However, the density data indicate that Eros is a very porous rubble pile. Spectral reflectivity data suggest an LL chon-drite composition although sulfur appears depleted. Our experience with NEAR-Shoemaker demonstrates a high degree of confidence in the new techniques required to maneuver spacecraft around asteroids. Asteroid shape, size, density, and rotation state are crucial parameters that must be known to implement these operations, thus these data must be available before detailed maneuvers can begin. Additionally, the design of the entire spacecraft and mission can be simplified if constraints on these asteroid parameters can be estimated early in the mission development phase.
Asteroids present new difficulties and new opportunities.
Their small size, high rotation rates and irregular shape and the abundance of dust make landing difficult, but at the same time their microgravity on surface makes hovering feasible. There has been considerable progress in designing sample containment devices to prevent contamination of the asteroid by terrestrial organisms, the transfer of organisms to Earth or the contamination of one asteroid sample with another. Although well-developed, these procedures require that such issues are addressed from the outset of mission planning. Experience with terrestrial ecosystems demonstrates that natural transfer is not equivalent to anthropogenic transfer, for example radiation doses experienced by meteorites during transfer are high enough to destroy living material while this would not be true for samples returned by missions. The increasing numbers of NEAs means that their accessibility as natural resources has improved.