When Mission Control in Houston lost all communication with the space shuttle Columbia during its return to Earth on Feb. 1, they knew something was terribly wrong.
In years past, the officials at Mission Control may not have blinked an eye when they lost a signal for a few minutes. But this was different because the communication blackout that used to occur as the shuttle entered the atmosphere no longer happens.
Back when a communication blackout was normal, the signal always came back after a few minutes, and Mission Control would hear the astronauts' voices again and would continue monitoring data.
This time, when communications and tracking of the shuttle were lost at 9 a.m. EST at an altitude of 203,000 feet over north central Texas, it never returned. The space shuttle had broken apart.
The media and other information services still report a communication blackout as a routine part of the shuttle's reentry into the atmosphere. But the blackout hasn't been an issue since December 1988, according to Roger Flaherty, deputy program manager for NASA's Tracking and Date Relay Satellite System.
The harshest environment
When the shuttle enters the Earth's atmosphere, tremendous heat builds up around the shuttle, and portions of the spacecraft's exterior reach 2,800 degrees Fahrenheit. The heat strips electrons from the air around the space shuttle, enveloping it in a sheath of ionized air that used to block communications anywhere from four to 16 minutes. The ionized particles around the shuttle are from ionization of the atmospheric gases as they are compressed and heated by the shock, or heated within the adjacent boundary layer. When the electron density rose high enough to exceed the critical plasma density of the link frequency, the result was significant attenuation or blackout.
The maximum heat buildup for Columbia probably occurred somewhere near an altitude of 200,000 feet while it was traveling around 12,500 miles per hour, which is about the time communication was lost for good.
Before the end of 1988, the shuttle entered blackout about 30 minutes before touchdown anywhere from 400,000 feet to about 200,000 feet. Radio signals between the spacecraft and the ground could not penetrate that sheath of ionized particles.
John Glenn reported in Newsweek that when he was watching the NASA channel in anticipation of the Columbia landing, he knew the agency had serious problems.
“Back in the old days, it was normal to lose contact for about four minutes during the highest heat of reentry, but the blackout period now is less,” wrote the former astronaut, who first orbited the Earth in 1962. “I turned to my wife, Annie, and said, ‘This is big-time trouble.’”
Of course, the blackout period is non-existent now, but older missions faced the loss of signals.
“In a sense, the blackout — the famous blackout — was part of space lore, the way it happened,” Flaherty said.
Mercury, Gemini and Apollo experienced several-minute blackouts during their atmospheric reentry phases. According to an article in The Interplanetary Network Progress Report from California Institute of Technology's Jet Propulsion Laboratory, NASA conducted several experiments in the 1960s involving Earth atmospheric reentry. The article, written by D. D. Morabito, stated that the use of an X-bandsystem over lower frequency bands was proposed. Another scientist compared predicted and measured communications blackout boundaries of atmospheric density and velocity profiles for Apollo.
No more blackouts
The solution came about after NASA launched the Tracking and Data Relay Satellite System. The first satellite was launched in 1983, and the next two went into orbit in 1988 and 1989. (One TDRS satellite was lost in the 1986 Challenger accident.) The system was built to provide communications for all space flights, from launch to reentry.
When the shuttle enters the atmosphere, the brunt of the heat is on the underside of the orbiter. The thermo protection tiles are facedown, so the plasma or ionization layer is open at the trailing end behind the shuttle, providing a hole through which communications with the shuttle can be maintained with the TDRS. Even if the TDRS satellites had been in use when Mercury, Gemini and Apollo were in flight, the spacecrafts still may have experienced blackouts because of their body shapes.
“The worst was the old Mercury, Gemini and Apollo because they were round hemispherical domes,” said John Wickman, president of Wickman Spacecraft and Propulsion.
The shuttle is shaped more like a plane, so as it comes into the atmosphere belly-down, “the coolest part of the orbiter has no plasma or radiation that will interfere with the radio frequency communications from that edge,” Flaherty said.
NASA found that if the radio signal was sent back up to the satellite and then down to the ground, they didn't even need to try to communicate through the plasma layer.
“Ionization still occurs, but we don't try to send that signal through that layer. We send it up through the atmosphere, so we're sending it away from the layer. The layer is on the bottom where the orbiter is the hottest,” said Catherine Watson, NASA spokesperson.
The orbiter, or shuttle, has multiple conformed, S-band medium-gainon a blister on the skin of the orbiter, according to Flaherty.
“The top of the orbiter has four S-band antennas — front left, front right, rear left, rear right — so that those antennas are commanded to optimize communications paths,” he said.
The orbiter primarily uses the S-band at 1,700 to 2,300 MHz for communications during entry. It supports voice, commands, telemetry and data files. The data rate is up to 192 kbps. The shuttle uses the Ku band (13.755 and 15.003 GHz) for communications while in space.
As the orbiter changes attitudes while preparing for landing, it automatically selects the antenna to optimize the communications link with the satellite. However, the antenna selection can be controlled from the ground if needed.
Even with a drop out here or there, communications is much more fluid than a total blackout for minutes.
NASA did not direct all of its plans for TDRS around solving this specific blackout, however.
“I think it was sort of serendipitous,” Flaherty said. “I think it may have been something people had thought about and said, ‘Gee, you know we're not shooting up through the plasma layer anymore, and there's a possibility by looking down on the orbiter here that we may be able to maintain communications right through what was then known as the blackout period.”
NASA launched what is known as the TDRS-3 satellite on Sept. 30. 1988, and the first two missions shortly after that would not have experienced the blackout period. The satellite's position as “west” supported shuttle communications.
Nine TDRS satellites have been launched into orbit since 1983. The ground station for this communications network is located in White Sands, N.M. This station contains the ground terminal communications relay equipment for the command, telemetry, tracking and control equipment of the TDRS.
“It's a very, very reliable system,” Flaherty said. “We're 99.91 percent proficient and that is for every minute of scheduled data over minutes of actual support.”
Each TDRS satellite contains two steerable single access antennas that provide dual-frequency single-access telecommunications at K and S band. The space-to-ground link antenna on the satellite provides the link between TDRS and the ground station. It's a two-meter parabolic reflector antenna. The satellite also has multiple-access antennas and an S-band omni directional antenna.
The initial fleet of TDRS satellites (TRDS 1-7) was built by TRW of Redondo Beach, Calif., but the last three (TDRS 8, I, J) were built by Boeing Satellite Systems.
“TDRS 8 is an operational spacecraft, and TDRS I and J have been launched and are in the process of being accepted by NASA at this time,” Flaherty said.
TDRS 8 was the first of the second-generation replenishment satellites.
With TDRS being fully operational, Mission Control was receiving telemetry and voice communications up until the moment the shuttle was seen breaking apart. With no blackout, continual communications may help investigators solve the mystery of the accident.
“The more communications, the more data you have, the better off we always are. For instance, we have gone to using TDRS and space-based relay to support launching vehicles,” Flaherty said. “We can get launch data throughout the entire launch period. If something does go wrong at least you can go back and look at the data and see if there are any telltales.”
As Columbia entered California air space, in fact, the first hints of trouble had surfaced, according to NASA. In the final seven minutes of the flight, Jeff Kling, the maintenance, mechanical arm and crew systems officer, reported a sudden and unexplained loss of data from spacecraft sensors, according to transcripts released by NASA.
“I just lost four separate temperature transducers on the left side of the vehicle, the hydraulic return temperatures,” Kling said.
The flight director then asked if there was anything common to the sensors. Kling said there was no commonality, suggesting a general failure.
At that point, everything appeared normal with shuttle flight control.
Kling then said that landing gear tires had lost pressure.
Houston: “Columbia, Houston. We see your tire pressure messages, and we did not copy your last.”
Commander Rick Husband: “Roger, buh…”
He was cut off at 8:59 EST.
A capsule communicator tried a series of radio calls to Columbia with no response.