This story was updated at 12:24 p.m. ET.
The seven astronauts on space shuttle Discovery are hoping for clear skies over Florida Monday morning so they can return to Earth, though there's a chance rainy weather may keep them in orbit an extra day.
Mission Control radioed Discovery's crew early Sunday to say that the chance of rain too close to their runway at NASA's Kennedy Space Center in Florida could delay their landing. The shuttle is due to land at the seaside space center in Cape Canaveral, Fla., at 8:48 a.m. EDT (1248 GMT) to wrap up a busy two-week delivery mission to the International Space Station.
"It's always a great time to spend time on orbit and 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," Discovery commander Alan Poindexter said Sunday morning during a series of radio interviews broadcast by NASA.
NASA flight rules forbid shuttle landing attempts if rainstorms creep within a 30-mile perimeter of the runway since the rain can damage an orbiter's heat-resistant tiles, among other risks. Discovery has two chances to land in Florida on Monday.
"We're worried about rain showers and we'll evaluate what we'll get tomorrow," entry flight director Bryan Lunney told reporters Sunday.
The Kennedy Space Center and a backup runway in California will be available on Tuesday in the event of a delay. The weather in Florida is expected to be slightly better.
Another runway at White Sands Space Harbor in New Mexico may be too wet to use, Lunney said.
Discovery's re-entry into Earth's atmosphere should be visible across parts of North America to skywatchers from the ground as it crosses from the northwest to the southeast on its way to Florida, Lunney said.
The shuttle will appear as a bright, fast streak across the sky to observers on the western half of the United States leading a plasma trail from the searing heat of re-entry. It should offer an even more impressive light show to the eastern half of the country.
"I've heard from other people who have seen it, that it is pretty neat to see," Lunney said.
Iceland volcano ash cloud
The potential for rain and clouds are the only obstacle standing between Discovery astronauts and their return to Earth. NASA officials said the shuttle's re-entry and landing will not be affected at all by the vast volcanic ash cloud from Iceland's Eyjafjallajökull volcano that has brought air traffic across Europe to a grinding halt.
Despite its significant impact to life in Europe and global air travel, the volcano's ash cloud has been hard for astronauts to spot from space because it is too far north to be seen from the orbit of the International Space Station.
"We've been looking for it and we haven't seen the volcanic ash plume yet," Poindexter said. "We hope to soon and perhaps maybe take a few photographs of it."
Discovery launched on April 5 and is flying one of NASA's last few shuttle missions before the space agency retires the shuttle fleet in September. After this mission, just three more missions remain, each one the final flight for the orbiter flying it.
Despite a main antenna failure, sticky bolts on spacewalks and other hurdles, the astronauts successfully delivered nearly 8 tons of supplies, science equipment and other gear to the space station. They performed three spacewalks and set two new space records for the most women astronauts in space at the same time (four in all, three on Discovery and one on the station) and the most Japanese astronauts together at one time (two, one from each spacecraft).
The astronauts were also in space last week during the announcement of President Barack Obama's new space exploration vision, which challenges NASA and the United States to send astronauts to an asteroid by 2025 and on to the moon in the mid-2030s.
"It's cool to think that we're back in the exploration business," said Discovery astronaut Dorothy "Dottie" Metcalf-Lindenburger, a former high school teacher who is making her first spaceflight.
Discovery's flight has been especially poignant for Metcalf-Lindenburger and the two other members of the shuttle crew who are making their first-ever spaceflights on one of NASA's last planned shuttle missions. Reaching space had been a life-long ambition for shuttle pilot Jim Dutton, an Air Force colonel, who flew Discovery on a victory lap around the space station after the two spacecraft undocked on Saturday.
"It was just absolutely beautiful to see that manmade creation up there floating above the Earth," Dutton said. "It is just a gorgeous vehicle to behold and we think that this has been a fantastic program."
SPACE.com is providing complete coverage of Discovery's STS-131 mission to the International Space Station with Managing Editor Tariq Malik and Staff Writer Clara Moskowitz based in New York. Click here for shuttle mission updates and a link to NASA TV.