STS-51 was a Space Shuttle Discovery mission that launched the Advanced Communications Technology Satellite ACTSin September 1993. The flight also featured the deployment and retrieval of the SPAS-ORFEUS satellite and its IMAX camera, which captured spectacular footage of Discovery in space. A spacewalk was also performed during the mission to evaluate tools and techniques for the STS-61 Hubble Space Telescope servicing mission later that year. STS-51 was the first shuttle mission to fly a GPS receiver, a TrimbleTANS Quadrex. It was mounted in an overhead window where limited field of view and signal attenuation from the glass severely impacted receiver performance. (Full Triple-redundant 3-string GPS would not happen until 14 years later with STS-118.)

The Advanced Communications Technology Satellite was deployed on flight day 1. This satellite served as a test bed for advanced experimental communications satellite concepts and technology. Its Transfer Orbit Stage (TOS) upper stage fired on time 45 minutes after deployment and boosted the satellite to geosynchronous altitude on the first day of the mission.

The first attempt to deploy ACTS was delayed by the crew when two-way communications were lost with Mission Control about 30 minutes before the deploy time. Flight controllers could receive telemetry and voice communications from Discovery, however the crew could not receive communications from the ground. The crew waived off the 2:43 pm CDT deploy when they did not receive a "go" from Mission Control as called for in preflight plans made for just such an occurrence. After the waive off of deploy, the crew changed the shuttle's S-Band communications system to a lower frequency and restored two-way communications with the ground. The two-way communications had been lost for a total of about 45 minutes. After consulting the crew, flight controllers began immediately planning for the second, and ultimately successful deploy.

During the deployment on 12 September, two Super*Zip explosive cords in the Airborne Support Equipment cradle (ASE) designed to release the spacecraft, one primary and the other a backup, simultaneously detonated. This caused minor tears in two dozen insulation blankets mounted on the bulkhead between the payload bay and the AFT near the #3 APU. The ASE ring holding the TOS was damaged as well, and ejected debris was visible as the stack moved away from the orbiter.

The Advanced Communications Technology Satellite (ACTS), a significant activity of the NASA Space Communications Program, provided for the development and flight test of high-risk, advanced communications satellite technology. Using multiple spot beam antennas and advanced on-board switching and processing systems, ACTS pioneered new initiatives in communications satellite technology. NASA Glenn Research Center was responsible for the development, management, and operation of ACTS as part of a long legacy of experimental communications satellites.

After fulfilling its original mission as a key part of the ACTS Gigabit Satellite Network, the spacecraft continued operations through a partnership between the space agency and a nonprofit consortium. It was shut down 28 April 2004 after funding dried up. The satellite was put into a flat spin with its solar array edges facing the sun, which should theoretically prevent it from ever being restarted. The spacecraft was moved to its final resting place at 105.2 degrees west longitude – where it poses minimal risk to other satellites – after NASA concluded in 2000 that it probably lacked the fuel to move to a higher graveyard orbit. Nevertheless, ACTS should not re-enter the atmosphere for thousands of years, according to Richard Krawczyk, the ACTS operations manager at Glenn Research Centre.