Distant retrograde orbits and the asteroid hazard

Distant Retrograde Orbits (DROs) gained a novel wave of fame in space mission design becauseof their numerous advantages within the framework of the US plans for bringing a large asteroid samplein the vicinity of the Earth as the next target for human exploration. DROs are stable solutions of thethree-body problem that can be used whenever an object, whether of natural or artificial nature, is requiredto remain in the neighborhood of a celestial body without being gravitationally captured by it. As such,they represent an alternative option to Halo orbits around the collinear Lagrangian points L 1 and L 2 .Also known under other names (e.g., quasi-satellite orbits, cis-lunar orbits, family-f orbits) these orbitalconfigurations found interesting applications in several mission profiles, like that of a spacecraft orbitingaround the small irregularly shaped satellite of Mars Phobos or the large Jovian moon Europa. In thispaper a basic explanation of the DRO dynamics is presented in order to clarify some geometrical propertiesthat characterize them. Their accessibility is then discussed from the point of view of mission analysisunder different assumptions. Finally, their relevance within the framework of the present asteroid hazardprotection programs is shown, stressing the significant increase in warning time they would provide in theprediction of impactors coming from the direction of the Sun.