Space shuttle Columbia's mission was aborted 6 1/2 seconds before its launch this morning when launch controllers received a reading of high levels of hydrogen gas in an aft engine compartment.

Eileen Collins, the first female space shuttle commander, and the four other astronauts were safe. Immediately after the shutdown, they began turning off all shuttle systems and getting out of the shuttle, whose main engines contained more than 500,000 gallons of liquid hydrogen fuel and liquid oxygen.

The indication of an increased concentration of hydrogen gas came just before the three main engines ignited. Shuttle computers automatically stopped the countdown.

NASA engineers suspected a faulty reading, and NASA commentator Bruce Buckingham said that depending on the problem, another launch attempt might be made Thursday. At the very least, technicians must replace six igniters, needed to burn off hydrogen vapor below the engines before they are fired.

It was the first time in years that trouble struck so late in the countdown. "Engineers in the firing room will continue to monitor the situation and strive to understand exactly what happened that caused this abort," Buckingham said.

The shuttle had been poised to to take off at 12:36 a.m. from Pad 39B at the Kennedy Space Center, on the 30th anniversary of the Apollo 11 moon landing, carrying a $1.6 billion X-ray telescope into orbit in the first shuttle flight commanded by a woman. First lady Hillary Rodham Clinton, the world champion U.S. women's soccer team and Sally Ride, America's first female astronaut, were on hand to watch.

The goal of the flight was to deploy NASA's Chandra X-ray observatory, a Hubble-class telescope designed to study radiation emitted by some of the most violent phenomena in the universe. The telescope and its two-stage rocket booster tipped the scales at 50,162 pounds, the heaviest payload ever placed aboard a shuttle.

The Chandra observatory, the most sophisticated X-ray telescope ever built, was designed to give astronomers a new view of the universe. Unlike visible light, X-rays from deep space are almost totally absorbed by Earth's atmosphere and can only be studied from space.

"The discovery capabilities are immense," theorist Michael Turner said of Chandra's potential.

Unlike Hubble, however, Chandra cannot be repaired in orbit if anything goes wrong. To cut costs, it was designed to operate in an orbit with a low point of 6,000 miles and a high point of 87,000 miles, well above the shuttle's maximum altitude.

The flight plan called for Chandra and its two-stage solid-fuel booster to be ejected seven hours and 17 minutes after launch. One hour later, the inertial upper stage booster was scheduled to fire, boosting Chandra into an initial orbit with a far point of 46,000 miles and a near point of 755 miles. An onboard rocket system was to fire five times over the next 10 days to boost Chandra to its final orbit.

Collins, a veteran of two shuttle flights, is the first female space commander since Soviet cosmonaut Valentina Tereshkova in 1963 and the first to be given major responsibility.

It is a role she has relished. "I wish that we could have had women in the space program a lot sooner," said Collins, a former test pilot, Air Force Academy math instructor and mother of a three-year-old girl. "But that's just the way our society worked and that's just the way it was. We can't change that now, but we can make it better in the future."

"Sorry about the delay. We'll do it in a couple days, hopefully," a launch controller told Collins before she climbed out of the shuttle.

Special correspondent William Harwood contributed to this report.

NASA's Great Observatories

Hubble Space Telescope

Cost: $1.5 billion

Launch: 1990, aboard space shuttle Discovery

Cost increased to $2 billion after mirror repairs and improvements in orbit in 1993 and 1997.

Observes visible light and, to a lesser degree, ultraviolet and infrared radiation.

Compton Gamma Ray Observatory

Cost: $557 million

Launch: 1991, aboard space shuttle Atlantis

Not built for servicing by astronauts.

Chandra X-Ray Observatory

Cost: $1.6 billion

Launch: Today, aboard space shuttle Columbia

Destined for oval orbit extending one-third of the way to the moon, meaning it can't be reached by astronauts.

Space Infrared Telescope Facility

Cost: $458 million

Launch: 2001, aboard a Delta rocket

Not built for servicing by astronauts.

SOURCE: Associated Press

CAPTION: Pilot Jeffrey Ashby, left, and commander Eileen Collins headed to space shuttle Columbia at Kennedy Space Center before Tuesday's aborted launch. Behind them were mission specialists Michel Tongnini, Catherine Coleman and Steven Hawley.