The descent is the subject of intense speculation and scrutiny from NASA.
The tensest moments of the shuttle Discovery's high-stakes mission should start around 3:45 a.m. EDT Monday, 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 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 for the first time in this mission.
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 landing procedure
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.
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 motion of wing parts called elevons, which guide the ship's banking maneuvers.
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 the damage will not pose a threat, such things can change how the shuttle interacts with the ever-denser atmosphere as it descends.