PGM-17 Thor: Three Lost Mice, Explorer 6's Earth Photo and the GRAB Spy

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The PGM-17 Thor intermediate-range ballistic missile became the core of one of the most important space launch families, whose Delta descendants retired only in 2024. Its first space work tested reentry vehicles for the Atlas missile: the Thor-Able topped the Thor core with an Able stage built around the Aerojet AJ10-40 engine taken from the Vanguard program. The first Thor-Able, flight 116 on April 23, 1958, was lost to a main engine turbopump failure, and the next two flights also failed to recover their reentry vehicles; each of the three carried a mouse, and all three died. A third stage, the Altair X-248 solid motor, was added, but in August 1958 another turbopump failure doomed the 84-pound Pioneer 0 lunar probe 73.6 seconds after launch. On August 7, 1959, a Thor-Able launched Explorer 6, the first satellite to send back pictures of Earth from orbit.

The restartable Able-Star stage, burning hypergolic propellants fed from gas-pressurized tanks, launched Transit 1B on April 13, 1960, the forerunner of satellite navigation, and GRAB-1 on June 22, 1960. Publicly a solar radiation satellite, GRAB-1 carried a classified package to detect Soviet air defense radars, making it the first successful electronic intelligence satellite. The Thor-Agena launched early Corona reconnaissance satellites from Vandenberg between June 1959 and May 1963, and to lift heavier spy satellites engineers created the Thrust Augmented Thor, with three Castor solid boosters of 53,000 pounds each, then a long-tank version holding 49,000 more pounds of propellant, which led to the Thorad-Agena and the Delta family. On June 29, 1961, an Able-Star stage that had delivered a Transit satellite exploded in orbit, the first object to unintentionally explode in space, creating at least 294 trackable pieces of debris.

• Turbopump cavitation, vapor bubbles collapsing in the liquid oxygen flow, could strip impeller blades and starve the engine of fuel.
• Hypergolic fuels ignite on contact, so the Able-Star needed no turbopumps or spark igniter to restart in orbit.
• GRAB, for Galactic Radiation and Background, flew weeks after Francis Gary Powers' U-2 was shot down and almost two months before Corona first returned film.
• The Agena's horizon sensors and cold gas thrusters held Corona cameras steady enough for high-resolution photos.
• Strapped-on solid boosters let the proven Thor core place 1,400 to 2,800 pounds into 100-nautical-mile polar orbits.