The tensest moments of the shuttle Discovery's high-stakes mission should start around 3:45 a.m. tomorrow, when the spacecraft is slated to break orbit and begin brushing the uppermost molecules of the Earth's atmosphere.
This is the part of the flight when the shuttle gradually ceases to be a spacecraft and becomes something even more precarious - a hypersonic glider, subject to unimaginable stresses and temperatures as high as 3,000 degrees. Air rushes by at speeds up to 27 times the speed of sound, so fast that it is not merely wind but a chemical reactant, breaking down into atoms as it smashes against the ship in a process that can increase heating.
It was during re-entry that the last shuttle, Columbia, took in hot plasma gases through a hole in its left wing and melted in minutes from the inside out.
Although NASA says Discovery is safe to make its return, everyone involved concedes that there will be new layers of worry and emotion this time. More than the launch two weeks ago, the descent will show whether the space program can start to emerge from the shadow of its last tragedy.
The shuttle's luminous re-entry has lost some of its beauty for astronaut Scott Altman, who was commander of a Columbia flight to the Hubble Space Telescope in 2002.
"I used to always love to talk about the light show that goes on," Altman said. "I think now the crew and the folks on the ground won't be looking at that the same way we did."
The survival of Discovery and its crew will depend on precise execution of its banking descent and the soundness of repairs made.
NASA will try to observe the re-entry in more detail than ever before, taking infrared images of Discovery from three aircraft flying at altitudes up to 60,000 feet. Even top experts in the field known as aerothermal dynamics say they have surprisingly little idea of what happens after the shuttle hits the first wisps of atmosphere 400,000 feet above Earth.
"We don't have wind tunnels simulating these kinds of altitudes, these speeds we're talking about," said Tom Horvath, an aerothermal dynamics expert from NASA's Langley Research Facility in Virginia. Horvath was part of a team of Langley engineers who came to Houston to assess the re-entry effects of tile damage, including two protruding gap fillers that astronauts removed last week.
The shuttle begins its journey home with a three-minute engine burn that slows the spacecraft enough for Earth's gravity to start hauling it back. Mission managers decide whether to start re-entry based on weather conditions along its path to landing sites at Florida's Kennedy Space Center and Edwards Air Force Base in California.
The decision is irreversible; the shuttle cannot reach orbit again once it has started down.
"At that point the vehicle is going to impact the Earth somewhere, and your job is to make sure there's a runway under you when it happens," said Altman, a former Navy fighter pilot from Pekin, Ill.
The crew's main preparation for the return of gravity is to drink copious amounts of fluid - about a half-gallon each in the hour before the de-orbit burn. Weeks of zero gravity trick the body into thinking it has too much fluid, and the body responds by retaining less water. If astronauts do not drink a day's worth of salt-laden fluid just before re-entry, they can get light-headed when normal gravity returns.
"Basically, the crew compartment gets wallpapered with drink bags," said former astronaut Janice Voss, now working at NASA's Ames Research Laboratory in California on a planet-finding probe.
After flying with the ship's underbelly facing the void of space, the crew fires small jets that move the nose until it is flying bottom side down, with the nose pointing up at a 40-degree angle.
At the highest altitudes of re-entry, Discovery still moves like a spaceship, using small rockets to control its trajectory. As it hits the atmosphere, the orbiter automatically starts converting manual movements of the control stick into the motion of wing parts called elevons, which guide the ship's banking maneuvers.
During Columbia's final flight, the crew took time to admire the growing radiance outside, which tends to progress from a greenish glow to a yellow-orange hue marked by graceful eddies, Altman said. In a crew videotape that survived Columbia's destruction but ended four minutes before the first sign of trouble, pilot William McCool described "a bright orange-yellow out over the nose, all around the nose." Altman said the bright flashes during his last mission looked as if someone was using a strobe light outside the cockpit.
Like many shuttle rookies, McCool watched in fascination as a card floated slowly to the floor, a sign that gravity was returning.
Columbia broke up at around 200,000 feet while tearing along at 19 times the speed of sound - the time of maximum heating during descent, experts said.
Normally at this point, the ride inside the shuttle is still smooth and remarkably silent, astronauts said. There are no sudden drops or sharp turns beyond what an airliner would do. It's simply the fastest glider on Earth in a steady fall, free from the whine of engines.
Aside from the gap fillers that the Discovery crew removed while in orbit, the main re-entry concern among NASA engineers involves several small gouges in tiles on the shuttle's underside. Although managers concluded that the damage will not pose a threat, such dings can change how the shuttle interacts with the ever-denser atmosphere as it descends.
Even scraping the thin black surface off the tiles can increase heating of the orbiter. The shuttle's shock wave breaks apart nitrogen and oxygen molecules, leaving free atoms that can react with the tiles if the coating has been removed.
"If they recombine at the tile surface, it gets hotter," said Steven Schneider, a professor at Purdue University's School of Aeronautics and Astronautics.
Schneider was part of a team of experts from around the country who advised NASA during this mission about the potential effects of tile damage on re-entry heating. They were concerned that the protruding gap fillers could disturb the flow of air around the orbiter at a dangerously early stage of descent.
Normally, that switch to a turbulent air flow, called a boundary layer transition, occurs when the shuttle is moving at Mach 8 and is about 125,000 feet from the ground. The gap fillers could have caused a transition far earlier, when the shuttle was moving at 24 times the speed of sound, exposing the skin to higher temperatures for a dangerously long portion of re-entry.
If there is sufficient harm to the heat-resistant tiles, the damage sites will spawn wedges of heat that grow and move back along the orbiter's underside. NASA engineer Horvath said he hopes the new infrared images from the chase aircraft will reveal for the first time how specific kinds of damage affect temperatures on the orbiter.
Once the shuttle reaches Mach 18, its rate of descent slows, as the thickening atmosphere exerts the greatest braking pressure. By the time it slows to Mach 10, the light show is gone, and the pilot and commander start to increase their activity in preparation for the final approach to the landing strip.
In an interview before the shuttle launched, Discovery astronaut Andy Thomas said commander Eileen Collins has a seamless rapport with pilot James Kelly, whose call signal, "Vegas," stems from his poker skills. Thomas said their teamwork showed in the flight simulator cockpit whenever Collins had to stop flying to address a simulated crisis.
"She says, 'Vegas, you've got the airplane,'" Thomas said. "She hands off, she starts working [the problem], she comes back and says, 'Vegas, I've got the airplane.'"
"So they change their roles just like that, bang-bang," Thomas said, snapping his fingers. "It's kind of poetic to watch it. It's impressive as hell."
By the time Discovery slows to Mach 1, Collins will take over for good, Altman said; the shuttle's commander has guided the vehicle to the landing strip on every mission.
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