NASA successfully launched the Space Shuttle Columbia mission STS93 on 23 July after its second launch attempt on 22 July was called off because of bad weather.

The Columbia, with its Chandra X-ray Observatory payload, was originally due for launch on 20 July - the 30th anniversary of the Apollo 11 moon landing. The launch was aborted at T-7s in the countdown when the Columbia's hazardous-gas detection system indicated a high concentration of hydrogen in the aft engine compartment. The system showed a 640 parts per million (ppm) concentration of hydrogen - more than double the amount allowed. System engineers at the Kennedy Space Center initiated a manual cut-off of the ground launch sequencer less than half a second before the Shuttle's three main engines were due to start.

System and data evaluation, including a complete review of the shuttle's main propulsion system and related sensors, found that the hydrogen concentration indication was false and the actual level was about 114ppm, which is within allowable limits.

Because the external ignitors had been lit, these had to be replaced before the second launch attempt. The ignitors burn off the hydrogen concentration outside the orbiter, near the main engines of the shuttle.

The Chandra X-ray Observatory remained powered up inside the orbiter, while eight middeck payloads were removed, reserviced and installed back inside the orbiter during the down period.

The primary objective of the STS93, the first US space mission to be commanded by a woman, is the deployment of the $1.5 billion Chandra X-ray Observatory. The Chandra - the most sophisticated space telescope ever built - will allow study of the origin, structure and evolution of the universe in greater detail than ever before.

Source: Flight International