ATS-4

ATS-4 also known as ATS-D was a communications satellite launched by NASA on August 10, 1968 from Cape Canaveral through an Atlas-Centaur rocket.

Objectives

The objective of ATS-4 was to investigate the possibilities of a gravity gradient stabilization system.

The satellite has a cylindrical shape with a diameter and height with the surface covered by solar panels that generated a maximum of 350 W of power, and stabilized by gravity gradient. It was based on the Hughes Aircraft HS-306 bus.

A total of four experiments were conducted during the mission:

The Atlas and Centaur stages performed satisfactorily and placed the Centaur/ATS-4 in an elliptical parking orbit. However the Centaur stage failed to re-ignite after a 61-minute coast. The failure was determined to be freezing of the hydrogen peroxide supply lines to the Centaur engines.
High atmospheric drag due to the low altitude of the achieved orbit precipitated the orbital decay of the spacecraft. ATS-4 still achieved good results in some of the experiments, but the primary objective of achieving gravity gradient stabilization of a satellite was not reached.
ATS-4 reentered the atmosphere on 17 October 1968.