Uhuru (satellite)
Encyclopedia
Uhuru was the first satellite
launched specifically for the purpose of X-ray astronomy
. It was also known as the X-ray Explorer Satellite, SAS-A (for "Small Astronomy Satellite" A, being first of the three-spacecraft SAS series), SAS 1, or Explorer 42.
The observatory
was launched on 12 December 1970 into an initial orbit of about 560 km apogee, 520 km perigee
, 3 degrees inclination, with a period of 96 minutes.
The mission ended in March 1973.
Uhuru was a scanning mission, with a spin period of ~12 minutes.
It performed the first comprehensive survey of the entire sky for X-ray
sources, with a sensitivity of about 0.001 times the intensity of the Crab nebula.
s, each with ~0.084 m2 effective area.
The counters were sensitive with more than 10% efficiency to X-ray photon
s in the ~2-20 keV
range.
The lower energy 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 energy limit was determined by the transmission properties of the counter filling gas.
Pulse-shape discrimination and anticoincidence
techniques were used to reduce the background due to particles and high-energy photons.
Pulse-height analysis in eight energy channels was used to obtain information on the energy spectrum
of the incident photons.
The two sets of counters were placed back to back and were collimated to 0.52° X 0.52° and 5.2° X 5.2° (full width at half maximum) respectively.
While the 0.52° detector gave finer angular resolution, the 5.2° detector had higher sensitivity for isolated sources.
s such as Cen X-3, Vela X-1
, and Her X-1, the identification of Cygnus X-1
, the first strong candidate for an astrophysical black hole
, and many important extragalactic sources.
The Uhuru Catalog, issued in four successive versions the last being the 4U catalog, was the first comprehensive X-ray catalog, contains 339 objects and covers the whole sky in the 2—6 keV band.
The final version of the source catalog is known as the 4U Calalog; earlier versions were the 2U and 3U catalogs. Sources are referenced as, eg, "4U 1700-37
".
", is the Swahili
word for "freedom". It was named in recognition of the hospitality of Kenya
from where it was launched, from the Italian/Kenyan San Marco launch platform
near Mombasa.
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
launched specifically for the purpose of X-ray astronomy
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
. It was also known as the X-ray Explorer Satellite, SAS-A (for "Small Astronomy Satellite" A, being first of the three-spacecraft SAS series), SAS 1, or Explorer 42.
The observatory
Space observatory
A space observatory is any instrument in outer space which is used for observation of distant planets, galaxies, and other outer space objects...
was launched on 12 December 1970 into an initial orbit of about 560 km apogee, 520 km perigee
Perigee
Perigee is the point at which an object makes its closest approach to the Earth.. Often the term is used in a broader sense to define the point in an orbit where the orbiting body is closest to the body it orbits. The opposite is the apogee, the farthest or highest point.The Greek prefix "peri"...
, 3 degrees inclination, with a period of 96 minutes.
The mission ended in March 1973.
Uhuru was a scanning mission, with a spin period of ~12 minutes.
It performed the first comprehensive survey of the entire sky for X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
sources, with a sensitivity of about 0.001 times the intensity of the Crab nebula.
Objectives
The main objectives of the mission were:- To survey the sky for cosmic X-ray sources in the 2-20 keV range to a limiting sensitivity of 1.5 × 10−11 ergs/(cm2-sec), 5 × 10−4 the flux from the Crab Nebula
- 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 arc minutes
- To determine gross spectral features and variability of X-ray sources
- To, wherever possible, perform coordinated and/or simultaneous observations of X-ray objects with other observers.
Instrumentation
The payload consisted of two sets of proportional counterProportional counter
A proportional counter is a measurement device to count particles of ionizing radiation and measure their energy.A proportional counter is a type of gaseous ionization detector. Its operation is similar to that of a Geiger-Müller counter, but uses a lower operating voltage. An inert gas is used to...
s, each with ~0.084 m2 effective area.
The counters were sensitive with more than 10% efficiency to X-ray photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s in the ~2-20 keV
Kev
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...
range.
The lower energy 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 energy limit was determined by the transmission properties of the counter filling gas.
Pulse-shape discrimination and anticoincidence
Nuclear electronics
Nuclear electronics is a subfield of electronics concerned with the design and use of high-speed electronic systems for nuclear physics and elementary particle physics research, and for industrial and medical use....
techniques were used to reduce the background due to particles and high-energy photons.
Pulse-height analysis in eight energy channels was used to obtain information on the energy spectrum
Spectral analysis
Spectral analysis or Spectrum analysis may refer to:* Spectrum analysis in chemistry and physics, a method of analyzing the chemical properties of matter from bands in their visible spectrum...
of the incident photons.
The two sets of counters were placed back to back and were collimated to 0.52° X 0.52° and 5.2° X 5.2° (full width at half maximum) respectively.
While the 0.52° detector gave finer angular resolution, the 5.2° detector had higher sensitivity for isolated sources.
Results
Uhuru achieved several outstanding scientific advances, including the discovery and detailed study of the pulsing accretion-powered binary X-Ray sourceX-ray binary
X-ray binaries are a class of binary stars that are luminous in X-rays.The X-rays are produced by matter falling from one component, called the donor to the other component, called the accretor, which is compact: a white dwarf, neutron star, or black hole.The infalling matter releases...
s such as Cen X-3, Vela X-1
Vela X-1
Vela X-1 is a pulsing, eclipsing high-mass X-ray binary system, associated with the Uhuru source 4U 0900-40 and the supergiant star HD 77581....
, and Her X-1, the identification of Cygnus X-1
Cygnus X-1
Cygnus X-1 is a well-known galactic X-ray source in the constellation Cygnus. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux density of 2.3 Wm−2Hz−1...
, the first strong candidate for an astrophysical black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
, and many important extragalactic sources.
The Uhuru Catalog, issued in four successive versions the last being the 4U catalog, was the first comprehensive X-ray catalog, contains 339 objects and covers the whole sky in the 2—6 keV band.
The final version of the source catalog is known as the 4U Calalog; earlier versions were the 2U and 3U catalogs. Sources are referenced as, eg, "4U 1700-37
4U 1700-37
4U 1700-37 is one of the stronger binary X-ray sources in the sky.It was discovered by the Uhuru satellite.The "4U" designation refers to the Fourth Uhuru catalog....
".
Naming
The satellite's name, "UhuruUhuru
Uhuru may refer to:, a Lake Victoria ferry in East Africa*Uhuru , campaigns for and achievement of national independence in Africa especially in Kenya, Uganda and Tanzania*Uhuru Kenyatta , Kenyan politician...
", is the Swahili
Swahili language
Swahili or Kiswahili is a Bantu language spoken by various ethnic groups that inhabit several large stretches of the Mozambique Channel coastline from northern Kenya to northern Mozambique, including the Comoro Islands. It is also spoken by ethnic minority groups in Somalia...
word for "freedom". It was named in recognition of the hospitality of Kenya
Kenya
Kenya , officially known as the Republic of Kenya, is a country in East Africa that lies on the equator, with the Indian Ocean to its south-east...
from where it was launched, from the Italian/Kenyan San Marco launch platform
San Marco platform
The Luigi Broglio Space Centre is an Italian-owned spaceport near Malindi, Kenya, named after its founder and Italian space pioneer Luigi Broglio. Developed in the 1960s through a partnership between the University of Rome La Sapienza's Aerospace Research Centre and NASA, the BSC served as a...
near Mombasa.
External links
- SAS-A (Explorer 42) Press Kit
- Uhuru Satellite at (GSFC. NASA)
- NSSDC Master Catalog: Uhuru
- http://heasarc.gsfc.nasa.gov/docs/uhuru/archive/uhuru_archive.html HEASARC archive