Mission Overview

High-Energy Astronomy Observatory 1 (HEAO 1) was the first in a series of three satellite observatories designed to continue the X-ray and gamma-ray studies initiated by ANS, OAO 3, UK 5, the OSO series, the SAS series, and the gamma-ray burst discoveries of the Vela satellites. This mission was specifically designed to map and survey the celestial sphere for X-ray and gamma-ray sources in the energy range of 150 eV to 10 MeV, to establish the size and precise location of X-ray sources to determine the contribution of discrete sources to the X-ray background, and to measure time variations of X-ray sources. This observatory consisted of a common spacecraft equipment module (SEM), which carried most of the spacecraft operational equipment, and a unique experiment module (EM), which carried some elements of the electrical distribution system in addition to the four experiments: Large Area X-ray Survey, Cosmic X-ray Background Experiment, Scanning Modulation Collimator, and Hard X-ray and Low Energy Gamma-Ray Experiment. Continuous celestial scans were made perpendicular to the satellite-sun vector during the initial phase of the mission. Scan rate was 0.03 rpm. The entire celestial sphere would be scanned in 6 months. When passing over the South Atlantic Anomaly (SAA) of the inner Van Allen Belt, high-voltage supplies were turned off or reduced to prevent damage caused by saturation effects. The six-sided HEAO 1 was 5.68 m high and 2.67 2.67 m in diameter, and weighed 2552 kg, which included 1220-kg experiments. Downlink telemetry was at a data rate of 6.5 kb/s for real-time data and 128 kb/s for either of the two tape recorder systems. The mission lifetime was August 12, 1977, to January 9, 1979.

Launch Information
Launch Date: 1977-08-12 at 06:39:00 UTC
Launch Vehicle: Atlas-Centaur
Launch Site: Cape Canaveral, United States
Decay Date: 1979-03-14

Trajectory Description
Type: Orbiter
Central Body: Earth
Epoch start: 1977-08-13 00:00:00 UTC

Orbital Parameters
Periapsis 432.0 km
Apoapsis 432.0 km
Period 93.5 minutes
Inclination 23.0°
Eccentricity 0.0

Regions Traversed
Trapped particle belts


Large Area Cosmic X-Ray Survey (A-1)

This instrument was a modular assembly of seven large-area, thin-window, proportional counter sensor modules to record incident X-ray fluxes. The objectives were to map the celestial sphere in the energy range from .15 to 20 keV with greater sensitivity than achieved previously, and to measure the spectra, location, and time variations of X-ray sources with a 0.1- to 1-deg angular resolution. Each of the sensor modules consisted of a proportional counter body frame on which was mounted a window support structure, counter back structure with integral control counter, collimator assembly, and electronic subassemblies. A honeycomb cell construction for the basic counter provided X-ray collimation of 80-deg by 4-deg FWHM. A back layer of the three-layered counter provided anticoincident protection against charged particle events. The front layer was the main X-ray sensor for most of the energy ranges. The collimator for each of the counters viewed the sky. The collimator on sensor modules 1 through 4 provided 1-deg by 4-deg FWHM FOV, on sensor modules 5 and 6 provided 1-deg by 0.5-deg FWHM FOV, and on sensor module 7 provided 8-deg by 2-deg FWHM FOV. Each of the sensors included movable radioactive calibration sources to provide a check on counter operation and channel position. There was also a magnet assembly to deflect low-energy radiation belt electrons. The control counter was a small counter at the back of the assembly that shared the counting gas with the main counter. It was excited by an Fe 55 source and served to generate the proper operating voltage on the main counter to compensate for gas density changes and high-voltage drifts.

Cosmic X-Ray Experiment (A-2)

This experiment was designed to measure the diffuse X-ray background in the energy range of 0.15 to 60 keV. Objectives were to measure relative diffusion and absorption of diffuse hard and soft X-rays at high galactic latitudes and correlate these measurements with radio and optical studies; to determine discrete source background contribution; to detect large-scale global anisotropies associated with solar system motion with respect to distant emission sources; to make broadband spectral classifications of diffuse and discrete X-ray sources; and to establish temporal variations of multicomponent spectral sources. Three types of multianode, multilayer counters were used for this experiment. Three high-energy detectors (HED) with xenon-filled counters covered the energy range of 3 to 60 keV with an effective area of 900 sq cm. The minimum detectable flux in a 1E3-s observation was 1E-4/sq cm-s-keV for energy bands 3 to 20 keV and 20 to 60 keV. One medium energy detector (MED) with an argon-filled counter covered the energy range 1.5 to 15 keV. The effective area of this counter was 900 sq cm. The minimum detectable flux was the same as for the HEDs. The two low-energy detectors (LED) were thin-window, propane gas flow counters to cover the energy range of 0.15 to 3 keV. The LED used permanent magnets to prevent incident electrons from reaching the detector window and a sunshade whenever direct sunlight was near the field of view. It had a 600-sq cm effective area. The minimum detectable flux for a 1E3-s observation was 1E-3/sq cm-s-keV for the 0.15- to 0.28-keV band and for the 0.5- to 3.0-keV band. The LEDs were shut down in May 1978 because of depletion of gas in the system.

X-Ray Scanning Modulation Collimator (A-3)

This experiment used a scanning modulation collimator (SMC) instrument to determine, for selected X-ray sources, their position within 5 arc-s and their angular size to a precision of 5-10 arc-s in three energy intervals from 1 to 15 keV, and to study the structure of their X-ray emission to a precision of 10 arc-s in three energy intervals from 1 to 15 keV. The SMC was composed of two parts, each containing four wire grid planes. Each provided a location and angular size measurement in one dimension. An additional collimator located forward to the front grid restricted the overall instantaneous field of view to 4-deg x 4-deg FWHM for each SMC. The outward view direction was perpendicular to the spacecraft spin axis (Z axis) and, hence, the instrument scanned a great circle band on the sky. The two parts of the SMC differed by having their plane of maximum transmission inclined +10 deg and -10 deg to the scan direction. Precise two-dimensional locations were determined by the intersections of the locations obtained from each of the collimators. The angular response of the two SMC components was 30 and 120 arc-s, which extended the dynamic range up to 16 arc-min, over which angular size and structure measurements were made. The SMC instrument was capable of detecting X-ray sources with an intensity of 1E-3 that of the Crab Nebula. This experiment was also equipped with two aspect sensors to provide data on the stellar orientation of the collimator axes to achieve the 5-arc-s position of sources.

Low-Energy Gamma-Ray and Hard X-Ray Sky Survey (A-4)

This experiment measured point and diffuse sources of X-rays and gamma rays in the 10-keV to 10-MeV range. The instrument consisted of seven NaI(Tl)/CsI(Na) Phoswich scintillators surrounded by eight large CsI(Na) scintillators that provided shielding and defined the fields of view. There were three detector types. The intermediate-energy detectors had an energy range of 10 to 200 keV, an area of 225 sq cm, CsI shielding of 2 in., and a field of view (FWHM) of 1 deg x 20 deg. The slat collimators of the intermediate-energy detectors were positioned at 60 deg relative to the scan direction, allowing point source determination to 1 deg over the approximately 40-deg-wide band scanned during each spacecraft rotation. The point-mode detectors had an energy range of 0.1 to 5 MeV, an area of 180 sq cm, CsI shielding of about 4 in., and a field of view (FWHM) of 20 deg. Sources detected were identified with low-energy sources by spectral similarity with measurements made by the intermediate-energy detector at about 100 keV. The diffuse-mode detectors had an energy range of 0.2 to 10 MeV, an area of 125 sq cm, CsI shielding of about 6 in., and a field of view (FWHM) of 10 deg. Point sources measured by the diffuse-mode detectors were related to those with similar spectra in the point-mode detectors. Each of the detectors was equipped with a pulse-shape analyzer and discriminator that detected and vetoed CsI(Na) events. The combination of shield upper-and lower-level discriminators (nominal settings of 5 MeV and 0.1 MeV) used for detector anticoincidence was selectable by command. Event time was nominally known to 0.1-s accuracy. This could be improved to 5 ms or 2E-5 s by command. The instrument also contained three particle monitors, which measured proton and electron fluxes in three energy ranges. There was a high-resolution timing system that measured cosmic gamma-ray bursts.


The data produced a spectrum of the diffuse background over the energy range 14-200 keV.