NASA Experimental Communications Satellites (original) (raw)
ATS 2
(101K)A key objective of the original five satellite ATS series was the development of gravity-gradient attitude stabilization for synchronous satellites. Gravity-gradient stabilization was chosen because it is cheap in terms of on-board power and propellant requirements. The real goal, however, was to move away from spin stabilized spacecraft to three-axis stabilization.
The advantage of spin stabilization is that it is a very simple way to keep the spacecraft pointed in a certain direction. The spinning spacecraft resists perturbing forces just like a gyroscope or a top. In space, forces that slow the rate of spin are very small, so that once the spacecraft is set spinning, it just keeps going.
With a spinner, there are inherent inefficiencies because only some of the solar cells are illuminated at any one instant and because most of the radio wave energy, radiating from the nondirectional antennas in all directions, is not directed at Earth. With a three-axis stabilized spacecraft, solar panels can be kept pointing at the sun and a directional antenna (that concentrates the radio energy into a beam) can be kept pointing at the Earth.
Gravity-gradient stabilization uses the Earth's gravitational field to keep the spacecraft aligned in the desired orientation. The spacecraft is designed with a mass distribution (e.g., on ATS- 2: two long booms) that keeps one end closer to the Earth. Gravity-gradient techniques take advantage of the decreasing strength of the gravity field (the gradient) as you move away from the Earth. The part of the spacecraft nearest the Earth (on one end of the boom) is in a slightly stronger portion of the gravity field. Although these techniques had been used in low Earth orbit before ATS, the question to be addressed was whether or not the gradient was too weak to be useful at higher altitudes.
ATS-2 was to investigate gravity-gradient stabilization at a 6000 n. mile orbit in addition to communication experiments and space environment investigations. It was launched on April 5, 1967 atop an Atlas-Agena D. Unfortunately, the Agena upper stage malfunctioned and the spacecraft was not injected into a circular orbit and only a few experiments were able to return data. ATS-2 reentered the atmosphere on September 2, 1968.