About Uhuru

Uhuru: "Freedom"

"Uhuru" is the nickname for the "Small Astronomy Satellite-1" (SAS-1), originally named the "X-Ray Explorer." This nickname was chosen because the satellite was launched from San Marco, off the coast of Kenya, on Kenya's independence day - the word "uhuru" being Swahili for "freedom."

Uhuru's Place in the X-Ray Exploration Program

The "X-Ray Explorer" was the third phase of a five-phase program developed in 1963 by Riccardo Giacconi of the American Science and Engineering group (AS&E). This program was designed to continue the broad surveys of the sky for x-ray sources, since at the time only two were known, Sco X-1 ("Scorpius X-1", a binary star found in the Scorpio constellation) and the Crab Nebula. The first and second stage, which Giacconi knew would be approved, were to continue the current rocket program (phase 1) and to include an x-ray experiment on the fourth Orbiting Solar Observatory (phase 2). The other three stages - the X-Ray Explorer orbiting satellite (phase 3), a spacecraft containing a focusing x-ray telescope (phase 4) and a large orbiting x-ray observatory carrying a 1.2-meter x-ray telescope (phase 5) - were not expected to be approved, but would eventually occur if the field of x-ray astronomy continued. Fortunately, the "X-Ray Explorer" was also approved.

The Proposal

"Notwithstanding the paucity of information, I was young and enthusiastic enough to preset to Dr. Nancy Roman, Cheif of the Astronomy Branch of NASA, a long-range program for X-ray observations extending over a decade... it was a delightful surprise to hear Dr. Roman express the opinion that NASA might, in fact, be interested in considering an X-ray Explorer fully devoted to a search for celestial X-ray sources." - Riccardo Giacconi

On April 8, 1963, Giacconi submitted a detailed proposal, "An X-Ray Explorer to Survey Galactic and Extragalactic Sources" to NASA. The mission was to last 18 months with a launch date of December 5, 1965. The satellite was to make a detailed survey of x-ray sources. One of AS&E's requirements was for the satellite to spin slowly. This allowed the detector to scan small angles of the sky and to keep x-ray sources in the satellite's field of view for a long time.

When the proposal was approved, NASA decided that AS&E should construct the scientific payload, but that the spacecraft itself was to be constructed by the Applied Physics Laboratory of John Hopkins University. NASA's Goddard Space Flight Center was to manage the project.

In late 1966, NASA thought that the X-Ray Explorer idea should be generalized. The "Small Astronomy Satellites" program was begun, and the X-Ray Explorer was the first, hence it's official name "SAS-1."

Uhuru's Hardware

When Uhuru was completed, it weighed 64 kg. It held independent detection systems, consisting of a set of beryllium-window proportional counters with collimators in front.

An exploded schematic of the instrumentation in Uhuru. (American Science and Engineering)

The detection systems pointed in opposite directions, one with a field of view of 0.5 degrees by 5.0 degrees, and the other with a field of view of 5.0 degrees by 5.0 degrees. The effective x-ray-collecting area was about 700 square centimeters.

The Launch

The launch of Uhuru on December 12, 1970. (NASA)

The Uhuru satellite was launched by a Scount rocket on December 12, 1970 into an orbit of ~560 km apogee, 520 km perigee, 3 degrees inclination and with a period of 96 minutes. The launch took place from the San Marco platform, an oil-drilling rig three miles off the coast of Kenya which had been modified for rocket launches by the Italian Space Agency. This location was chosen because it was very close to the equator, where the earth's rotation is fastest and it's easier to put objects into orbit. This also allowed for a heavier payload, and kept the satellite away from the regions in the earth's magnetic field where the concentration of trapped charged particles is high.

The launch was not without problems. A battery failure two days before an earlier scheduled launch caused a further delay of the mission. A simple replacement fixed this problem, but the scientists had all the more anxiety.

How Uhuru Worked

Uhuru spun slowly, usually making one revolution every 12 minutes, but with the use of a magnetic torquing system (which reacted against the earth's magnetic field), it would be slowed to a spin of less than one revolution per hour. The satellite was usually pointed at one direction for about a day, and then moved (by the magnetic torquing system) to point at another position.

The Uhuru satellite. This drawing shows the X-ray detectors, star sensors and satellite systems for control. The detectors scan a region of the sky as the satellite rotates. (R. Giacconi and colleagues)

The Uhuru detectors survey in the energy band of 2 to 20 keV. Sources as weak as 1/10,000th the strength of Sco X-1 can be detected. Star sensors are onboard to determine the position of any detected signals.

[ About this figure ]

End of the Mission

Within two years, a catalog of over 150 X-ray sources had been created. By 1974, one year after the March, 1973 ending of the Uhuru mission, thirty-five of the X-ray sources discovered by Uhuru were identified optically. The survey performed by Uhuru made it possible for later satellite experiments to make detailed observations of the X-ray sources that Uhuru found.

Continue on to the Discoveries section...

Astronomy 305 Fall 1996
Assignment 1
Uhuru By Bill Kendrick
September, 1996