Explorer 33

Did You Know?

• Explorer 33 was intended to be the first U.S. spacecraft to enter lunar orbit.
• 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 Earth's magnetic field and solar wind speed near Earth.

Spacecraft

Explorer 33 was spin-stabilized with its spin axis parallel to the ecliptic plane and a spin period varying between 2.2 and 3.6 seconds. Its main body was an octagonal prism 71 cm across and 20.3 cm high, with a mass of 93.4 kg. Four n/p solar cell arrays produced an average of 43 watts, and two 183 cm magnetometer booms extended from the bus. Four whip antennas were mounted on top, and a 35.8 kgf thrust retrorocket (Thiokol TE-M-458) was mounted on the bus. Power was stored in silver-cadmium batteries, and communication used a 7-watt transmitter with PFM-PM telemetry and a digital data processor.

Experiments

The Ames magnetometer experiment used a boom-mounted triaxial fluxgate magnetometer with orthogonally mounted sensors, one along the spin axis. A motor interchanged a spin-plane sensor with the axial sensor every 24 hours for inflight zero-level determination. The noise threshold was about 0.2 nT, with three ranges covering ±20, 60, and 200 nT full scale per vector component, digitized to 1% of the full range. Measurements were taken every 2.05 seconds, with the same range repeated every 6.14 seconds. Three EON type 6213 Geiger–Müller tubes (GM1, GM2, GM3) and a silicon solid-state detector (SSD) measured solar X rays (2–12 Å) and charged particles. The GM tubes detected electrons above 45–50 keV and protons above 730–830 keV. The SSD had four thresholds: PN1 detected protons 0.31–10 MeV and alphas 0.59–225 MeV; PN2 detected protons 0.50–4 MeV and alphas 0.78–98 MeV; PN3 detected protons 0.82–1.9 MeV and alphas 1.13–46 MeV; PN4 detected alphas 2.1–17 MeV. GM1 and the SSD were parallel to the spin axis, GM3 antiparallel. Data from GM1 and PN1 were divided into quadrants relative to the Sun. Readout intervals were 82 or 164 seconds. High temperatures adversely affected SSD particle data from 16 September to 14 January and 16 March to 14 July each year after 16 September 1966, but alpha data were unaffected. On fewer than ten occasions, a GM tube produced high spurious counts for several hours during extremely high particle and X-ray fluxes.

Launch

Explorer 33 launched on 1 July 1966 from Cape Kennedy, Florida, on a Thor-Delta E1. The second and third stages delivered excess thrust, giving the spacecraft about 21.3 m/s extra velocity toward the Moon, too much for the retrorocket to achieve lunar orbit. The retrorocket placed it into a highly elliptical Earth orbit with an inclination of 28.9°, a perigee of 265,680 km, and an apogee of 480,763 km—the highest orbit of any satellite up to that time. It came within 35,000 km of the Moon on its first orbit and within 40,000 km on subsequent approaches in September, November, and December 1966. All experiments operated successfully until September 1971.

MOSFET-based telemetry system

Explorer 33 was part of a series where a major leap in spacecraft electronics came from using MOSFETs, a type of transistor NASA chose for the IMP program in 1964. These MOSFET blocks came from General Microelectronics, which had just started selling MOS technology that same year and got NASA as its first customer. MOS technology made chip fabrication simpler, allowing more transistors on a single chip, which solved the problem of needing more onboard electronics for communication and other tasks. At Goddard Space Flight Center, engineers built circuits from MOSFET blocks and resistors, which made up 93% of the AIMP-D's electronics. This shift boosted the transistor count from 1,200 on the first three IMP spacecraft to 2,000 on AIMP-D, and communication channels from 175 to 256. It also cut the number of non-resistor parts from 3,000 on IMP-A to 1,000 on AIMP-1, even though the satellite was twice as complex electrically. Earlier IMP spacecraft used some integrated circuits, but their encoders still relied on one transistor per package. AIMP-1 packed 4,200 semiconductors into just 700 packages, reducing both component count and space, with parts arranged in cordwood modules. It also upgraded the Digital Data Processors and added an Optical Aspect Computer that could switch between power-saving modes to ease the load on batteries and solar panels.