Mission Overview

UK 5 was the fifth scientific satellite in a U.K./U.S. collaborative space research program. The satellite was the fifth such satellite and the third to be wholly built in the United Kingdom. It carried six experiments (five U.K. and one U.S.) for cosmic X-ray studies that measured the spectra, polarization, and pulsar features of X-ray sources. The tasks of the scientists with experiments on the UK 5 satellite were, in general, to improve the accuracy of position measurement of X-ray stars and to measure their energy spectra. The spacecraft was spin stabilized. Two experiments scanned the sky perpendicular to the spin axis, while four experiments pointed parallel to the spin axis. When fully equipped, UK 5 weighed 300 lb. The satellite was approximately cylindrical in shape, 38 in. in diameter by 34-in. high. UK 5 was launched into a quasi-circular orbit at a height of 400 to 500 km. During operation in orbit UK 5 spun at a rate of 10 revolutions per minute. The satellite was designed to operate over a restricted range of solar aspects with the sun within 45 deg of normal to the spin axis. To enable various parts of the sky to be observed, the pointing direction of the spin axis could be altered by a pulsed gas jet system. The UK 5 data system generated sector information with respect to the sun's position to enable the position of X-ray sources to be determined. Data were stored on board the spacecraft in a core storage and dumped to ground stations once per orbit. All satellite operations were directed from a control center at the Appleton Lab, U.K.

Launch Date: 1974-10-15 at 07:47:00 UTC
Launch Vehicle: Scout
Launch Site: San Marco Platform, Kenya
Decay Date: 1980-03-14

Trajectory Details
Type: Orbiter
Central Body: Earth
Epoch start: 1974-10-15 00:00:00 UTC

Orbital Parameters
Periapsis 512.0 km
Apoapsis 557.0 km
Period 95.30000305175781 minutes
Inclination 2.9000000953674316°
Eccentricity 0.0032530000898987055


Rotation Modulation Collimator (RMC)

This experiment combined the function of observing X rays in different energy ranges with that of star tracking. The experiment contained a rotation collimator, utilizing the satellite spin, behind which there were three detectors. The field of view was a cone with a half-angle of 10 deg to 20 deg, depending on the type of radiation viewed by the different detectors. The three detectors functioned as follows: (1) a visible-light photomultiplier enabled the spin axis to be accurately determined by viewing the background of optical stars; (2) an array of channel electron multipliers, with selectable filters, covered the wavelength range 0.3 to 6 keV; (3) a group of proportional counters covered the range 2.5 to 30 keV. It was estimated that source positions could be determined to within 2 arc min for bright sources.

2- to 10-KeV Sky Survey Instrument (SSI)

This experiment consisted of a large-area proportional counter arranged to view in a direction perpendicular to the satellite spin axis. The satellite rotation, therefore, allowed a scan of a 360-deg band of the sky. When the satellite spin axis was arranged to point at a galactic pole, the whole of the Milky Way could be scanned at once. The experiment covered the photon energy range 1.5 to 20 keV and effected a high-sensitivity survey, obtaining source locations, intensity, and spectra. A number of different modes of operation were used in which the available storage space in the core stored obtained spatial information at the expense of spectral resolution, or the converse. The sensitivity of the experiment allowed the detection of sources of the order of 1.E-4 times the intensity of Sco X-1, within the time of about 1 day. The ability of the survey instruments to determine the position of a source depended on the strength of the source and the number of other sources in a given part of the sky. A source of 5E-3 times the strength of Sco X-1 could be located with a precision of about 15 arc-min.

High-Resolution Source Spectra

This experiment consisted of a high-resolution, proportional-counter spectrometer with a 128-channel pulse-height analyzer, and responded to photons in the 2- to 30-keV energy range. The spectra of sources were examined in greater detail than had been previously possible. Line emission for certain elements (e.g., iron) could also be identified. The detector viewed in a direction parallel to the spin axis and, therefore, continued to observe the same piece of sky for as long as the position of the satellite spin axis remained unaltered. The experiment axis pointed approximately 2 deg off the spin axis, so that when observing a source also 2 deg off the spin axis the source passed in and out of the field of view during each rotation. This permitted the background flux to be sampled every spin period by recording the spectral information in four sets of locations, each corresponding to a quadrant of the spin cycle. The should have overcome the lack of information on possible fluctuations in the background flux during an orbit's integration. The experiment could also have been operated in a mode in which periodicities in the range typical of pulsar frequencies were detected.

Bragg Crystal Spectrometer (BCS)

This experiment was a polarimeter/spectrometer operating in the 2- to 8-keV range. It used two large plane crystals, lithium hydride and graphite, in a Bragg spectrometer with a honeycomb collimator. It was mounted to view along the satellite spin axis and to examine the radiation of individual X-ray sources for possible polarization and/or the existence of line emissions. In a source of the brightness of the Crab nebula, a polarization of 2.5% could be detected. The experiment also conducted searches for pulsar activity. The nature of the experiment made it possible to examine the polarization of the pulsar itself by looking for different pulsar behavior in the separate polarization components.

High-Energy Cosmic X-Ray Spectra

This experiment was designed to extend the spectral information on selected X-ray sources in the energy region above 20 keV. The detector was an 8 sq cm x 4 cm csi (na) scintillator actively collimated to provide 8 deg fwhm. Measurements were possible up to 2 MeV, although the efficiency of the detector fell steeply at this energy. The detector axis was inclined a few deg with respect to the satellite spin axis so that it coned as the satellite spun. The counting rate resulting from a point source a few deg from the spin axis was thus modulated with the spin period. This modulation was detected by dividing the spin cycle into four sectors and analyzing the different counting rates in each. In this way, the source intensity could be determined from the amplitude of the modulation. For pulsar observations, a large energy window at the lower end of the detector range was used. The observations in this energy region were analyzed for a pulsar periodicity in a special system that was part of the spacecraft handling electronics.

All-Sky Monitor

The purpose of this experiment was to monitor the entire sky continuously for transient X-ray phenomena and, at the same time, to monitor all the strong X-ray sources in the sky for long-term temporal effects. The experiment utilized two X-ray pin-hole cameras to image the sky. Position-sensitive proportional counters recorded the photons imaged through the pinholes. The fan beam response of the cameras allowed the whole sky to be monitored at least once per spacecraft rotation. The energy window was 3-6 keV. It was a valuable aid in programming satellite maneuvers so that transient events in the X-ray sky, such as nearby novae and X-ray flares, could be rapidly made available for study with greater resolution by the other experiments and other spacecraft.