CAPE CANAVERAL -- The fourth countdown for space shuttle Columbia began yesterday as scientists prepared for the long astronomy mission and tried to keep their excitement in check.

Columbia has been grounded for 3 1/2 months because of leaking hydrogen. Officials will not know for sure whether all of the leaks have been plugged until the shuttle is fueled for Tuesday's early morning launch.

"This is the fourth countdown we're going through," said mission scientist Ted Gull. "We know we're ready, and that's a very positive thing to say."

Since the last launch attempt 1 1/2 weeks ago, NASA has replaced two leaking components in Columbia's aft compartment. A new set of recirculation pumps also was installed, but engineers later determined there was nothing wrong with the ones that were removed.

Hydrogen flooded Columbia's aft compartment during fueling for the Sept. 6 launch attempt. But no leaks were detected around new fuel lines between the orbiter and external tank, the site of extensive leaking during fueling for the first launch attempt in late May.

Columbia's second countdown was halted late last month after communication with one of four telescopes aboard the shuttle was disrupted. The telescopes constitute a $150 million observatory that will probe ultraviolet light and X-rays emitted from some of the hottest objects in the universe.

The mission originally was scheduled for March 1986 but was put on indefinite hold when Challenger exploded earlier that year.