CAPE CANAVERAL, Fla. – Their mission almost complete, space shuttle Discovery's astronauts checked out their ship's flight systems Friday to ensure a safe return to NASA's spaceport.
Discovery and its crew of seven were due back Saturday afternoon, ending a nearly two-week mission that left the international space station fully powered with a new set of solar wings. Favorable landing weather was expected.
"We'll keep our fingers crossed," said shuttle commander Lee Archambault.
Late Friday afternoon, NASA cleared Discovery's heat shield for re-entry based on the astronauts' inspection the day before with a laser-tipped boom.
Also Friday, the astronauts exchanged a warm "aloha" with students at the Honolulu school President Barack Obama graduated from 30 years ago.
Archambault passed on Obama's regards — the president asked the crew in a phone call to do so earlier this week.
Astronaut Richard Arnold II, a former schoolteacher, told the students it was satisfying to see the space station after all the construction work.
"We pulled away and saw the solar arrays that we installed and then deployed," he said.
Added crewmate John Phillips: "We're all really proud to bring that extra power to the station."
Discovery had to be gone from the space station by Wednesday to make room for a Russian Soyuz spacecraft that's due to arrive Saturday with a fresh crew.
During their eight days there, the shuttle astronauts installed the space station's final set of solar wings. The extra electrical juice is needed for all the science experiments that are planned once the station population doubles, to six, in late spring.
The shuttle also delivered a new urine processor that got the space station's water-recycling system working, and an iodine flush that got rid of the bacteria that had tainted the lines.
The space station also got a new crew member, a Japanese astronaut who took the place of Sandra Magnus. She is finally headed home after 4 1/2 months in orbit.
During re-entry, Discovery will test out a thermal tile that is deliberately flawed: It has a "speed bump" molded into it.
NASA wants to gauge the amount of disturbance in the flow of hypersonic air over the quarter-inch-high bump, and determine the amount of downstream heating.
The experimental tile, under the left wing, will be exposed to nearly 3,000 degrees Fahrenheit during descent. NASA officials say the tiles in this area are 3 inches thick and they have no concern about damaging the wing.
A gash in the left wing led to Columbia's destruction in 2003 and the deaths of all seven astronauts.
A Navy plane will fly below Discovery as the shuttle heads over the Gulf of Mexico toward Florida, and monitor heating on the bottom of the shuttle with an infrared camera.
This new type of tile was developed as a potential improvement for the space shuttles, scheduled to retire by the end of next year.
It's also being considered for NASA's shuttle replacement, a craft called Orion that is intended to carry crews to the space station and eventually the moon and Mars.