STS-51-C (formerly STS-10) 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, 1985. 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.

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STS-51-C

Discovery in orbit; in-flight photography of the Department of Defense support mission is limited.
Names	Space Transportation System-15
Mission type	SIGINT satellite deployment
Operator	NASA, U.S. Department of Defense
COSPAR ID	1985-010A
SATCAT no.	15496
Mission duration	3 days, 1 hour, 33 minutes, 23 seconds (achieved)
Distance travelled	2,010,000 km (1,250,000 mi)
Orbits completed	49
Spacecraft properties
Spacecraft	Space Shuttle Discovery
Launch mass	113,802 kg (250,890 lb)
Landing mass	(Classified)
Payload mass	(Classified)
Crew
Crew size	5
Members	Thomas K. Mattingly II Loren J. Shriver Ellison S. Onizuka James F. Buchli Gary E. Payton
Start of mission
Launch date	January 24, 1985, 19:50:00 UTC
Rocket	Space Shuttle Discovery
Launch site	Kennedy Space Center, LC-39A
Contractor	Rockwell International
End of mission
Landing date	January 27, 1985, 21:23:23 UTC
Landing site	Kennedy Space Center, SLF Runway 15
Orbital parameters
Reference system	Geocentric orbit[1]
Regime	Low Earth orbit
Perigee altitude	332 km (206 mi)
Apogee altitude	341 km (212 mi)
Inclination	28.45°
Period	91.30 minutes
STS-51-C mission patch Back row: Gary E. Payton, James F. Buchli, Ellison S. Onizuka Front row: Loren J. Shriver, Thomas K. Mattingly II Space Shuttle program ← STS-51-A (14) STS-51-D (16) →

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STS-51-C launched from Kennedy Space Center (KSC) on January 24, 1985, at 19:50:00 UTC, and was the first of nine shuttle missions in 1985. 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's length of three days was shorter than the week or longer of most civilian shuttle flights. It was the first dedicated to the U.S. Department of Defense (DoD),^[3] 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 U.S. Air Force only stated that the shuttle successfully launched its payload with an Inertial Upper Stage (IUS) on the mission's seventh orbit. It is believed that the payload was a Magnum SIGINT satellite into geosynchronous orbit.^[4] 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".^[5] Payton was a USAF Manned Spaceflight Engineer (MSE); the USAF declined a NASA offer to fly another MSE on the mission.^[6]

Also, according to Aviation Week, the shuttle initially entered a 204 × 519 km (127 × 322 mi) orbit, at an inclination of 28.45° 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 (322 mi).

The mission lasted 3 days, 1 hour, 33 minutes, and 23 seconds. Discovery touched down on SLF Runway 15 at KSC on January 27, 1985, at 21:23:23 UTC. IMAX footage of the STS-51-C launch was used in the 1985 movie The Dream is Alive.

The crew insignia for STS Flight 51-C includes the names of its five crewmembers. The STS 51-C mission marked the third trip of the Space Shuttle Discovery into space, which is referenced by the three colored trailing strips behind the orbiter in the United States red, white and blue. It was the first Space Shuttle mission totally dedicated to the U.S. Department of Defense, hence the DoD central eagle on the mission patch. The five stars on the upper part of the golden band of the DoD insignia represent the five astronauts. As this mission was classified, the patch includes no further detail as to the mission's payload or nature. For similar reasons, the name of the used orbiter was omitted from the patch.

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 Space Shuttle 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.^[7]

This information is 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 12 °C (54 °F).^[7]

• List of human spaceflights
• List of Space Shuttle missions