Instruments
The original Vela satellites were equipped with 12 external X-ray detectors and 18 internal neutron and gamma-ray detectors. They were equipped with solar panels generating 90 watts.
The Advanced Vela satellites were additionally equipped with two non-imaging silicon photodiode sensors called bhangmeters which monitored light levels over sub-millisecond intervals. They could determine the location of a nuclear explosion to within about 3,000 miles. Atmospheric nuclear explosions produce a unique signature, often called a "double-humped curve": a short and intense flash lasting around 1 millisecond, followed by a second much more prolonged and less intense emission of light taking a fraction of a second to several seconds to build up. The effect occurs because the surface of the early fireball is quickly overtaken by the expanding atmospheric shock wave composed of ionised gas. Although it emits a considerable amount of light itself it is opaque and prevents the far brighter fireball from shining through. As the shock wave expands, it cools down becoming more transparent allowing the much hotter and brighter fireball to become visible again.
No single natural phenomenon is known to produce this signature, although there was speculation that the Velas could record exceptionally rare natural double events, such as a meteoroid strike triggering a lightning superbolt in the Earth's atmosphere, as may have occurred in the Vela Incident.
They were also equipped with sensors which could detect the electromagnetic pulse from an atmospheric explosion.
Additional power was required for these instruments, and these larger satellites consumed 120 watts generated from solar panels. Serendipitously, the Vela satellites were the first devices ever to detect cosmic gamma ray bursts.
Some controversy still surrounds the Vela program since on 22 September 1979 the Vela 6911 satellite detected the characteristic double flash of an atmospheric nuclear explosion. Still unsatisfactorily explained, this event has become known as the Vela Incident. The now-declassified report of the scientific panel appointed to evaluate the event concluded that it was "Probably not the event of a nuclear explosion"
Read more about this topic: Vela (satellite)
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