Image: Hexagon canister
CIA
According to the declassified CIA document, "On 10 July 1971, the third Hexagon RV was lost. The parachute failed to reef before fully deploying, and it was snapped off at the swivel. The RV entered ballistically and impacted the water with a force of approximately 2600 G's and settled in 16,000 feet of water."
By
updated 8/10/2012 11:56:01 PM ET 2012-08-11T03:56:01

It's a plot worthy of a Hollywood action movie: 40 years ago, the U.S. Navy carried out a daring mission to retrieve a top-secret film capsule that had settled more than 16,000 feet (4,876 meters) underwater on the ocean floor. At the time, the expedition was the deepest undersea salvage operation ever attempted.

Documents released by the Central Intelligence Agency on Wednesday detail the capsule's incredible recovery, using what was at the time the Navy's most sophisticated deep-sea submersible.

On July 10, 1971, a classified U.S. satellite, code-named Hexagon, attempted to return a mysterious "data package" to Earth by ejecting a capsule over the Pacific Ocean. The capsule's parachute failed, and the canister slammed into the water with an excruciating 2,600 G's of force.

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Hexagon satellites, which were declassified in 2011, were photo reconnaissance spacecraft that were part of an American Cold War-era spy program. Since these satellites preceded today's era of digital technology, Hexagons recorded images on film, sending them back to Earth in capsules that re-entered Earth's atmosphere and were recovered within a designated zone near the Hawaiian Islands in the Pacific Ocean.

During the first Hexagon mission in 1971, the parachute attached to one of these capsules broke. The capsule sank to a depth of about 16,400 feet (almost 5,000 meters) in the Pacific.

To recover the sunken capsule, the U.S. Navy crafted a bold rescue mission to be carried out by its Trieste II Deep Sea Vehicle, or DSV-1. At the time, this mini-submarine was the Navy's best deep-sea submersible. [Photos: 1972 Spy Satellite Capsule's Deep-Sea Rescue]

The newly declassified CIA documents include a report of the undersea mission and pictures of the Navy's DSV-1 and the film capsule at the bottom of the ocean. The report explains how and why the CIA decided to retrieve the Hexagon Recovery Vehicle (RV) in the first place.

"The decision was made to attempt the deep-sea recovery of the RV primarily for the intelligence value of the film record and secondly to establish a capability for deep oceanographic recovery," intelligence officials wrote.

Image: Hexagon RV
CIA
A Hexagon Recovery Vehicle (RV) is shown in its pre-launch state.
Image: Trieste II
CIA
The Trieste II bathysphere, with its recovery hook attached, was used during three secret attempts to retrieve the sunken Hexagon Recovery Vehicle in 1971 and 1972.
Image: Hexagon film stack
CIA
The damaged Hexagon spy satellite film stack sits on the ocean floor, before the 1972 U.S. recovery effort.

According to the documents, the Trieste II made three attempts to salvage the film capsule: first on Nov. 3, 1971; then on Nov. 30, 1971; and finally, in a successful third try, on April 25, 1972.

The mission's planners had four primary areas of concern, beginning with the ability to pinpoint the impact area. At that time, no object the size of the film canister had ever been detected by sonar and been searched for underwater. Officials were also unsure how much damage the capsule had suffered upon impact, and after being submerged in sea water. Finally, the Trieste II had yet to venture below 10,000 feet (3,048 meters) before then.

These obstacles were eventually overcome: Before dawn on April 26, 1972, the Trieste II emerged from the water about 350 miles (563 kilometers) north of the Hawaiian Islands with the remains of the Hexagon film capsule.

"The third attempt was successful in locating and securing the film stacks; however, as the Trieste was surfacing, the film broke into pieces," officials wrote in their report. "Twenty-five feet was recovered."

The film stack underwent extensive analysis, and it was determined that the Hexagon's Recovery Vehicle broke apart as it crashed into the water. The spools of film were separated from the capsule, and several pieces were cut and floated away.

All was not lost: The mission proved to be a valuable test of the Navy's ability to carry out deep-sea recovery expeditions. In the report, CIA officials discussed some of the lessons learned, particularly from the setbacks that were experienced on all three attempts.

"The third dive, the mechanical arm failed to work, almost preventing operation of the recovery device," the report said. "The on-board computer has never worked. Much more attention is required to the use of high reliability parts and extensive subsystem testing to assure confidence in any given operation."

Over the course of the operation, the focus shifted from the potential value of the film's reconnaissance to the value of testing the capabilities of the Trieste II submersible.

"All of the men involved remained enthusiastic and determined throughout the many frustrations and are to be commended for their fine efforts," the report concluded.

Follow Denise Chow on Twitter @denisechow or Space.com @Spacedotcom. We're also on Facebook and Google+.

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