STS-51-C

STS-51-C was the 15th flight of NASA's Space Shuttle program, and the third flight of Space Shuttle Discovery. It launched on January 24, 1985, and made the fourth shuttle landing at Kennedy Space Center, Florida, on January 27. STS-51-C was the first shuttle mission to deploy a dedicated United States Department of Defense (DoD) payload, and consequently many mission details remain classified.

Mission summary
STS-51-C launched from Kennedy Space Center (KSC) on January 24, 1985 at 14:50 EST, and was the first of nine shuttle missions that year. It was originally scheduled for January 23, 1985, but was delayed because of freezing weather. Challenger had been scheduled for this flight, but Discovery was substituted when problems were encountered with Challenger ' s thermal protection tiles. STS-51-C marked the 100th human spaceflight to achieve orbit.

The mission was the first shuttle flight dedicated to the Department of Defense (DoD), and most information about it remains classified. For the first time, NASA did not provide pre-launch commentary to the public until nine minutes before liftoff. The Air Force only stated that the shuttle successfully launched its payload with an Inertial Upper Stage on the mission's seventh orbit. It is believed that the payload was a Magnum/ORION ELINT satellite into geosynchronous orbit. Other DoD flights STS-33 and STS-38 could have carried similar payloads. Payton stated in 2009 that STS-51-C's payload is "still up there, and still operating."

Also according to Aviation Week, the shuttle initially entered a 204 km x 519 km orbit, at an inclination of 28.45 degrees to the equator. It then executed three Orbital Maneuvering System (OMS) burns, the last being executed on the fourth orbit. The first burn was conducted to circularize the shuttle's orbit at 519 km.

The mission lasted 3 days, 1 hour, and 33 minutes. Discovery touched down on Runway 15 at KSC on January 27, 1985, at 16:23 EST. IMAX footage of the STS-51-C launch was used in the 1985 movie The Dream is Alive.

Connection to the Challenger disaster
Almost exactly a year after STS-51-C, Space Shuttle Challenger was destroyed with all hands on board during the STS-51-L mission including Ellison Onizuka, a crew member on both flights. As part of the investigation into the disaster, it was reported to the Rogers Commission that during the launch of STS-51-C, the worst solid rocket booster (SRB) blow-by effects of any mission prior to STS-51-L occurred, indicating conclusively that the Viton O-rings were not sufficiently sealing the hot gases inside the combustion chambers of the SRBs while firing. After they were recovered post-flight, the O-rings in both the right and left SRBs showed some degree of charring, but analysis of the center field joint of the right SRB showed an unprecedented penetration of the primary O-ring and heavy charring on the secondary O-ring.

This information was significant to the established consensus that low air temperature was a major factor in Challenger's destruction because the temperature at STS-51-C's launch was also, up to its time, the coldest recorded during a shuttle launch, at only 53 °F.