 Uhuru (X-ray Explorer Satellite) | |
| Mission type | Astronomy |
|---|---|
| Operator | NASA |
| COSPAR ID | 1970-107A |
| SATCATno. | 4797 |
| Website | heasarc |
| Mission duration | 3 years |
| Spacecraft properties | |
| Dry mass | 141.5 kilograms (312 lb) |
| Start of mission | |
| Launch date | 12 December 1970, 10:53:50 (1970-12-12UTC10:53:50Z) UTC |
| Rocket | Scout B S175C |
| Launch site | San Marco |
| End of mission | |
| Last contact | March 1973 (1973-04) |
| Decay date | 5 April 1979 |
| Orbital parameters | |
| Reference system | Geocentric |
| Regime | Low Earth |
| Eccentricity | 0.002956 |
| Perigee altitude | 520.0 kilometers (323.1 mi) |
| Apogee altitude | 560.0 kilometers (348.0 mi) |
| Inclination | 3.0 degrees |
| Period | 95.70 minutes |
| Epoch | 12 December 1970, 05:54:00 UTC[1] |
| Instruments | |
| All-Sky X-Ray Survey | |
Uhuru was the firstsatellite launched specifically for the purpose ofX-ray astronomy. It was also known as theX-ray Explorer Satellite,SAS-A (forSmall Astronomy Satellite A, the first of the three-spacecraft SAS series),SAS 1, orExplorer 42. TheNASAobservatory was launched on 12 December 1970 into an initial orbit of about 560 kmapogee, 520 kmperigee, 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 forX-ray sources, with a sensitivity of about 0.001 times the intensity of theCrab nebula.
The main objectives of the mission were:[2]
The payload consisted of two sets ofproportional counters, each with ~0.084 m2 effective area. The counters were sensitive with more than 10% efficiency to X-rayphotons in the ~2–20keV 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 andanticoincidence techniques were used to filter out emissions of particles and undesirable high-energy photons in the background. Pulse-height analysis in eight energy channels was used to obtain information on the energyspectrum of the incident photons. The two sets of counters were placed back to back and were collimated to 0.52° × 0.52° and 5.2° × 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.[2]
Uhuru achieved several outstanding scientific advances, including the discovery and detailed study of the pulsing accretion-poweredbinary X-Ray sources such asCen X-3,Vela X-1, andHer X-1, the identification ofCygnus X-1, the first strong candidate for an astrophysicalblack 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.[3]The final version of the source catalog is known as the 4U Catalog;[4] earlier versions were the 2U and 3U catalogs. Sources are referenced as, e.g., "4U 1700-37".
The satellite's name, "Uhuru", is theSwahili word for "freedom". It was named in recognition of the hospitality ofKenya from where it was launched, using the ItalianSan Marco launch platform near Mombasa.[citation needed]
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