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The HEASARC and NuSTAR teams are greatly saddened by the sudden passing of Katja Pottschmidt. Most recently Katja was the lead scientist for the NuSTAR Guest Observer Facility (GOF), a role she had supported for many years. During her science career she worked on many other high energy astrophysics missions and played an integral role in advancing our knowledge of the universe. She was a wonderful colleague and friend and will be keenly missed by all who knew her.
ORS 4
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.