| ESSA 4 | |
| Mission Type: | Weather |
| Operator: | NASA |
| Cospar Id: | 1967-006A |
| Satcat: | S02657 |
| Launch Date: | 21 January 1967, 17:31 GMT |
| Launch Rocket: | Thor-Delta E |
| Launch Site: | Vandenberg Space Launch Complex 2 |
| Entered Service: | 21 January 1967 |
| Deactivated: | 5 May 1968 |
| Orbit Reference: | Geocentric orbit |
| Orbit Regime: | Low Earth orbit |
| Orbit Inclination: | 102° |
| Orbit Period: | 113.48 minutes |
| Orbit Eccentricity: | 0.00740 |
| Apsis: | gee |
| Programme: | ESSA program |
| Previous Mission: | ESSA-3 |
| Next Mission: | ESSA-5 |
ESSA-4 (or TOS-B) was a spin-stabilized operational meteorological satellite. Its name was derived from that of its oversight agency, the Environmental Science Services Administration (ESSA).
ESSA 4 was launched to replace ESSA 2, launched February 3, 1966, which had drifted into an orbit of limited usefulness. The satellite was financed, managed, and operated by the Environmental Science Services Administration (ESSA).[1]
The cartwheel-shaped spacecraft carried two Automatic Picture Transmission (APT) systems, with which it could instantly transmit photos of Earth's cloudcover to APT ground stations.[1]
ESSA-4 was launched on January 26, 1967, at 17:31 UTC. It was launched atop a Delta rocket from Vandenberg Space Launch Complex 2,[2] into Sun-synchronous orbit. ESSA-4 had an inclination of 102°, and an orbited the Earth once every 113.4 minutes. Its perigee was 1328km (825miles) and its apogee was 1443km (897miles).
The satellite properly aligned itself with respect to the Earth during its 18th orbit, whereupon its first photos were transmitted. A two-week spacecraft checkout and evaluation program ensued.[1] One of the APTs failed after launch, but the other performed normally.[3]
ESSA 4 was turned operationally off on December 6, 1967, being finally deactivated on May 5, 1968.[3]