On July 23, 1999, the space shuttle Columbia launched into orbit to deploy the Chandra X-ray Observatory.
This space telescope can detect X-ray radiation emitted by things in the universe that are hot and have a lot of energy, like supernova explosions and black holes.
When it launched, Chandra was the most sophisticated X-ray telescope ever built.
But there’s another reason why this shuttle mission is considered a major milestone for NASA.
STS-93 crewmember Eileen Collins also became the first woman to command the space shuttle on this flight.
Source: https://www.space.com/39251-on-this-day-in-space.html