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New Kind of Orbit Could Ease Mars Communications
japan_dan writes "An interesting way to enable Earth-Mars communication when the Sun occludes the direct radio line-of-sight: ESA proposes placing a pair of continuous-thrusting relay satellites, using a solar electric propulsion system — one in front and ahead of Mars, the other behind and below — with both following non-Keplerian, so-called 'B-orbits'. This means the direction of thrust is perpendicular to the satellites' direction of flight, allowing them to 'hover' with both Earth and Mars in view. Quoting from the Q&A: 'We found that a pair of relay satellites would only have to switch on their thrusters for about 90 days out of every 2.13-year period, and this solution would only increase the one-way signal travel time by one minute, so it could be effective.'" Here is the paper describing non-Keplerian orbits (PDF).
Would it?
Mars has an aphelion (maximum distance from sun) of 250 Gm, and the Earth has an aphelion of 150 Gm. So when the sun is occluding their line of sight, they are on opposite sides of the sun and are separated by at most 400 Gm. If you had a satellite in the Earth's L4 or L5 point, then this would form a 150,350,400 Gm triangle with Mars. Thus the total signal distance would be 500 Gm. This would add 100 Gm, increasing the transit time by 5.5 minutes (from 22.2 to 27.7 minutes). Not as good as the solution presented but not twice as long.
Placing these in the Earth's orbit, rather than Mars', would have the added advantage of solving the solar occlusion problem for anything we send out into the solar system, not just for things on Mars.
Yes, and that's the whole point - when the planet is blocked, the Lagrange points would be visible to use for a relay.
Look up 1940's science fiction about the Venus Equilateral Relay Station by George O. Smith http://en.wikipedia.org/wiki/Venus_Equilateral
"Surely there is a stable point somewhere above the sun?"
No.
Gravity is always pulling you down, but there are places in the solar system where gravity balances out. These are called Lagrange points and space agencies use them as stable places to put spacecraft. If you're not in one of those places, you're happily going to fall on/in-to the object or end up in some sort of orbit going around the object, but you're not going to be motionless or synced up with anything.
All stable points within our solar system (L1/L5) are on the ecliptic plane iirc.
"Kill 'em all and let Root sort 'em out"
The receiver/transmitter on these satellites and space probes are very small. Generally they transmit using only a few watts, and we rely on huge antennas like in the ubiquitous dishes in the Deep Space Network to gather enough of that minuscule signal to distinguish it from background noise. Going the other way, we use the same huge antennas to blast commands to these spacecraft at anywhere between 5-500 kW. By the time the signal reaches the spacecraft, it has dissipated substantially, but its original broadcast power was high enough that the spacecraft's relatively small antenna can still collect enough of it to distinguish the signal.
Putting a repeater spacecraft at the L4 or L5 points would place them a substantial distance from Mars. Consequently the repeater would need a very large antenna and large amounts of power (though not as big/much as earth-based antennas) in order to relay signals to/from a spacecraft on Mars. The idea presented in the paper is more akin to what we do right now with the two Mars Rovers and several of our Mars orbiters. The Rovers themselves have weak antennas and can't communicate directly with Earth except at low data rates. Instead, they transmit their data to the orbiters (same antenna can achieve higher bandwidth since the distance is much less), which then relay it to Earth using their much larger and more powerful antenna.
(Introduction to channel capacity for those who may be wondering what the relationship is between data transmission speed and signal to noise ratio.)