Complex sentence: Like the Apollo LM, Altair's crew module is based on a sideways cylinder, but unlike its two-man Apollo ancestor, Altair is designed to carry the entire four-man mission crew to the surface while the Orion Spacecraft remains unmanned in lunar orbit.
Split into simpler sentences: Like the Apollo LM, Altair's crew module is based on a sideways cylinder. Unlike its two-man Apollo ancestor, Altair will carry the entire four-man mission crew to the surface while the Orion Spacecraft remains unmanned in lunar orbit.