ATK didn't own Thiokol until 2001. Just one of a long history of splits and mergers.

1929: Thiokol Chemical Company founded.
1949: Thiokol produce the TX-18 Falcon missile, the world's first solid-fueled missile system.
1957: In anticipation of the forthcoming Minuteman contract, the company builds its plant at Brigham City, Utah.
1957: Thiokol Huntsville builds XM33 Pollux missile
1958: Merger with Reaction Motors Inc. (RMI), makers of liquid propellant rocket motor systems.
1958: Thiokol awarded contract to build the TU-122 rocket motor for the first stage of the LGM-30 Minuteman ICBM system.
1959: Thiokol Huntsville produces CASTOR strap-on booster rocket, used on the Atlas rocket.
1974: Thiokol wins the contract to build the solid rocket booster (SRB) for the Space Shuttle
1978: The company sells its ski lift division to CTEC and its snow equipment division to Logan Manufacturing Company (LMC), owned by John DeLorean.
1980: Acquires Carlisle Chemical Company of Cincinnati, Ohio.
1982: Thiokol merges with Morton-Norwich products (owners of the Morton salt concern, the Simoniz automotive products brand, and various chemical concerns). The merged company is called Morton Thiokol Incorporated (MTI).
1986: An O-ring fault in an MTI SRB destroys space shuttle Challenger in flight
1989: Morton Thiokol splits, with most of the chemical concern going with Morton. The propulsion systems divsion becomes Thiokol Inc.
1998: Thiokol changes name to Cordian Technologies.
2000: Thiokol merges with two divisions of Alcoa and with Howmes Castings and Huck Fasteners to become AIC Group (Alcoa Industrial Components).
2001: Alliant Techsystems (ATK) Inc. (a company formed when Honeywell spun off its defense division) spends $2.9 billion buying Thiokol and related businesses from AIC/Alcoa. ATK built the third stage of the Trident missile and had earlier bought Hercules Aerospace Co., builder of the second stage. With the purchase of Thiokol, who make the missile's first stage, ATK controls the lion's share of the US solid-rocket market.