MarsSat: Assured Communication with Mars

Abstract

The author developed the MarsSat concept during the 1990s. For this task, he designed a class of orbits to solve the problem of communicating with crews on Mars when the planet is in solar conjunction as seen from Earth, a planetary configuration that occurs near the midpoint of a conjunction class mission to Mars. This type of orbit minimizes the distance between Mars and the communications satellite; thus, minimizing the size, weight, and power requirements, while providing a simultaneous line-of-sight to both Earth and Mars. The MarsSat orbits are solar orbits that have the same period as Mars, but are inclined a few degrees out of the plane of the Mars orbit and also differ in eccentricity from the orbit of Mars. These differences cause a spacecraft in this orbit to rise North of Mars, then fall behind Mars, then drop South of Mars, and then pull ahead of Mars, by some desired distance in each case-typically about 20 million kilometers-in order to maintain an angular separation of a couple of degrees as seen from a point in the orbit of Earth on the opposite side of the Sun. A satellite in this type of orbit would relay communications between Earth and Mars during the period of up to several weeks, when direct communication is blocked by the Sun. These orbits are far superior for this purpose when compared to stationing a satellite at one of the Sun-Mars equilateral Lagrangian points, L(4) or L(5), for two reasons. First, L(4) and L(5) are 228 million kilometers from Mars, about 10 times the distance of a spacecraft in one of the MarsSat orbits, and by virtue of the inverse-square law, all other things being equal, the signal strength received at L(4) or L(5) would be one percent of the signal strength received by a spacecraft in one of the MarsSat orbits. Thus, a relay satellite stationed at L(4) or L(5) would have to be that much more powerful to receive data at the same rate, with concomitant increases in spacecraft size and weight. Second, a number of Martian Trojan asteroids have been discovered at the Sun-Mars L(4) and L(5) points, and there are probably countless smaller objects that have collected in these regions that pose a significant threat to any spacecraft located there.