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Shuttle probe follows a trail of data

A more detailed picture of Columbia’s final moments could point investigators toward what caused the disaster.
/ Source: NBC News

Investigators looking into the loss of the shuttle Columbia and its seven astronauts on Feb. 1 are putting the finishing touches on the collection of raw data from the final moments of flight. Now their analysis is shifting to interpreting those findings and “walking back” the reconstruction of these events to try to find the cause of the catastrophe.

Laborious decoding efforts have revealed much about the last 32 seconds of telemetry signals sent back in a form too garbled to be understood in real time. Results of this analysis were contained in two “timelines” released last week.

Furthermore, sources within the investigation have told that investigators hope an avionics box recovered several weeks ago may contain critical data about the exact moment Columbia broke apart. Part of the shuttle’s Global Positioning System receiver, the device has been sent to the vendor in Iowa in an attempt to read the last “state vector” — the precise position and time when the power was cut off by the cabin’s separation from the rest of the fuselage.

Telemetry from the shuttle details a graphic sequence of events as Columbia’s autopilot struggled to maintain control against the growing air drag on its left wing. In the final seconds, the shuttle was already tumbling and pieces were breaking off. first described these preliminary interpretations on Feb. 24, based on sources familiar with the investigation. But for two weeks afterward, representatives of the Columbia Accident Investigation Board continued to say the data they had seen showed that the shuttle was flying under control of its aerosurfaces and steering thrusters right up until the end of data transmission. These most recent timelines now show this could not have been true.

Interrupted communications
During entry into the atmosphere, communications between Columbia and Mission Control in Houston were transmitted via a relay satellite high over the Pacific Ocean. Ionized plasma surrounded the spacecraft and blocked direct radio transmissions from the shuttle to the ground, creating a “blackout” during this period. But the plasma was thinner above and behind the shuttle, and for the past dozen years or so NASA has adapted its space-to-space communications relay system to circumvent the blackout effect.

Normal communications with Columbia were maintained until 7:59:32 a.m. Texas time. Since the shuttle was in a steep left bank and the relay satellite was “setting” — getting low to the horizon — the line of sight from the transmitter to the satellite passed near the shuttle’s tail. As a result, the signal was interrupted. NASA operators, aware of this geometric relationship, were not alarmed at first. But the radio link was never fully restored, even though some unreadable data was recorded at a ground station.

The first five seconds of garbled data were later reconstructed by analysts. It shows that the autopilot declared a “Master Alarm,” but the exact code could not be read. This told the crew that something was seriously wrong.

Analysis also shows that the shuttle’s orientation began to shift, with the “sideslip” — the direction of air across the wings — changing by several degrees. ”[Autopilot] drops left wing to compensate for increasing aerodynamic [twist],” NASA’s timeline notes. The shuttle was also firing two steering jets to keep the nose from being pulled to the left.

In a last desperate attempt to maintain proper pointing, the autopilot activated two more steering thrusters, all it had available. The ailerons on the wings were commanded to higher and higher deflection, until the very last indication. “The rate of change had reached the maximum allowed,” says the report. Then — silence. No signal at all, not even a garbled one, was received.

All investigators who have talked with privately presume that this total radio cutoff indicates the shuttle had begun veering to the left. This could have pointed its antenna away from the relay satellite.

Messages within garbled data
Twenty-five seconds later, at just three seconds after 8 a.m. CT, a final two-second burst of garbled data began. It contained information about Columbia’s current situation, as well as a log of error messages that had been sounded during the 25 seconds of total silence.

Nine seconds into the silence, the computers had sounded a “Roll Reference” alarm, which informed the crew that the autopilot was having difficulty keeping the shuttle in proper roll orientation. “Message generation less than ten seconds after start of [all] yaw jets firing suggests rapid change in Lift to Drag ratio,” NASA’s report states. In plain English, the shuttle was literally changing shape, and the computers knew it.

Within seconds, a series of error messages came from Columbia’s left thruster pod, atop the fuselage back at the base of the tail. Thrusters were leaking, then fuel tanks were leaking, and finally most of the telemetry readings from the entire pod went out.

The most common interpretation for these readings and for ground observations is that the left wing folded up, impacting the thruster pod. Within seconds, according to NASA’s report, “Large debris [was] seen falling away from the Orbiter.”

This was probably the wing, which held together most of the way down, as shown by the small “scatter” pattern of pieces that were later recovered.

Without the left wing, the shuttle would be twisted into a hard left roll under the lift forces from the right wing. Combined with the left yaw that began soon after its thrusters and ailerons lost their battle to preserve stability, this would create a rapid end-over-end cartwheel motion.

NASA’s reconstruction of the last two seconds was expressed in dispassionate engineering terminology that could not disguise the horrible situation. “Data suggests vehicle was in an uncommanded attitude,” the report states, “and was exhibiting uncontrolled rates.” Sensors for angle rates have a maximum value of 20 degrees per second, and the yaw sensor reading was “pegged high” — the actual rate could have been much higher. The pitch and roll rate measurements were unreadable but probably below the maximum value.

The report confirmed earlier partial announcements that the shuttle’s hydraulic power system had totally failed, with zero pressure and fluid levels in all three systems. This is another indication that the left wing was totally gone.

Although the autopilot was still in command of the shuttle, there is some indication that one of the control sticks was deflected, an action that can return the computers to manual mode. But it’s equally possible that the deflection was caused by an inadvertent bump by one of the pilots’ knees.

Final breakup
Thirteen seconds after the end of this burst of data, video taken from the ground shows more big pieces coming off the main body of the shuttle. Forty miles high, Columbia was crossing Interstate 45 just north of Corsicana, Texas. The NASA report specifies “Vehicle Main Body break-up” at between 21 and 25 seconds after 8 a.m. CT.

More precise timing of the breakup may be extracted from the Global Positioning Satellite unit recovered two weeks after the crash. According to reports from independent sources familiar with the investigation — but unconfirmed by the Columbia Accident Investigation Board — the unit was in such good shape that its emergency battery may have preserved the last frame of data prior to loss of electrical power. Reportedly the unit has been shipped to the vendor in Iowa for data recovery.

Since power to the crew cabin is provided from fuel cells underneath the shuttle’s payload bay in the mid-fuselage, investigators tell that the moment of power loss would probably coincide with the time the fuselage broke into pieces.

Reconstruction of what became of Columbia’s pieces and contents after breakup will require analysis of the scatter patterns and physical conditions of related items on the ground. The Columbia Accident Investigation Board has told NBC News that this has not yet been done.

But a more precise understanding of the final moments of the shuttle’s flight, developed through data analysis, is proving useful in deducing what physical abnormalities were present, based on a much more detailed understanding of their effects on Columbia’s flight. Experts associated with NASA and the Columbia Accident Investigation Board are now focusing on an assessment of how these abnormalities originated and developed.

James Oberg, space analyst for NBC News, spent 22 years at the Johnson Space Center as a Mission Control operator and an orbital designer.