Imagine the Universe!

Satellite Showcase

Vela 5B

Vela 5B
Credit: NASA

The Mission

The Vela 5B 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. There were 12 Vela satellites, launched in 6 pairs, put into orbit during the lifetime of the program. Vela 5A and 5B were placed into nearly circular orbits (roughly 180 degrees apart) at a geocentric distance of ~118,000 km on 23 May 1969; the orbital period was ~112 hours. Each satellite rotated about its spin axis with a ~64-sec period. Identical X-ray detectors were on each satellite, located ~90 degrees from the spin axis so that the entire celestial sphere was observed twice per satellite orbit. Data were telemetered in 1-sec count accumulations. Outliving its twin by a significant period of time, Vela 5B operated until 19 June 1979, although telemetry was often not recorded after mid-1976.

The Instrumentation

The scintillation X-ray detector (XC) aboard Vela 5B 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 sq-cm. Sensitivity to celestial sources was severely limited by the intrinsic detector background of ~36 cts/sec. The Vela 5B X-ray detector yielded ~40 cts/sec for the Crab, so 1 Vela ct/sec ~25 UFU~4.5E-10 ergs/sq-cm/sec in the 3-12 keV response band.

Both Vela 5A and 5B also carried 6 gamma-ray detectors. They had a total volume of ~ 60 cu-cm of CsI and could detect photons in the 150-750 keV energy range. It was in 1969-70 that these detectors on board the Vela spacecraft first discovered gamma-ray bursts.

The Science Results

While modest in its size and limited by its high background, Vela 5B's long lifetime afforded it unique opportunities to provide major scientific contributions. It was one of the first satellites to report the existence of gamma-ray bursts, and co-discovered (with ANS) X-ray bursts. Many papers have been published concerning the long-term variabilities of X-ray binaries and the appearances of X-ray transients. The Vela 5B light curve of one such transient, V0332+53, is shown below.

Ten year X-ray light curve of V0332+53
Credit: NASA
The ten-year light curve of the transient source V0332+53 as seen by the Vela 5B all-sky monitor. The source became very bright in 1973, and was not seen again until 1983.

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

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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
Project Leader: Dr. Barbara Mattson
Curator: Meredith Gibb
Responsible NASA Official: Phil Newman
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This page last updated: Friday, 28-Sep-2007 15:39:14 EDT