The Ranger program was a probe series to transmit close-up black and white
photographs of the Moon before crashing into the Lunar surface. Three of
the nine Rangers (7,8,9) were successful. Rangers 1 through 5 experienced
technical problems which affected the success of the missions: the launch
vehicles malfunctioned for Ranger 1 and 2, Rangers 3 and 5 missed the
Moon, and Ranger 4 landed on the back side of the Moon and returned no
Ranger 3 was launched on 26 January 1962. It carried an instrument designed
to measure gamma-rays coming from the surface of the moon. It was intended
that the instrument would take some 60 hours of data during the flight
before lunar impact in order to determine the interplanetary gamma-ray
flux, and to determine the background for the lunar measurements. Twelve
hours of data were to be taken with the detector in a stowed position, and
the remaining data were to be taken with the detector extended on a 6 ft.
boom. These data permitted a determination of the satellite body effect on the
measured gamma-ray flux. Missing the moon, the satellite went into a solar
orbit (apogee 1.163 AU; perigee 0.9839 AU) and returned no further data.
Ranger 5 was launched on 18 October 1962. Missing the Moon by 725 km, it
went into a heliocentric orbit, with apogee 1.052 AU and perigee 0.949 AU, at
orbital inclination 0.39 degrees. Five hours of data were taken of the
cosmic gamma-ray background, with the detector in the stowed position only.
Both Ranger 3 and Ranger 5 carried the same gamma-ray instrument. The
experiment consisted of 3 units: a detector, a 32-channel pulse height
analyzer, and a high-voltage power supply. The detector was a beveled 3-in
diameter CsI crystal, surrounded in a phoswich arrangement by a 1/8-in
thick plastic scintillator. This was coupled to a 3-in photomultiplier
tube. The pulse height analyzer provided storage of 2e16 pulses in each
channel. Channel 0 recorded all pulses larger than those appearing in
channel 31. Alternate spectra were taken over the energy ranges 0.1-3.0 MeV and
20-600 keV. Data were integrated for 459.5 s. On-board radioactive sources
allowed for calibration.
An average cosmic gamma-ray countrate of 0.27 cts/sq-cm/s between 0.5-2.1
MeV, and 0.67 cts/sq-cm/s above 2.1 MeV was observed. These values were in
good agreement with previous balloon and rocket measurements. The continuum
fell roughly as E^-2.4 up to 1 MeV and was essentially flat thereafter.