Launch from Cape Canaveral (KSC); landing on Cape Canaveral (KSC).

STS-91 marked the 9th and final MIR docking mission. Following a two day solo flight the Discovery docked with the MIR space station on June 04, 1998. A common flight with the 25th MIR resident crew (June 04, - June 08, 1998) was performed. The MIR-25 and STS-91 crews transferred more than 500 kilograms (1,100 lb) of water, and almost 2,130 kilograms (4,700 lb) of cargo experiments and supplies were exchanged between the two spacecraft. During this time, long-term U.S. experiments aboard the MIR were moved into Discovery’s middeck locker area and the SPACEHAB single module in the orbiter’s payload bay, including the Space Acceleration Measurement System (SAMS) and the tissue engineering co-culture (COCULT) investigations, as well as two crystal growth experiments. The crews also conducted Risk Mitigation Experiments (RMEs) and Human Life Sciences (HLS) investigations.

Andrew Thomas (final U.S. astronaut on MIR) returned to Earth with the Discovery. The astronauts and cosmonauts performed an experiment to find the leak in the module Spektr, but this failed.

The KU-band system failure was determined to be located in a component that was not accessible to the crew. The failure prevented television transmission throughout the mission. Television broadcasts from MIR were prevented by a problem between a Russian ground station and the mission control center outside of Moscow, limiting communications to audio only on NASA television.

Several secondary payloads: The Space Acceleration Measurement System (SAMS) and the tissue engineering co-culture (COCULT) investigations, as well as two crystal growth experiments. A prototype of the Alpha Magnetic Spectrometer (AMS) was carried. It was designed to look for dark and missing matter in the universe.

Other experiments conducted by the Shuttle crew during the mission included a checkout of the orbiter’s robot arm to evaluate new electronics and software and the Orbiter Space Vision System for use during assembly missions for the ISS. Also on board in the payload bay were eight Get Away Special experiments, while combustion, crystal growth and radiation monitoring experiments were conducted in Discovery’s mid-deck crew cabin area.