STS-93 in 1999 marked the 95th launch of the Space Shuttle, the 26th launch of Columbia, and the 21st night launch of a Space Shuttle. Eileen Collins became the first female shuttle Commander on this flight. Its primary payload was the Chandra X-ray Observatory. It would also be the last mission of Columbia until March 2002. During the interim, Columbia would be out of service for upgrading, and would not fly again until STS-109. The launch was originally scheduled for 20 July but the launch was aborted at T−7 seconds. The successful launch of the flight occurred 3 days later. The payload was also the heaviest payload ever carried by the Space Shuttle system, at over 22.7 tonnes (25 tons).