Cape Canaveral . Biological, microgravity experiments aboard Spacelab 2. When it cleared and the count resumed, a failure in an Air Force Range Safety command message encoder verifier at the Range Control Center canceled the launch at the T-31 seconds mark. This system is used to transmit a vehicle destruct signal if it should become necessary.
Flight rules require that both communication transponders be functional for launch.
Launch occurred at just ten seconds inside the scheduled liftoff window. The minimal delay was due to a stray U.S. Navy aircraft in the range safety restricted zone. No serious technical issues were worked during the countdown. Preliminary data indicate that the flight performance of both RSRMs was well within the allowable performance envelopes, and was typical of the performance observed on previous flights. Nominal pressure is 650 psi for that time frame.