Vela was the name of a group of satellites developed as the Vela Hotel element of Project Vela by the United States to monitor compliance with the 1963 Partial Test Ban Treaty by the Soviet Union, and other nuclear-capable states. It means watchman in Spanish.

Vela started out as a small budget research program in 1959. It ended 26 years later as a successful, cost-effective space system. In the 1970s, the nuclear detection mission was taken over by the Defense Support Program (DSP) system, and in the late 1980s, by the Navstar Global Positioning System (GPS) satellites. The program is now called the Integrated Operational Nuclear Detection System (IONDS).

The total number of satellites built was 12 — six of the Vela Hotel design, and six of the Advanced Vela design. The Vela Hotel series was to detect nuclear explosions in space, while the Advanced Vela series was to detect not only nuclear explosions in space but also in the atmosphere.

All spacecraft were manufactured by TRW and launched in pairs, either on a Atlas-Agena or Titan III-C boosters, and placed in 63,000 to 70,000 mile orbits, well above the Van Allen radiation belts. The first Vela Hotel pair was launched in 1963, three days after the Test Ban Treaty was signed, and the last in 1965. They had a design life of six months, but were actually shut down after five years. Advanced Vela pairs were launched in 1967, 1969 and 1970. They had a nominal design life of 18 months, later changed to 7 years. However, the last satellite to be shut down was Vehicle 9 in 1984, which had been launched in 1969 and had lasted nearly 15 years.

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: 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, so the amount of light it emits increases with its surface area. No natural phenomenon is known to produce this signature.

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 Gamma ray bursters.

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.

Includes material from NASA Goddard's Remote Sensing Tutorial