SOFT LANDING SYSTEM: Project engineers are developing an air-bag system (above) that inflates underneath the Orion crew module to cushion the shock of landing on hard terrain, if the vehicle should do so. If the air-bag technology proves unsuitable, however, they have several alternatives in the works, including retrorocket systems.

OVERALL TRADE-OFFS: Engineers must find the optimal balance between the maximum payload masses that the Ares boosters can deliver, on the one hand, and the combination of the widest safety margins, greatest system redundancy and most useful crew accommodations on the other.

HEAT SHIELD: Although the engineers favor PICA (phenolic impregnated carbon ablator) as the capsule’s heat-shield material, building such a large structure could pose problems. Heat-resistant space shuttle tiles or Apollo’s ablator material could serve as substitutes.