CAPE CANAVERAL, Fla. -- When NASA cancels a space shuttle countdown, as it has twice in the past two weeks, the delay usually means a little training, some shopping or just plain relaxing for the shuttle's crew.
The consequences are a bit more extreme, however, for the small, four-legged comrades of the seven astronauts on the shuttle Columbia, which is set for a third launch attempt at 8 a.m. tomorrow.
Sitting through a scrub means certain death. The only question is whether they wind up in a trash heap or fed to wild birds.
NASA assigns 163 rats to each launch attempt; if something interferes with liftoff for two days running, the animals must be replaced with a whole new batch before the space agency tries again.
Meanwhile, 123 members of the old crew are killed and disposed of in such a way as to keep radioactive tracer chemicals in their bodies from contaminating the environment.
The others are killed and sent to the University of Florida Zoological and Wildlife Veterinary Clinic in Gainesville, where they are fed to eagles, falcons and other birds of prey.
"Which is a far better thing than just throwing them in the incinerator," noted Jane Hutchison, spokeswoman for NASA's Ames Research Center in California.
The 2,478 baby jellyfish that have been loaded and unloaded from Columbia these past two weeks also are replaced with a new batch each time. But they can all be returned to their home laboratory in Virginia.
Both animals are to be studied after Columbia's nine-day flight to help scientists learn how and why muscles, blood, bone and nervous systems are affected by the weightlessness of space.
Many of the experiments planned for the mission -- the most comprehensive medical-research space flight yet undertaken -- will be conducted on Columbia's seven crew members.
But each set of orbiting rats, and a matching set of Earth-bound counterparts, is injected before each launch attempt with radioactive chemicals that attach to blood, muscle and bone tissue.
However, those chemicals decay and are excreted by the rats within a few days. In addition, the rats grow too big for the experiment.
So if Columbia isn't launched within two days of the rats going aboard -- as has happened twice already -- researchers must get rid of the old ones and replace them with new ones.