SAS A (Small Astronomy Satellite A, also known as Explorer 42 or Uhuru) was the first in a series of small spacecraft whose objectives were to survey the celestial sphere and search for sources radiating in the X-ray, gamma-ray, UV, and other spectral regions. The primary mission of Uhuru was to develop a catalog of celestial X-ray sources by systematic scanning of the celestial sphere in the energy range from 2 to 20 keV.

The orbiting spacecraft was in the shape of a cylinder approximately 56 cm in diameter and 116 cm in length. Four solar paddles were used to recharge a 6 amp-h, eight-cell, nickel-cadmium battery, and to provide power to the spacecraft and experiment. The spacecraft was stabilized by an internal wheel, and a magnetically torqued commandable control system was used to point the spin axis of the spacecraft to any point in the sky. The aspect sensing system consisted of both a star and sun sensor that shared the same processing electronics. The system was designed with a heavy emphasis on redundancy, not only in the more obvious areas such as aspect sensors and high- and low-voltage power supplies but also in signal switching and high-voltage distribution. The resulting instrument was capable of sustaining several simultaneous major failures without seriously compromising the scientific objectives. Data were stored on a one-orbit storage tape recorder and telemetered during a 3.4-minute playback cycle. A 1000 bps PCM/PM system was used.