HEASARC Staff Scientist Position - Applications are now being accepted for a Staff Scientist with significant experience and interest in the technical aspects of astrophysics research, to work in the High Energy Astrophysics Science Archive Research Center (HEASARC) at NASA Goddard Space Flight Center (GSFC) in Greenbelt, MD. Refer to the AAS Job register for full details.
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.
HEASARC Staff Scientist Position - Applications are now being accepted for a Staff Scientist with significant experience and interest in the technical aspects of astrophysics research, to work in the High Energy Astrophysics Science Archive Research Center (HEASARC) at NASA Goddard Space Flight Center (GSFC) in Greenbelt, MD. Refer to the AAS Job register for full details.
