NASA Readies Space Shuttle for Third Launch Attempt : Science: If successful, Columbia’s 14-day medical research mission will set a fleet record. The crew will study body changes in weightlessness.

NASA loaded a fresh batch of 48 rats into the space shuttle Columbia on Sunday for the third launch attempt in less than a week.

Columbia was due to lift off at 7:53 a.m. PDT today on a 14-day medical research flight, the longest planned for a shuttle.

The first two launch attempts last Thursday and Friday were thwarted by computer and communications system failures. Officials said both problems were resolved.

“In this business, you have to go when you’re ready and you can’t be anxious or antsy or anything like that. That’s not the way it works,” said Bill Dowdell, National Aeronautics and Space Administration test director.

Rain and thunderstorms added to last week’s troubles, but meteorologists forecast an 80% chance of acceptable conditions today.

The seven astronauts spent the weekend relaxing and reviewing flight plans.

Two of the four scientists--veterinarian Martin Fettman and biochemist Shannon Lucid--left in place the catheters intended to help measure blood pressure during the flight.

Each of the astronauts will submit to numerous medical tests aimed at studying space motion sickness and body changes in weightlessness.

The rats are intended for similar tests.

NASA program scientist Frank Sulzman said rats undergo physical changes in weightlessness faster than humans, making the animals valuable research tools.

A previous batch of 48 rats was removed from Columbia on Saturday after the earlier launch delays. Those rats were killed and either discarded because of radioactive isotopes in their blood or donated to a rehabilitation program for birds of prey. NASA has nearly 1,000 rats on standby in case of more launch delays.