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).
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