Mission Overview

Uhuru (SAS-A) was the first in the 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 spacecraft was launched December 12, 1970, from the San Marco platform off the coast of Kenya, Africa, into a near-circular equatorial orbit. 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 of 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 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.

Launch Information
Launch Date: 1970-12-12 at 10:54:00 UTC
Launch Vehicle: Scout
Launch Site: San Marco Platform, Kenya
Decay Date: 1979-04-05

Trajectory Details
Type: Orbiter
Central Body: Earth
Epoch start: 1970-12-12 10:54:00 UTC

Orbital Parameters
Periapsis 531.0 km
Apoapsis 572.0 km
Period 95.69999694824219 minutes
Inclination 3.0°
Eccentricity 0.0029559999238699675

Instrumentation

All-Sky X-Ray Survey

The X-ray instrument aboard SAS-A (Explorer 42) consisted of two nearly identical sides, both physically and electronically. Each side contained an X-ray detection system composed of a collimator, proportional counters, associated processing electronics, and an aspect sensing system. The high spatial resolution side had a viewing angle of 0.5-deg by 5-deg FWHM and a detection range from 1 to 20 keV. The other side had a high-sensitivity collimator with a viewing angle of 5-deg by 5-deg FWHM. This side had a detection range from 1 to 10 keV. Six proportional counters, composed of a beryllium shell with 2.5-mm beryllium foil windows, were behind each collimator. The interior contained a 2-mm tungsten anode wire and a gas composition of 90% argon, 9.5% carbon dioxide for quenching, and 0.5% helium at a pressure of 940 mm of mercury. Low-intensity radioactive sources were used for inflight calibration of the instrument. The spin axis of the spacecraft was held fixed in the sky for about a day at a time. During this period a band of approximately 10 deg about the equator of the spin axis was scanned. The primary data reduction objective was to superimpose the X-ray data recorded as “count rate vs time” to “count rate vs azimuth” so that the superimposition data would be equivalent to a single sweep through the observing 10-deg band with a total observing time of 1 day. An array was created of X-ray superimposition (representing the 360-deg circle scanned) broken into 4320 elements of azimuth of 5 min each for the 0.5-deg detector and 1080 elements of azimuth of 20 min each for the 5-deg detector.

Science
Summary