Octahedral Research Satellite 4 (ORS 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°. 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° 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°. 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.

Mission Characteristics

Payload

Science Highlights

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.