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.

Vela 5A was one of two spin-stabilized, polyhedral satellites that comprised the fifth launch in the Vela program. The orbits of the two satellites on each launch were basically circular at about 17 earth radii, inclined at 60 deg to the ecliptic, and spaced 180 deg apart, thus providing a monitoring capability of opposite sides of the earth. The objectives of the satellites were (1) to study solar and cosmic X rays, extreme ultraviolet radiation (EUV), solar protons, solar wind, and neutrons, (2) to carry out research and development on methods of detecting nuclear explosions by means of satellite-borne instrumentation, and (3) to provide solar flare data in support of manned space missions. Vela 5A, an improved version of the earlier Vela series satellites, had better command capabilities, increased data storage, improved power requirements, better thermal control of optical sensors, and greater experimentation weight. Power supplies of 120 W were provided by 22,500 solar cells mounted on 24 of the spacecraft's 26 faces. A rotation rate of 78 rpm during transfer orbits and 1 rpm after final orbit insertion maintained nominal attitude control. Eight whip antennas and four stub antenna arrays at opposite ends of the spacecraft structure were used for ground commands and telemetry.

These capabilities enabled qualitatively new discoveries about dynamical timescale phenomena related to neutron stars and black holes, phenomena which probe basic physics in the most extreme environments of gravity, density, and magnetic fields. RXTE has extended its lifetime by applying the proportional counter area selectively and maintains schedule flexibility by making use of the distribution of targets around the sky.

Launch-Orbit Information

Launch Information
Launch Date: 1969-05-23 at 07:57:01 UTC
Launch Vehicle: Titan III-C
Launch Site: Vandenberg AFB, United States

Trajectory Description
Type: Orbiter
Central Body: Earth
Epoch start: 1969-05-23 07:55:00 UTC

Orbital Parameters
Periapsis 110900.0 km
Apoapsis 112210.0 km
Period 6703.0 minutes
Inclination 32.79999923706055°
Eccentricity 0.005553999915719032


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.

Two Extreme Ultraviolet Detectors 30 to 150 A, 120 to 900 A
Two extreme ultraviolet (EUV) detectors were mounted in the apex positions of the spacecraft to measure solar EUV radiation. Both detectors used retarding potential analysis of photo electrons to obtain approximate spectral data of the solar flux. The XUO (X-ray Ultraviolet Open) detector was an open-window device designed to cover the wavelength range from 120 to 900 A using ten retarding potential steps from 7 to 250 V. The XUW (X-ray Ultraviolet Window) detector covered the range 30 to 150 A in ten analyzer steps from 75 to 1000 V. Both detectors were equipped with repeller grids for charged particle suppression. The acceptance aperture was 20 deg in one dimension, allowing approximately a 3-s scan of the sun, and +50 deg to -50 deg in the other dimension. The XUO detector was read out in real time only. The XUW was read out in real time, and it also stored data at about one fourth the rate of real-time data.

Solar X-Ray Detectors, 0.5 to 3.0 A, 1 to 8 A, 1 to 16 A, 44 to 60 A
This experiment was designed to monitor the solar ambient and flare-produced flux of X rays in the 0.3- to 60-A wavelength region. Two identical X-ray sensor units were mounted at diametrically opposed apex positions on the satellite. Each unit contained four detectors: three ion chambers and a thallium-activated sodium iodide (NaI[Tl]) scintillation detector. Since each ion chamber had a hemispherical window, the combined output signals from identical chambers in each sensor unit approximated the response of an ideal detector with a 4-pi-sr field of view. The ion chambers had the following window materials, gas fills, and wavelength responses. Chamber 1: 0.127 mm of beryllium, 0.9 atm of argon plus 0.1 atm of helium, 1 to 8 A. Chamber 2: 6.35 micrometers of mylar overcoated with about an 8500-A layer of aluminum, 0.5 atm of nitrogen, 1 to 16 A. Chamber 3: 6.35 micrometers of mylar, 0.5 atm of nitrogen, 1 to 16 A and 44 to 60 A. This combination of ion chambers allowed solar X-ray flux measurements in the bands 1 to 8 A, 1 to 16 A, 8 to 16 A, and 44 to 60 A to be obtained upon suitable analysis of the data. The scintillation detector used for the 0.3- to 3-A wavelength region consisted of a NaI(Tl) crystal optically coupled to a PMT, the output of which fed a five-level, integral, pulse-height analyzer. Unlike the ion chambers, the two scintillation detectors in the two sensor units were not identical. The more sensitive detector had a 1.27-cm-diameter, 1-mm-thick crystal covered by a flat 0.25-mm-thick beryllium window. The less sensitive detector (1E-2 ergs/sq cm-s) had a 6.35-mm-diameter, 1-mm-thick crystal and a 2.03-mm-thick beryllium dome window in addition to the flat 0.25-mm window mounted on the face of the crystal. Both ion chambers and scintillation detectors were capable of observations with time resolutions of 2 s. The average detection efficiencies for the ion and scintillation detectors were of the order of 20% and 60%, respectively.

Solar Particle Telescopes
The solar telescope experiment was designed to measure the energy spectrum and angular distribution of solar protons between 0.3 and 50 MeV, and of solar alpha particles between 2 and 100 MeV. In addition, the experiment was designed to identify and monitor the flux of deuterium, tritium, and helium-3 nuclei which may be emitted during a solar particle flare and to monitor the intensity of more heavily ionized particles. There were three telescopes in a single plane oriented at angles of 45 deg, 90 deg, and 135 deg relative to the spacecraft spin axis. Each instrument consisted of a collimating tube (providing an angular view of 30 deg) in front of a solid-state dE/dx vs E particle detector. The telescope data system failed on May 29, 1969. Electron Detectors
Two sets of three solid-state electron detectors in a telescopic arrangement with an angular view of 30 deg were used to observe electrons over the range 30 to 150 keV. Protons of energy less than 300 keV or greater than 50 MeV could also be detected. One set of detectors viewed the particles directly. The other utilized a scatter geometry to improve ability to observe electrons in the presence of much larger fluxes of protons. Each of the three direct-view detectors and each of the three scatter-geometry detectors lay in a single plane and they made angles of 45 deg, 90 deg, and 135 deg with the spacecraft spin axis. The data system failed on May 29, 1969.

Solar Wind
Two electrostatic analyzer-electron multiplier units were used to study the interplanetary solar wind (including heavy ions) and protons and electrons in the magnetotail. Energy analysis was accomplished by charging the plates to known voltage levels and allowing them to discharge with known RC time constants. Particles in a 6-deg by 100-deg, fan-shaped angular range were accepted for analysis during a decaying voltage cycle. The 100-deg dimension was parallel to the spacecraft spin axis for both detectors. One analyzer-multiplier unit studied solar wind electrons in the energy range from 7.5 eV to 18.5 keV and solar wind positive ions (mainly protons and alpha particles) in an energy-per-charge range from 120 eV/Q to 5 keV/Q. The other unit studied magnetotail protons or electrons between 20 eV and 33 keV and solar wind heavy ions in the energy-per-charge range from 1 keV/Q to 8.3 keV/Q.

Cosmic X-Rays
The cosmic X-ray detector was a large-area (26 sq cm) sodium iodide scintillator with a 5-mil(0.127 mm) beryllium window. The experiment was designed to provide measurements of the location, intensity, and intensity variations of nonsolar X-ray sources over a long period of time. The detector was sensitive to X-ray photons in two energy intervals (3 to 6 keV and 3 to 12 keV), and was sufficiently sensitive to monitor from 6 to 12 galactic X-ray sources. Any one source was viewed for approximately 1 h, and every 2 days each source was back in view. Three modes of readout were available: (1) the real-time normal mode, in which counts from each energy channel were transmitted every second, (2) the high-resolution mode, in which only the 3- to 12-keV channel was transmitted eight times per second, (3) the store mode, in which only the 3- to 12-kev channel was stored.

Neutron Detector
The neutron detector consisted of a large (about 3.6 kg) polyethylene moderator surrounding two helium-3-filled proportional counters. Neutrons between 1 and 100 MeV were thermalized by the moderator and detected by the counters. The instrument was also sensitive to protons above 25 MeV.

Gamma-Ray Astronomy
This experiment consisted of six 10-cc cesium iodide scintillation counters distributed to achieve nearly isotropic sensitivity. Individual detectors responded to energy depositions of 0.2 to 1.0 MeV with a detection efficiency ranging from 17% to 50%. The scintillators were shielded against direct penetration by electrons below 0.75 MeV and protons below 20 MeV. No active anticoincidence shielding was provided. Normalized output pulses from the six detectors were summed into counting and logics circuitry. Logical sensing of rapid, statistically significant count-rate increases initiated the recording of discrete counts in a series of logarithmically increasing time intervals. This capability provided continuous temporal coverage, which, coupled with the isotropic response, is unique in astronomy. A time measurement was also associated with each record. The data accumulations included a background component due to cosmic particles and their secondary effects. The observed background rate, which was a function of threshold energy, was about 150 counts/s.


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