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I've heard that rovers on the surface of Mars can only communicate with and send data to orbiters for only 8 minutes at a time.

Wouldn't two areostationary_orbit satellites orbiting opposite to each other allow every rover on the surface of Mars, no matter where it is, to have a constant and unlimited communication line to send data?

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Two satellites is never enough. In the picture below two satellites can cover the surfaces of Mars between the pairs of tangent lines enveloping each satellite, but a belt (shown in blue cutting through Mars) is left uncovered because it is outside the region between the pairs of tangents. On Mars or any other round body, you always need at least three satellites to have at least one of them communicating at every longitude around the Equator.

enter image description here

Moreover, the method of multiple areostationary orbits cannot work at all latitudes. This is basically because the satellites in this method must be centered on the Equator and the tangents I have drawn can never reach the poles from there. In practical Martian terms this means we would not be able to rove the polar ice caps freely with an areostationary configuration. There are configurations that cover the entire planet such as this approach with four satellites, but they do not use the areostationary orbit.

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    $\begingroup$ It's not necessarily to scale, I just made a sketch. If you are comparing the synchronous height to Earth, remember that Mars also has less than half the radius of Earth. $\endgroup$ Dec 28 '20 at 0:23
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    $\begingroup$ And even three will leave gaps at the poles. Not very important on Earth, potentially more so on Mars. $\endgroup$ Dec 28 '20 at 0:57
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    $\begingroup$ With the Draim Tetrahedral Configuration, though, you wind up with full coverage with four satellites in inclined, elliptical orbits that each satellite has constant line-of-sight to the other three, and every point on the surface has line-of-sight to at least one of them at all times. $\endgroup$
    – notovny
    Dec 28 '20 at 1:29
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    $\begingroup$ Agree @uhoh, even after I referenced a tetrahedral configuration. $\endgroup$ Dec 28 '20 at 1:54
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    $\begingroup$ It's also true that, even on Mars, using a comminications satellite that is only just above the horizon is a problem -- hills and boulders can block your view very easily. $\endgroup$ Dec 28 '20 at 10:50

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