Countdown for Shuttle Begins; Helium Leak Held Insignificant

KENNEDY SPACE CENTER, Fla. —  NASA on Monday cleared the space shuttle Discovery for flight and started the countdown for its trip into orbit with an atmosphere research satellite.

Test director Mike Leinbach said a leak in the orbital steering system was too small to be of concern.

The countdown clock began ticking at 2 p.m. PDT. Discovery is scheduled to blast off at 3:57 p.m. PDT Thursday.

The five astronauts are to release the multimillion-dollar Upper Atmosphere Research Satellite midway through the five-day mission.

The observatory is the first satellite to be launched in NASA’s Mission to Planet Earth program, a long-term study of the environment from space.

One of the satellite’s primary jobs will be to study the ozone layer surrounding Earth. Scientists fear that this stratospheric shield against dangerous ultraviolet rays is being destroyed by industrial chemicals.

This satellite and others will make up an Earth Observing System, an international effort being coordinated at the Goddard Space Flight Center in Greenbelt, Md.

The satellite’s instruments will measure the chemical compounds, which make up less than a millionth of 1% of the volume of the upper atmosphere.

The launch had been threatened late last week when workers discovered the leak in a helium line valve. Although an alternate line is available, NASA wanted both lines in good condition.

Helium is used to pressurize the fuel tank for jets used to steer the shuttle in orbit.

Tests over the weekend showed the leak was “very small” and that both helium lines were operational, Leinbach said. A shuttle successfully has flown once before with a similar-size leak.