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Satellite Showcase

Uhuru, The First Small Astronomical Satellite (SAS-1)

Uhuru (SAS-1)
Credit: NASA

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
Uhuru measurement of the rotational period of Cen X-3
Credit: NASA
Uhuru determination of the neutron star rotation period from the binary Cen X-3

Figure 2
Uhuru measurement of the orbital period of Cen X-3
Credit: NASA
Uhuru observation of the orbital modulation found in the X-ray emission of Cen X-3

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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