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Vela 5A

Vela 5A in the clean room
Credit: Los Alamos National Laboratory

* Mission Overview

The Vela-5A nuclear test detection satellite was part of a program run jointly by the Advanced Research Projects of the U.S. Department of Defense and the U.S. Atomic Energy Commission, managed by the U.S. Air Force. It and its twin, Vela-5B, were placed ~180 degrees apart in nearly circular orbits at a geocentric distance of ~118,000 km on 23 May 1969. The orbital period was ~112 hours. The satellite rotated about its spin axis with a ~64-sec period. The X-ray detector was located ~90 degrees from the spin axis, and so covered the celestial sphere twice per satellite orbit. Data were telemetered in 1-sec count accumulations. The X-ray detectors on Vela 5A failed on 24 July 1970.

* Instrumentation

The scintillation X-ray detector (XC) aboard Vela 5A consisted of two 1-mm- thick NaI(Tl) crystals mounted on photomultiplier tubes and covered by a 5-mil-thick beryllium window. Electronic thresholds provided two energy channels, 3-12 keV and 6-12 keV. In front of each crystal was a slat collimator providing a FWHM aperture of ~6.1x6.1 degrees. The effective detector area was ~26 cm2. Sensitivity to celestial sources was severely limited by the high intrinsic detector background.

Both Vela 5A and 5B also carried 6 gamma-ray detectors. They had a total volume of ~60 cm3 of CsI and could detect photons in the 150-750 keV energy range. It was in 1969-70 that the Vela spacecraft first discovered gamma-ray bursts. The gamma-ray detectors continued to function until at least 1979.

* Science

The four Vela satellites (5A & B, 6A & B) recorded 73 gamma-ray bursts in the ten year interval July 1969 - April 1979.

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