Vela was the name of a group of satellites developed as the Vela Hotel element of Project Vela by the United States to detect nuclear detonations to monitor compliance with the 1963 Partial Test Ban Treaty by the Soviet Union.

Environmental Research Satellites (ERS) 17 & 18, part of the Octahedral Research Satellite Mark III series, were designed to provide complete monitoring of radiation in space and to study the radiation background problems associated with the nuclear test detection devices on the Vela Satellite Program. ERS 17/18 were developed to measure ionizing radiation throughout the magnetosphere up to the altitudes of the Vela satellites.

ERS 17 was launched into a highly elliptical orbit along with the Vela 3A and 3B satellites. The initial apogee altitude was 112,200 km and the perigee altitude was 192 km, which resulted in an orbital period of 43 hours. The orbital plane was inclined about 58° southward from the ecliptic plane and about 80° from the earth-sun line. The initial satellite spin rate was 6 RPM and inclined about 30° from the earth-sun line. ERS 17 returned data for about 3.5 months when a premature termination of the one-year end-of-life timer on November 3, 1965.