NASA vetoed an extra day in space for Columbia's astronauts because of concern over next week's landing weather and the weak condition of the remaining baby rats on board.
The decision Thursday means the space shuttle will return Sunday, 16 days after lifting off on a voyage to study the brain.
The astronauts, flight controllers and scientists had gone into the flight hoping for a 17th day of research. But 52 of the 96 baby rats aboard have died, and some of the survivors are weak.
Scientists want the rats back as soon as possible for dissection, before the animals deteriorate any further and spoil the findings. Rearchers have the minimum number of young rats with which to work.
"They're frustrated," said Joseph Bielitzki, NASA's chief veterinarian. "They put four years of solid effort into this and then had something unexpected happen. It kind of threw a monkey wrench into the works."
The surrogate mother rats could not or would not feed the young rodents, which in turn suffered low blood sugar, low body temperature and dehydration.
There was almost another casualty Thursday.
The astronauts were ordered to kill an adult rat after its electronic cap came off and its brain electrodes came out. But the veterinarian on board, Richard Linnehan, said the rodent was too healthy to kill. He applied ointment to the animal's head to prevent a small, exposed wound from becoming infected.
"It's not good for research, but my guess is he may be attached to this guy," Bielitzki said. "I don't blame him."
The Physicians Committee for Responsible Medicine, a non-profit group in Washington, criticized NASA for the "wasteful and unnecessary" experiments on the flight, particularly those involving animals.