In 1976, when Carl Berglund was almost 50 years old, a plan to send a spacecraft to Halley's Comet landed on his desk.

Berglund, an engineer at NASA's Jet Propulsion Laboratory, was used to seeing bold ideas, but this one was particularly ambitious. A spacecraft equipped with a square sheet of Mylar nearly a kilometer wide would harness the pressure of sunlight for propulsion, spiral closer to the Sun than Mercury, throw itself out of the plane of our solar system, and rendezvous with the world's most famous comet, which was returning to Earth's skies in 1986.

Berglund's formal project title was lead designer, but to hear him tell it, he was simply a "cog engineer." At JPL, he looked at preliminary spacecraft designs and helped figure out how specific components would work. He doesn't remember having a specific reaction to what he soon learned was a "solar sail"—he just went to work on the project like anything else.

"A lot of work, you know, we just did it because it came along," he told me last year.

Around the time Berglund joined the project, Carl Sagan appeared on the Tonight Show with a model of the kite-like spacecraft—one of two designs being considered. The program manager was the scientist-engineer Louis Friedman, and JPL director Bruce Murray supported the effort. Together, Sagan, Friedman and Murray would found The Planetary Society in 1980.

Berglund only worked on the solar sail for a few months. But during that time, he amassed a treasure trove of meeting notes, schematics and overhead slides—all of which he has saved to this day.

The documents provide new insights into what would have been the world's first solar sail, which laid the groundwork for The Planetary Society's Cosmos 1, LightSail 1 and LightSail 2 spacecraft.