ESA proposes ion drive Sun-dodge Mars commsat ploy Experts in interplanetary navigation have hit upon a novel scheme for maintaining communications with Mars, should a need to do so eventuate - as in the case of a manned mission to the red planet, for instance. Diagram of the proposed Mars commsat orbits, looking in the plane of the ecliptic. Credit: Universities of …
"...the time taken for messages to travel back and forth - already some hours when Earth and Mars are far apart."
The Earth is 'only' 8 light minutes from the Sun and is 1 AU from the Sun.
Mars orbit is about 1.5 AU from the Sun (you're getting the sense of scale here...).
So, how come it takes 'some hours' for a signal return trip Earth-Mars when Mars is at the other side of the Sun? It should take about 40 minutes. (I've drawn a diagram, I think I know what I'm doing here). Even allowing for elliptical orbits and wobbles etc, I don't see how they can say 'some hours'.
Interesting idea. It seems Mars-Sun L4 and L5 have asteroids, which might be a bit inconvenient. they're both 750 light seconds from Mars, so radio links would still be slow.
I don't have a chart to hand, so I'm not sure if Earth-Sun L4 and L5 would also be occluded during conjunction.
"...the time taken for messages to travel back and forth - already some hours when Earth and Mars are far apart."
No they don't - trip times to Mars vary from just over 3 minutes when Earth is closest to the planet to about 22 minutes when they're on opposite sides of the Sun. Double that for a round trip. If you're being put on hold for 'some hours' when calling Mars you should consider changing your operator.
The problem isn't the time it takes to get the information there, it's the amount you can get. Ever watched a video on youtube using dialup? Well, dialup is fast compared to what you can get
when you comunications over this long a distance, because signal strench will cause serious problems. That's why getting a message there takes a long time.
"Well, dialup is fast compared to what you can get when you comunications over this long a distance..."
Case in point, during the recent LCROSS mission, the controllers clearly dropped the bandwidth they were prepared to allocate to one of their live cameras to 0.1Mbps. And the moon is several hundred times closer than Mars, and radio signal strength falls off with distance squared (which is another reason why Lagrange points aren't as useful as they might seem).
Second case: the transmitter on Voyager 2 is about 30 watts and can still be picked up by the Deep Space Network, but only at very low bandwidths. During the fly past Neptune, they needed more bandwidth and had to use the Very Large Array instead. According to http://hireme.geek.nz/dsn-at-home.html, Voyager 2 is currently managing 7.2 kilobits per second (so yes, less than dial-up) when the weather is nice. (What NASA really needs is a whacking great receiver dish in Earth orbit, but that's not sexy.)
You have to extract signal from noise, so you can either send less signal (more clearly) or send a louder signal. The theoretical limit is given by the Shannon-Hartley theorem.
Sodding hell. Aint that why we have satellites?? And, which Lagrange Points (LP's)? Earth's, or Mars'? . How about parking the repeaters in moon's LP's? More sunlight == more power, Shirley.
(Oh, yep, I always wondered how NASA got a well-panned shot of the Apollo 11 LEM launching from the moon. Their explanation went something like "Camera operator anticipated the 4 seconds signal transmit time". Except, it ain't 4 seconds. It's 2.7. Only, that's return time, so it's really about 1.35. One way. Maybe moon seconds. Yo, that'll do it. NOT.
Not one to doubt Armstrong and Aldrin didn't come back with green cheese on their boots, but...)
Why not just put a relay in an inclined orbit close to, and around the sun? The delay would be short. Only need 1 sat. As a bonus, it could work as a relay for all planets. (Something like Venus to sun relay sat to Earth would be better than bouncing it off of the Mars sat.) I think the chances of Mars behind the sun and an inclined relay sat in a close orbit to the sun transiting or also being behind the sun are like, super rare.
The Register 
Biting the hand that feeds IT
Copyright. All rights reserved © 1998–2024
