NASA's Explorer 33 spacecraft, also known as
Anchored IMP A (AIMP A) and as IMP D, was launched into a geocentric
orbit in July, 1966. Explorer 33 was a spin-stabilized (spin axis
parallel to the ecliptic
plane, spin period varying between 2.2 and 3.6 s) spacecraft instrumented
for studies of interplanetary plasma, energetic charged particles
(electrons, protons, and alphas), magnetic fields, and solar X rays
at lunar distances. The spacecraft failed to achieve lunar orbit but
did achieve mission objectives. Lunar
perturbations caused significant variations in orbit parameters
The initial apogee occurred at about
1600 h local time. Over the first 3-yr period, perigee varied between
6 and 44 earth radii. Apogee varied between 70 and 135 earth radii,
and the inclination with respect to the equator of the earth varied
between 7 and 60 deg. Periods of principal data coverage (essentially
100%) are July 1, 1966 (launch), to January 14, 1970; February 21,
1970, to March 6, 1970; July 31, 1970, to September 14, 1970; January
15, 1971, to February 28, 1971; March 23, 1971, to May 31, 1971; and
August 23, 1971, to September 15, 1971. No data were obtained after
September 21, 1971.