CAPE CANAVERAL, Fla. — Discovery and seven astronauts plan to dive through Earth's atmosphere Monday morning on a trajectory that would send a shuttle crew hurtling across America's heartland for just the second time since the 2003 Columbia disaster.
If weather permits, Discovery is targeting a 6:48 a.m. MDT return to Kennedy Space Center to end a two-week International Space Station resupply mission. A second landing opportunity follows at 8:23 a.m.
But rain showers expected within 30 nautical miles of the runway could keep the crew in orbit for a 15th day.
"We're confident that the folks in Houston and the folks in Florida will do everything they can to get us home when the weather will allow it," said shuttle commander Alan Poindexter, a 48-year-old Navy captain.
Mission managers decided before Discovery's April 5 launch to land on a path that angled down from the north and carried the shuttle over much of the continental United States, instead of taking the more common approach from the south.
The change to a "descending node" approach lengthened the crew's work days during a busy mission that transferred tons of cargo to and from the shuttle, and set up a preferred daylight landing.
The selected ground tracks also met safety criteria established after Columbia broke up and left a trail of debris over east Texas and Louisiana.
After the accident, which did not injure anyone on the ground, NASA modeled "debris envelopes" for different failure times and paths, and selected routes that avoided flying over heavily populated areas.
"It's a difficult thing to talk about it, but it's important work that we do," said John Shannon, shuttle program manager. "What we have done here is take a very close look and made sure that the descending node opportunity, which will go over the United States, does not put any major population areas at risk, and our general population risk is below those limits that were set."
Of 18 post-Columbia flights, Discovery's November 2007 return home is the only other to fly over the country.
But the safety of people on the ground is not the only reason shuttles typically take an "ascending" approach that glides over Central America and the Gulf of Mexico en route to Florida's Space Coast.
That trajectory requires less fuel. And during summer months, it avoids flying through seasonal formations of high-altitude clouds laden with ice particles that could damage the orbiter.
In this case, Shannon said, "the significant advantage you get for the crew timeline really said that this was the right way to go, as long as we could meet our safety criteria."
The choice means a rare and possibly last opportunity for many Americans to see a returning shuttle streaking overhead. Only three more shuttle missions are planned.
Bryan Lunney, the NASA flight director overseeing Discovery's atmospheric re-entry, said people in the western half of the country might see the orbiter's plasma trail, a white streak moving fast and high in the dark sky.
In the eastern half, if visibility is good enough, he said viewers might see more of a "glowing cloud plowing through" a lighter sky.
"It could be really neat," he said. "I hope it clears up for folks."
If Lunney gives the "go" for Monday's first landing attempt, Discovery will fire its engines in the direction of travel at 7:43 a.m. to begin an hour-long plunge that will brake its speed from 17,500 mph to 225 mph at touchdown.
Lunney said the ash plume from an Iceland volcano that has grounded air travel in Europe posed no concern for Discovery.
The crew and spaceship would feel the first effects of the atmosphere a little more than 30 minutes before landing, as they fly over the northern Pacific Ocean.
The orbiter would cross into U.S. airspace over Montana and pass west of St. Louis, Mo., east of Memphis, Tenn., and west of Jacksonville, Fla., before descending from the north toward the space center's Runway 15.
If weather does not cooperate Monday, Discovery would probably land Tuesday at either Kennedy Space Center or Edwards Air Force Base in California's Mojave Desert.
The crew on Sunday took steps to power down unnecessary systems to preserve every hour of possible flight time through Wednesday.
Also Sunday, crew members tested Discovery's flight control systems, including the wing flaps and tail fin, and fired 38 primary steering jets to make sure they are ready for re-entry. All the systems performed well.
On the heels of Discovery's arrival, space center teams Monday night plan to roll Atlantis to its launch pad for what may be the last time. Atlantis is targeted to launch to the space station May 14.