Space shuttle Columbia and its crew returned to Earth on Sunday, ending two weeks of lab work that advanced brain research despite unexpected animal casualties. And the experiments were far from over.

Within an hour, the crew was hustled off to medical tests that were expected to go on for days. Six of the seven astronauts left on stretchers; doctors wanted them reclining to preserve their weightless state.At the same time, NASA rushed to unload the animals so scientists could begin dissecting the few dozen surviving baby rats, as well as the nearly 2,000 fish, snails, crickets and older rodents that flew. Most of the young rats died in orbit, victims of maternal neglect.

It was a race against gravity: The sooner the astronauts and animals could be examined, the greater the likelihood of observing space-induced changes in the nervous system.

"I'm sitting here like a little kid with ants in my pants," said Gay Holstein, a Mount Sinai School of Medicine researcher whose rats flew on Columbia. "I can't wait to get going on my experiment."

To everyone's relief, Columbia landed right on time at the Kennedy Space Center, where about 200 researchers waited with scalpels.

Columbia swooped through a clear noontime sky and touched down neatly on the runway. The trip added 6.4 million miles to the odometer of NASA's oldest shuttle.

Commander Richard Searfoss had only two functioning hydraulic power units for most of the hourlong descent. The cooling system for the third unit failed to work Saturday; Searfoss turned that unit on just minutes before touch down so it would not overheat.

Mission Control congratulated the astronauts for "a historic mission that elevated neuroscience research to record heights."