When the first of two NASA rovers arrives on Mars next Saturday, a Denver-built orbiter will be the key link in a new system to relay vital landing data back to Earth. Mars Global Surveyor will pass 250 miles overhead on Jan. 3 when the the first of the golf cart-sized rovers, Spirit, streaks to its landing site. Spirit is scheduled to arrive at Gusev Crater, a giant impact basin that may once have held a lake, at 9:35 p.m. MST. About a minute before it bounces to the surface, Spirit will begin transmitting detailed information about its status to Global Surveyor. About two hours later, those vital signs will be beamed back to Earth.