artist concept of Vela 5B

* Mission Overview

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. It was placed in a nearly circular orbit 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. Vela-5B operated until 19 June 1979, although telemetry tracking was poor after mid-1976.
While modest in its size and limited by its high background, Vela-5B's long lifetime afforded it unique opportunities 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. The long mission lifetime gave the opportunities to monitor the variability in X-ray binaries and records transient behaviour.

* Instrumentation

Vela-5B carried two set of instruments : the scintillation X-ray detector (XC) and 6 gamma ray detector. 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 cm2. 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~4.5X10-10 ergs/sq-cm/sec in the 3-12 keV response band.
One important detector performance characteristic which affects the Vela-5B data is a gain variation due to a ~60° C satellite temperature change from one side of the orbit to the other. If the data for a source were taken when the satellite was at one of its temperature extremes, a profound modulation is introduced into the count rate at the 56-hour timescale between observation sequences of the source. Additionally, the amplitude of the effect is modulated by the ~300-day precession period of the Vela-5B orbit. Lack of pre-launch testing precludes any quantitative post-launch compensation. A temperature time history is available to HEASARC users in a FITS file (VELA_TEMP) so that they may check any suspicious source data against the known times of temperature extremes.
The time history of the Crab detected flux decreased by ~15% between 1969 and 1979. It is believed that this decrease is due to a gain change in the XC detector as it aged. No attempt to correct for this trend has been made in the data processing. Users who desire to do so, or who want to express detected source intensities in units of crabs, will have to access the FITS file containing the Crab data to extract the necessary information.
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 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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