|Mission type||Magnetospheric research|
|Mission duration||1,876 days (5 years, 1 month and 21 days)|
|Manufacturer||Goddard Space Flight Center|
|Launch mass||212.0 kilograms (467.4 lb)|
|Start of mission|
|Launch date||July 1, 1966, 16:02:25UTC|
|Launch site||Cape Canaveral LC-17A|
|End of mission|
|Last contact||September 21, 1971|
|Perigee altitude||265,679 kilometers (165,085 mi)|
|Apogee altitude||480,762 kilometers (298,732 mi)|
|Argument of perigee||119.2000 degrees|
|Mean anomaly||21.7899 degrees|
|Epoch||May 12, 1971, 12:00:00 UTC|
Explorer 33 (also known as AIMP-D, IMP-D, AIMP 1, Anchored IMP 1, Interplanetary Monitoring Platform-D) was a spacecraft in the Explorer program launched by NASA on July 1, 1966 on a mission of scientific exploration.
Originally intended for a lunar orbit, mission controllers worried that the spacecraft's trajectory was too fast to guarantee lunar capture. Consequently, mission managers opted for a backup plan of placing the craft into an eccentric Earth orbit with a perigee of 265,679 km and an apogee of 480,762 km — still reaching distances beyond the Moon's orbit.
Despite not attaining the intended lunar orbit, the mission met many of its original goals in exploring solar wind, interplanetary plasma, and solar X-rays. Principal investigator James Van Allen used electron and proton detectors aboard the spacecraft to investigate charged particle and X-ray activity. Astrophysicists N. U. Crooker, Joan Feynman, and J. T. Gosling used data from Explorer 33 to establish relationships between the Earth's magnetic field and the solar wind speed near Earth.