ORS 4 (Octahedral
Research Satellite 4), also known as Environmental Research
Satellite-18 (ERS-18), was launched
on 28 April 1967. The elliptical orbit had an apogee of 111,553 km, a
perigee of 8631 km, and an inclination of 32.9 degrees. The orbital period
was 2840 minutes. The spacecraft was a spin stabilized octahedron that
weighed 7.8 kg and measured 29.3 cm along each triangular edge. Each of the
8 triangular faces contained solar cells, allowing for an average power
output of 4 Watts. The spin rate was initially 6 rpm. There was a
large coning during the early lifetime and owing to improper dynamic
balancing, the final stable spin axis was ~ 90 degrees from the intended
one. This change of orientation caused only minor effects in the data
interpretation. The satellite carried a solar aspect sensor that determined
the angle between the satellite-sun line and the satellite spin axis to
within 7.5 degrees. The spacecraft operated well from launch until 3 June
1968, when a preset timer turned off the transmitter.
The primary objectives of this satellite were to measure the cosmic gamma-
ray spectrum between 0.25-6 MeV, monitor the solar X-ray flux, obtain a
background measurement for a prototype space nuclear detonation detector,
and measure charged particles within the magnetosphere.
The gamma-ray experiment consisted of 2 separate detector systems. The main
system was used to measure the cosmic gamma-ray spectrum in the range 0.25-
6 MeV. It consisted of a 7.62 cm diameter by 6.35 cm long NaI crystal
surrounded on all sides but one by a 1 cm thick plastic scintillation
counter. The 2 crystals were optically separate and viewed by separate
photomultiplier tubes. The plastic counter served as a charged particle
rejector. A five channel differential pulse height analyzer and 2 integral
discriminators were used to measure the energy loss in the central
detector. Each segment was sampled for 4.7 s every 75.2 s. A second gamma-ray
detector system was used to determine the feasibility of a simple heavily
shielded NaI crystal to measure delayed gamma-rays from a nuclear
detonation in space. The sole purpose was to obtain the background counting
rate as a function of position in the magnetosphere.
The spectrum determined by the ORS 4 detector above 1 MeV was considerably
above the extension of the diffuse cosmic background X-ray spectrum
previously detected by Ranger III and other experiments. This led to the
belief that there was an additional cosmic or galactic gamma-ray component
above the one accounted for by the intergalactic electrons scattering on
the 3 degree Kelvin background radiation.
