An enormous volume of photographic imagery from the KH-9 HEXAGON intelligence satellites was quietly declassified in January and will be transferred to the National Archives later this year for subsequent public release.

The KH-9 satellites operated between 1971 and 1984. The imagery they generated should be of historical interest with respect to a wide range of late Cold War intelligence targets but is also expected to support current scientific research on climate change and related fields of inquiry.

The film-based KH-9 satellites were officially declared “obsolete” by the Director of National Intelligence in 2011. The KH-9 imagery was nominally approved for declassification in February 2012, and then it was finally declassified in fact this year.

ODNI spokesman Michael Birmingham said that approximately 97 percent of the satellite imagery that was collected from the 19 successful KH-9 missions was formally declassified by DNI James R. Clapper on January 11, 2013.

“The small amount of imagery exempted from this declassification decision will be removed prior to its accession to the National Archives (NARA) and will remain classified pursuant to statute and national security interests, and reviewed periodically to determine if additional declassification is warranted,” Mr. Birmingham said last week.

The imagery is being transferred to NARA in stages, with final delivery scheduled for September 2013, he said.

The transfer is being implemented pursuant to a November 2012 Memorandum of Agreement between the National Geospatial-Intelligence Agency (NGA) and the National Archives, under which the Archives is “responsible for providing public access to the declassified imagery.”

Reishia R. Kelsey of NGA public affairs confirmed that the imagery “will be made available to the public following its accession to NARA” later this year.

The National Archives was not prepared last week to set a precise date for public release. But an Archives official said that “NARA intends to make these records available to the public at our research room in College Park, MD as soon as possible following transfer.”

If successfully executed, the release of the KH-9 imagery will constitute a breakthrough in the declassification and disclosure of national security information. It will be one of several discrete but momentous shifts in secrecy policy during the Obama Administration that have often gone unrecognized or unappreciated. Though these declassification actions took years or decades to accomplish, they have been downplayed by the White House itself, which has seemed curiously ambivalent about them. They include the public disclosure of the size of the U.S. nuclear weapons arsenal, the routine publication of the annual intelligence budget request, the release of the Office of Legal Counsel “torture memos,” the declassification of the KH-9 satellite itself, and others.

The KH-9 imagery is being processed for public release pursuant to the 1995 Executive Order 12951 on “Release of Imagery Acquired by Space-based National Intelligence Reconnaissance Systems.” That order had been effectively dormant since the Clinton Administration, when the last major release of intelligence satellite imagery (from the CORONA, ARGON and LANYARD missions) took place.

The declassification of the KH-9 imagery is a massive undertaking, Mr. Birmingham of ODNI said last year.

“For context, and to grasp the scope of the project, the KH-9/HEXAGON system provided coverage over hundreds of millions of square miles of territory during its 19 successful missions spanning 1971-1984,” he said. “It is a daunting issue to address declassification of the program specifics associated with an obsolete system such as the KH-9, which involves the declassification of huge volumes of intelligence information gathered on thousands of targets worldwide during a 13 year time period.”