Caption: This is an end-on view of an alternative Mars/Earth communication relay architecture option, looking into the Ecliptic plane. Credit: Credits: ESA/University Strathclyde/University Glasgow
According to the paper, "Non-Keplerian Orbits Using Low Thrust, High ISP Propulsion Systems," an innovative solution to the Mars communication problem may be found by placing a pair of communication relay satellites into a very special type of orbit near Mars: a so-called 'B-orbit' (in contrast to an 'A-orbit', based on natural orbital laws).
Ion thrusters, powered by solar electricity and using tiny amounts of xenon gas as propellant, would hold the satellites in a B-orbit in full view of both Mars and Earth. The satellites could then relay radio signals throughout the Mars–Earth conjunction season, ensuring that astronauts at Mars were never out of touch with Earth.
The research was only the first step in understanding the complex details of such a mission. A lot more work must be done to understand in detail how the satellites have to apply the thrust - for example, taking into account the natural eccentricity of the Martian orbit. Also, failure scenarios must be studied, to have a back-up plan in case one of the ion thrusters failed. In addition, as part of their research, they catalogued other possible mission profiles.
One example would be to use continuous thrust to create a fixed, virtual 'truss' between two spacecraft perpendicular to their flight direction. It would be like having the two spacecraft connected by fixed bar or rod; this could be useful for certain applications.
Another example would be to hover near one of the Earth-Sun system Lagrange points. NASA studied just such a mission profile, called GeoStorm, back in the 1990s with a view to stationing a satellite closer to the Sun than the L1 Lagrange point so as to provide improved early warning of magnetic storms caused by solar coronal mass ejections. Such a mission would have used a solar wind sail for its thrust, but it could also be done using ion propulsion, which can offer control advantages compared to solar sails; this must be studied further.
There's still lots to be done, but this research will help pave the way for future robotic missions to places we've never been or for a human mission to Mars.