Astronauts aboard the space shuttle Discovery prepared Monday to collect data to help scientists monitor Earth's fragile ozone layer and conduct other scientific experiments.

On their 29th orbit around Earth, the five-member crew was awakened for their third day in space at 1:47 a.m. EDT by the Coast Guard Hymn, played in honor of astronaut Bruce Melnick, the first Coast Guard member to fly in space."Another good morning to you, Discovery. Another great day in space," controllers at Mission Control in Houston told the astronauts in a package of instructions and messages faxed to the spaceship.

With the Ulysses solar probe speeding smoothly into deep space, the crew planned to activate the Shuttle Solar Backscatter Ultraviolet experiment to measure ultraviolet radiation "backscattered" by Earth's atmosphere.

The data will be used to calibrate instruments aboard other satellites that guage the state of the ozone layer, which scientists believe is being damaged by pollution.

The astronauts also planned to check the health of 16 rats on board as part of a medical experiment aimed at evaluating whether biological changes caused by near-weightlessness could be used to help test new drugs.

In addition, crewmembers planned to work with a personal computer to assess the performance of machines that might be used on NASA's planned space station, and conduct experiments with a new voice-activated command system.

The crew members are commander Richard Richards, 44, co-pilot Robert Cabana, 41, William Shepherd, 41, Thomas Akers, 39, and Melnick, 40.

Richards and Cabana will guide Discovery to a landing Wednesday at 6:57 a.m. PDT at Edwards Air Force Base, Calif.

Back on Earth, engineers at the Kennedy Space Center planned to move the shuttle Columbia to launch pad 39B, where the ship will undergo exhaustive testing to find an elusive hydrogen fuel leak that has repeatedly grounded the shuttle.

Discovery's blastoff on Saturday marked NASA's first shuttle launch in more than five months. Attempts to launch the shuttles Atlantis and Columbia failed in May, July and September, primarily due to a series of hydrogen leaks.

Discovery's blastoff ended the frustrating hiatus in American manned space flight and enabled the astronauts to finally dispatch the European-built Ulysses probe to the sun, the mission's primary objective.