Rain and overcast skies prevented the space shuttle Discovery from returning to Earth on Monday, and Mission Control instructed the astronauts to spend a 15th day circling the world and awaiting better weather.
Mission Control radioed up the disappointing news after passing up two landing attempts.
"The folks really worked it hard down here. There was a lot of cause for optimism ... but in the end of the day" the clouds remained too low and too thick, Mission Control radioed.
"We appreciate everything you've done," shuttle commander Alan Poindexter replied, "and we'll be hopeful for better weather tomorrow."
Clearer skies were expected over Florida's Kennedy Space Center on Tuesday. If the clouds linger, however, NASA will try for the backup landing site in Southern California. Touchdown is scheduled for 7:33 a.m. ET, shortly after sunrise.
Discovery and its seven astronauts can remain in orbit until Wednesday. They're wrapping up a resupply mission to the International Space Station.
If Discovery aims for Kennedy, it will cross the United States on its way home, providing a rare visual treat. The streaking, glowing trail should be visible from the ground, weather permitting, as the shuttle passes over the Pacific Northwest and descends over America's interior.
The last time a returning shuttle flew over so much of the United States was in 2007. No further re-entries like this are planned as the shuttle program draws to a close. NASA has tried to keep continental flyovers to a minimum for public safety reasons, ever since space shuttle Columbia shattered over Texas in 2003.
Typically, a shuttle returns from the southwest, zooming up over the South Pacific, Central America, and the Gulf of Mexico. NASA changed Discovery's flight path before liftoff on April 5, to maximize the crew's work time in orbit and reduce fatigue.
A touchdown at Edwards Air Force Base in California would eliminate a coast-to-coast flyover.
The volcanic eruption in Iceland, at least, was not interfering with NASA's effort to bring Discovery home. The re-entry path does not go anywhere near the European airspace threatened by volcanic ash.
Discovery undocked from the space station on Saturday, leaving behind tons of science experiments and equipment so the orbiting outpost can operate for years to come. The astronauts' biggest contribution was a new tank full of ammonia coolant, which took three spacewalks to hook up.
A pressure valve in the space station's cooling system got stuck after the ammonia tank was plugged in. Astronauts will have to deal with the problem on a future spacewalk. For now, though, the lab complex is being cooled properly.
This is Discovery's next-to-last flight. NASA has only three shuttle flights left before retiring its fleet of three orbiters. Atlantis is next up in less than four weeks, and Endeavour's final turn is due in July. The last shuttle mission — by Discovery — is scheduled for September.
Once the shuttles are retired, the space station will have to rely on Russian spacecraft to transport supplies and astronauts, at least temporarily. Over the longer term, NASA is supporting the development of new U.S. commercial spaceships to resupply the space station. One of the rockets being built for the task, SpaceX's Falcon 9, is being prepared for an initial test launch from Cape Canaveral no earlier than May 8.
This report was supplemented by msnbc.com.
