Cape Canaveral: The shuttle Discovery crew scrambled aboard their ship on Monday for an early Tuesday launch to the International Space Station to deliver supplies, laboratory gear and mice for bone-loss experiments.
Liftoff was scheduled for 1:36 a.m. EDT (0536 GMT) on Tuesday from the Kennedy Space Center in Florida.
The mission, scheduled to last 13 days, will deliver final experiment racks to the orbital outpost, a $100 billion project of 16 nations, and supply the station with food and spare parts to keep it operating after the space shuttles are retired next year.
As the assembly of the space station winds down, U.S. space agency NASA and its partners -- Russia, Europe, Japan and Canada -- are shifting focus to research programs.
Two refrigerator-sized racks for fluid physics and materials science experiments are among the seven tons of new gear slated to be installed during the shuttle`s nine-day stay at the station.
Six mice, including three that were genetically enhanced with a double portion of a bone-building gene, will be left on the station as part of a study to find out why astronauts` bones break down in the gravity-free world of space.
The affliction is similar to the bone-destroying disease osteoporosis, which affects millions of people on Earth, particularly post-menopausal women, and occurs because bone breakdown outpaces replenishment.
Scientists aren`t sure what causes the imbalance.
Julie Robinson, NASA`s lead space station scientist, said there was a complex interaction between the hormones that regulate the way the cells operate.
"It`s so complicated that to some extent in treating osteoporosis on the ground we`re doing things that work and we`re not sure exactly why they work," Robinson said.
"Imagine down the road if you could ... tailor a treatment rather than giving everyone the same drug and hoping it works," she added.
The mice will be contained in a habitat developed by Italian aerospace company Thales Alenia Space in Genoa.
Although NASA has flown rodents on the shuttle and station previously, they have never been left behind for a long stay in space. The mice are scheduled to return aboard NASA`s next shuttle mission in November.
Discovery`s flight comes as the Obama administration begins to weigh options for the U.S. human space program, including extending the life of the space station beyond September 30, 2015, when funding is projected to end.
NASA spends about half of its $18 billion annual budget on human space programs, including about $2.5 billion a year to run the station, which is nearing completion after more than a decade of construction 220 miles above Earth.
The station will be as big as a football field and weigh nearly 900,000 pounds (408,000 kg).
"It`s clearly expensive to continue to operate and maintain a spacecraft of this magnitude in orbit. The benefits have to be worth that cost," said Mark Uhran, a NASA deputy administrator in charge of the space station.
"We`re confident that once we ramp up this R&D (research and development) program we will have ample justification and continue it," he said.