Explorer 33 was designed to become the first U.S. spacecraft to enter lunar orbit, where it was to study interplanetary charged particles, magnetic fields, and solar X-rays.
Explorer 33's second-stage launch gave it too much velocity, so its retrorocket was unable to slow it down enough to be captured by the Moon's gravity. Instead, it continued to fly in a very long, elliptical orbit around Earth that stretched more than 50,000 km beyond the Moon's orbit. Nevertheless, it returned key data on Earth's magnetic field, the interplanetary magnetic field, and radiation.
1 Jul 1966: Launch
21 Sep 1971: Contact Lost
Mission Type: Orbiter
Launch Vehicle: Thor-Delta E-1 (no. 39 / Thor no. 467 / DSV-3E)
Launch Site: Eastern Test Range / launch complex 17A, Cape Canaveral, USA
NASA Center: Goddard Space Flight Center
Spacecraft Mass: 93.4 kg
- fluxgate magnetometers
- thermal ion probe
- ion chamber
- tubes plus p-on-n junction
- Faraday-cup probe
Spacecraft Dimensions: Octagonal body 71 cm in diameter, 20 cm high
Spacecraft Power: 4 arrays of solar cells
Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, Monographs in Aerospace History No. 24, by Asif A. Siddiqi
National Space Science Data Center, http://nssdc.gsfc.nasa.gov/
Solar System Log by Andrew Wilson, published 1987 by Jane's Publishing Co. Ltd.
Explorer 33 was designed to become the first U.S. spacecraft to enter lunar orbit (planned parameters were 1,300 x 6,440 kilometers at 175° inclination), but the Thor Delta E-1 second stage accelerated too rapidly for compensation by the probe's retrorocket to achieve lunar orbit.
Instead, the spacecraft (56.7 kg by this time) went into an eccentric Earth orbit of 15,897 x 435,330 kilometers. The main solid-propellant retrorocket engine later stabilized the orbit to a less eccentric 30,550 x 449,174-kilometer orbit at 28.9° inclination. In its new orbit, the probe returned key data on Earth's magnetic tail, the interplanetary magnetic field, and radiation.