Satellite Showcase
Uhuru, The First Small Astronomical Satellite (SAS-1)
Credit: NASA
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The Mission
The Small Astronomical Satellite 1 (SAS-1) was the first earth-orbiting
mission dedicated entirely to celestial X-ray
astronomy. It
was launched from a seaborne platform off the coast of Kenya on 12 December
1970. In honor of being launched on Kenya's Independence Day, the satellite
was renamed "Uhuru", the Swahili word for freedom. The mission
ended in March 1973.
With Uhuru's instruments constantly monitoring the sky, scientists began to
understand just how violent a Universe we live in, far more so than we
believed from visible
light observations. Discovering over 300 sources, the statistical
evidence could then be put together for theorists to build an understanding of
the dynamics of X-ray generation in the cosmos. As a result, the field of
X-ray Astronomy began in earnest.
The Instrumentation
The X-ray detectors on board Uhuru consisted of 2 sets of proportional
counters, each with ~0.084 sq-m effective area. The counters were sensitive
to ~2-20 keV X-rays. The lower limit was determined by the attenuation of
the beryllium windows of the counter plus a thin thermal shroud that was
needed to maintain temperature stability of the spacecraft. The upper limit
was determined by the transmission properties of the filling gas.
The Science Results
Uhuru had a spin period of ~12 minutes and provided a comprehensive view
of the entire sky with a sensitivity of about 0.001 times the intensity of the
Crab nebula. The main objectives of the mission were to do such a sky survey;
to determine discrete source locations with a precision of a few square
minutes of arc for strong sources and a few tenths of a square degree at the
sensitivity limit; to study the structure of extended sources or complex
regions with a resolution of about 30 arcminutes; to determine gross spectral
features and variability of X-ray sources; and, wherever possible, to perform
coordinated and/or simultaneous observations of X-ray objects with other
observers.
The final Uhuru catalog contained over 300 objects -- primarily binary
stellar systems, supernova remnants, Seyfert galaxies, and clusters
of galaxies.
Below we see the discovery of the 4.8 second neutron star rotation period
(Figure 1) and the 2.09 day eclipsing binary period (Figure 2) in the Uhuru
data from Cen X-3. The solid lines represent the best mathematical fits to
the periodic behavior of the data.
Figure 1
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Credit: NASA
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Uhuru determination of the neutron star rotation period from the binary
Cen X-3
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Figure 2
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Credit: NASA
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Uhuru observation of the orbital modulation found in the X-ray emission of
Cen X-3
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