* Mission Overview

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.

* Instrumentation

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.

* Science

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.

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Page authors: Lorella Angelini Jesse Allen
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