It was hoped that Explorer 33, also known as the Anchored Interplanetary Monitoring Platform (AIMP), would become the first U.S. spacecraft to enter lunar orbit (planned parameters were 800 x 4,000 miles or 1,300 × 6,440 kilometers at 175 degree inclination), but the Thor Delta E-1 second stage accelerated too rapidly, ensuring that lunar orbit would not be possible.

Instead mission planners adopted an alternate mission which required the probe’s (Thiokol TE-M-458) 79 pound-force (35.8 kilogram-force) thrust retro-rocket to fire about 6.5 hours after launch. Under the new plan, the spacecraft was put into a highly elliptical Earth orbit of 279,000 x 19,000 miles (449,174 × 30,550 kilometers) at 28.9 degree inclination. In this orbit, Explorer 33 was high enough to be perturbed by the Moon’s gravitation.

In its new orbit, Explorer 33 approached the Moon on its very first orbit to a distance of 22,000 miles (35,000 kilometers) with subsequent close approaches in Sept., Nov., and Dec. 1966 varying from 25,000 to 37,000 miles (40,000 to 60,000 kilometers).

During this time, the probe returned key data on Earth’s magnetic tail, the interplanetary magnetic field, and radiation. In January 1967, Goddard Space Flight Center engineers used an electric “screwdriver” to restore power to the spacecraft after a temporary blackout. The emergency repair was the most remote repair carried out on a spacecraft to date.

The mission was declared a success by Jan. 1, 1967, although the spacecraft continued to return useful data until Sept. 15, 1971.


Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.

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