It had been a successful mission, and the crew prepared to slowly start the long controlled fall home after completing the retroburn from the Orbital Maneuvering System. But unbeknown to them, the first hint of trouble appeared a few minutes after Entry Interface, defined as 400,000 feet altitude.
The vehicle was still moving at about Mach 24 — twenty-four times the speed of sound. The yaw moment had changed almost imperceptibly, with a gentle tug to the left — there was a slight asymmetry in the aircraft’s aerodynamics, but no one on the ground or in the cabin noticed it at the time.
Twelve seconds later, a temperature sensor indicated that a hydraulic brake line in the left wing was warmer than it should have been, and was slightly out of specification in that regard. As the vehicle was gradually slowing down, perceived gravity slowly growing to an indiscernible hundredth of a gee, fiery hot plasma was infiltrating through the hole in the leading edge, insinuating itself into the interior of the wing and starting to damage it, but none realized it yet.
A minute or so later, as they approached the California coast from three hundred miles out, an off-nominal rolling moment appeared, more evidence of subtle changes to the vehicle’s outer shape. A few seconds later, Mission Control received a signal that several sensors were starting to indicate problems, but no one on board or in the control room was yet aware.
Half a minute later, still going Mach 23 after crossing the coast line, observers on the ground in California saw an eight-pound piece of the left wing separate from the vehicle, creating a luminescent trail in the plasma, though they didn’t know at the time what they were seeing. No one in Houston or in the cabin was aware of this. About the same time, the side-slip angle exceeded any previous entry experience, as the vehicle was no longer moving in a pure forward motion. A little over half a minute later, the left elevon started to trim to compensate for the off-nominal forces, but no one noticed at the time.
A few seconds later, as the craft crossed the border into Nevada, someone in Mission Control finally noted the temperature-sensor anomalies from a couple minutes earlier.
A few more seconds after that, as the crew was doing a pressure check on their suits in preparation for landing, the brightest piece of debris was shed, but the sensors indicated nothing, and no one on board saw it.
The crew started to really sense the deceleration — about a third of a gravity — a minute or so later, as the dynamic pressure increased to forty pounds per square foot. This was a value that allowed the aero surfaces to take over from the small rockets that had been controlling the vehicle’s attitude. Thirty seconds later, they commenced the first roll reversal to bleed off excess energy, from right-wing low to left-wing low, starting the standard series of S-turns as they approached their eventual landing site. For the veterans on board, the entry seemed to be going normally.
But a picture taken from an infrared camera in New Mexico showed some bulges in the flow field on the left wing that couldn’t be explained by the vehicle attitude. No one saw it until later. The trim on the elevon was now departing sharply from a standard entry, as it fought to maintain the nominal attitude in the face of increasing asymmetric aerodynamic loads.