon” is the carrier particle for both visible light AND radio waves, both of which are forms of electromagnetic energy, just at different frequencies. “Photonic energy” (which I’ve never heard of) would be synonymous with electromagnetic energy. Light, heat, gamma, radio, microwaves, ultraviolet – all are mediated by photon particles, but possess properties of waves as well, hence the “frequency” characteristic. Easiest to think of them as waves while they are traveling, but particles when they strike something.
And those numbers only apply to light traveling in vacuum. Light passing through matter travels measurably slower. I think some lab recently performed an experiment in which they “stopped light” inside a substance by slowing it waaaaaay down somehow. Don’t know how they did it, I didn’t read the whole thing.
would it stand to reason then, that certain frequencies of electromagnetic energy travel “faster” as they interact less with most forms of matter and thus have less interference when traveling large distances than other wavelengths traveling the same path and distance?
for example, inferred light compared to gamma-radiation, inferred light would have a higher chance of interacting with a medium compared to gammarays delaying the wavelength as it passes through minuscule amounts of matter. Could one say this would result in gammarays having a faster “net speed” on average?
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.
Just came across this:
Radio signals are electromagnetic waves, such as light or X-ray. The speed of electromagnetic waves in vacuum, is 300000km/sec (approximately).
In order to calculate the time of travel with this speed from Earth to Mars, we need to know the distance.
When the Mars and the Earth are at the opposite sides of the Sun, the distance is the largest: approximately: 378 million km. The time needed for an electromagnetic wave to cover this distance is approximately: 21 minute.
The closest distance between Mars and Earth is 78 million km, the time in this case is: 4.3 min.
So the time of travel between Earth and Mars is between 4.3 minutes and 21 minutes, depending on the actual distance between the two planets.
Shab, you are calculating as if it is 2D plane, while DoctorZuber already mentioned the earth and mars elipses are of different plane.
So when Mars is at the exact opposite of Earth and the Sun is directly in between. Would it be possible for radio to even transmit to Earth Via right thru the sun?
You are wrong. It is because the earth is often much further from Mars than Mars is from the sun.
Usually I mean. The distances vary greatly, of course.
yo wassup guys whats lite
Guys guys guys u are over anylizing everything. It’s cause mars is further from earth than the sun sometimes cause of orbits
Yes they do. (Buy better science books.)
No, it takes 13 minutes round trip. Half that from Earth to Mars (depending on relative positions of both planets in their orbits). To my knowledge, there is no such thing as “photonic.” Light is electromagnet waves (or photons depending on the nature of the experiment you use to determine that – see “double slit light experiments”). All electromagnet waves travel at the same speed in space.
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
Just multiply miles by 1.8 to get equivalent kilometers. Or kilometers by .6 to get miles. Now you should be covered unless someone here starts talking about rods or leagues or something weird like that.
Actually, 1 mile = 1.625 km.
Just to assure accuracy for the people visiting. If you decide that you would like to do any maths related to this subject, here are the actual conversion values for U.S. and Metric measurements of distance:
1 mile = 1.609334 km (1.609 is close enough)
1km = 0.621371192 mile (0.621 is OK also)
No, we won’t use metric exclusively. Metric isn’t simpler, and miles are well established not only in the USA but in the UK too, and the UK helps to fund the ESA and the EU, so the ESA and the EU should use the BOTH measurement systems, instead of trying to impose metric on everyone.
Metric has been established in the UK for decades. School science has used SI and not Imperial for decades. Trying to use both systems would waste time and risk dangerous errors.
Meters are part of the SI system of units (https://en.wikipedia.org/wiki/International_System_of_Units). It becomes very complicated to use any standard derived unit such as Newton (Force) expressed by miles and pounds.
Further, 1 cubic decimeter equals 1 liter of water weighing 1kg which can also come in quite handy in everyday life. This is quite a trick in inches, fluid ounces, and pound 😉
No we don’t. You can use your metric system and we can use our British/American system.
We don’t speak one language, and we don’t need one system of measurement.
It’s good to be fluent in multiple systems.
There is a good reason to stick to metric when discussing scientific matters. Several space missions have failed disasterously due to mix-ups translating between the two 😛
Please not distance from earth to mars is about 225 million km (not miles).
the change in distances are also one reason they have to plan specific dates when launching satellites to Mars or any other planets.
It’s more like very different because mars also orbits the sun, but slower than earth, sometimes it is from our view behind the sun and other cases behind us
That is incorrect. Distance to sun 96 million miles. Distance to Mars, 140 million.
The distance to Mars varies because the Earth and Mars are both orbiting the sun, but not at the same speed or the same distance from the sun. Sometimes we are on the opposite side of the sun from Mars, and the sun is therefore closer to us than Mars is.
dude, shit happens
If the earth is on one side of the sun and mars is on the other side, then the distance from the earth to mars is greater than from the earth to the sun. Seems pretty straight forward. All electromagnetic radiation travels at the speed of light. Gladson is wrong.
Both earth and mars are in orbit around the sun. We make the trip in 265 earth days. Mars requires 669 earth days. Mars mean orbital radius to the sun is 1.6 times that of earth. We can be MUCH closer to Mars than the sun, or 2.6 times as far away, depending on our relative orbital positions.
Round trip radio time depends on where earth and mars are relative to the sun. If we are at our closest point to mars, R/T transit time is 4 minutes. If we are at opposite sides of the sun, R/T time is 24 minutes.
365 earth days
It varies, because Earth and Mars orbit the sun at different speeds. Sometimes they are on the same side of the sun and, at closest approach, light only takes about 3 minutes to travel between the two. Sometimes they are on opposite sides of the sun and it takes up to about 22 minutes. As to the other responses, the difference between speed of light and radio waves is negligible over these short distances.
Depends on the positionsame of earth and Mars. If we are on the same side of sun we are closer. If the planets are on opposite sides of sun. The sun is closer. The sun distance from Earth is relatively constant. Mars is not.
It’s a two way signal. Query and Response.
Because mars can be on the complete opposite side of the sun compared to the earth
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
Now you are making sense, but at that point how much longer will it take the suns light to reach mars?
How much time could we have cut from our journey to mars if we timed it accordingly?
Well, you have to remember that both Earth and Mars are moving at different speeds while the rocket is en route. Google “hohmann transfer” for details on that if you’re interested.
Glad you didn’t bring in Einstein’s curved space theory! If you had then they’d be even more confused about how the Sun depresses Space/Time and therefore the distance light travels when Mars is on the opposite side of the sun as Earth then the curvilinear distance is actually greater than a straight line and the time for the message to get to Mars actually increases by the lending effect
Should be “Lensing effect!”
Really?
How do you determine the exact time when the sun spot is intitially emitted.
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.
Nobody is suggesting that there need to be fundamental changes in physics here. I’m not sure where you would get that idea.
I don’t understand. Didn’t Khan do this and end up getting the enterprise destroyed at one point?
12 minutes and 60 seconds? 13 minutes!
I just want to establish some facts here for those who are confused about physics.
1) Electromagnetic radiation and light are the same thing. That includes visible light, radio waves, and so forth. There is no such thing as “Photonic energy”.
2) All light, regardless of frequency, travels at the same speed in a vacuum, known as the “speed of light”. The speed of light in matter can vary, but in this case that is not important since all the space between mars, the earth, and the sun is vacuum. There is no air in space.
If earth and Mars are traveling in different orbits around our sun ,their distances between them is continually changing and so will be the communication speed and all this makes it even more complicated on a permanent basis to figure this out (just a bit to much for my math capabilities)
I really appreciate the skills of the modern scientists to make this all possible.
You are all the frontiersmen and women of humanity……thanks!
I was having a discussion with my Circuits professor about the speed of communication to the Rover being faster than light. My professor and I
wondered how that was possible. Thanks for all the great Info.
Bulldog6 Through quantum entanglement that would be possible however it has only been preformed on particles, scientists are yet to find a way for this to be used in a wave which is what would need to be done to communicate with the Rover at faster than light communication. For now communication with the Mars Rover happens at the speed of light until a better understanding of quantum entanglement comes into existence.
Can we discuss then through a light system rather than a radio system ?
Can We transmit using light? a laser light pointing to space satellites, and satellites to satellites by laser light ( like station to station ) then to the nearest satellites to the earth or mars…
using few laser light pointing at the same time to transmitt few data in the same time.. speed up the tranmittion.
laser light on going straight line.. so .. i guess not easy to aim..pointing from satellites to satellites.. or mars to satellites…..
its something like using scope on the satellite watch on top of the rover.. its kind like back to human early using light as signal.. but just by fire and now using laser gun which very command now..
Radio signals are a light wave and travel at the same speed as the light emitted from a laser. The advantage to radio waves is that they are more broad than a beam of light emitted from a laser so pin point accuracy is not needed to get a message from Earth to the rovers on Mars’ surface
Since we have established that light and radio waves go at the same speed, it would make no differences.
transmission time would still be the same
Wait until the planets about a hundred light years away pick up our 1960-2017 broadcasts.
They will probably shoot us with some kind of deadly weapon.Earlier broadcasts before 1960 will probably be fine, even pleasant, but anything after 1960, especially 2008 on, we are going to be in big trouble with superior species