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HEAO 2 (Einstein)


HEAO 2 (High-Energy Astronomy Observatory 2), also known as Einstein, was the second of three missions in a program of research in high-energy astronomical phenomena. The specific objectives of this mission were imaging and spectrographic studies of specific X-ray sources and studies of the diffuse X-ray background. The spacecraft bus was identical to the HEAO 1 vehicle, with the addition of reaction wheels and associated electronics to enable the telescope to be pointed at sources to within 1 min of arc. The instrument payload weighed 1450 kg.

A large grazing-incidence X-ray telescope provided images of sources that were then analyzed by four interchangeable instruments mounted on a carousel arrangement that could be rotated into the focal plane of the telescope. The telescope collected X-rays over an angular range of approximately 1 deg × 1 deg, with the focal plane instruments determining the limiting resolution up to a few arc-s for each measurement. The four instruments were a solid-state spectrometer (SSS), a focal plan crystal spectrometer (FPCS), an imaging proportional counter (IPC), and a high-resolution imaging detector (HRI). Also included were a monitor proportional counter (MPC), which viewed the sky along the telescope axis, a broadband filter, and objective grating spectrometers that could be used in conjunction with focal plane instruments and an aspect system.

The scientific objectives were

  • to locate accurately and examine X-ray sources in the energy range 0.2 to 4.0 keV. with high resolution;
  • to perform high-spectral-sensitivity measurements with both high-and low-dispersion spectrographs; and
  • to perform high-sensitivity measurements of transient X-ray behavior.

The spacecraft was a hexagonal prism 5.68 m high and 2.67 m in diameter. Downlink telemetry was at a data rate of 6.5 kb/s for real-time data and 128 kb/s for either of two tape recorder systems. An attitude control and determination subsystem was used to point and maneuver the spacecraft. Gyros, sun sensors, and star trackers were employed as sensing devices.

HEAO (High Energy Astronomical Observatory) 2 carried a X-ray telescope sensitive over the approximate energy range 0.2-3.5 keV. which focused energy on following  instruments:

  • High Resolution Imaging (HRI) - a digital X-ray camera which provided high spatial and
    temporal resolution in the 0.15-3.0 range;
  • Imaging Proportional Counter (IPC) - a position sensitive proportional counter sensitive between 0.4-4.0 keV.
  • Solid State Spectrometer (SSS) - a cryogenically cooled lithium-drifted Si(Li) detector with a range between 0.5-4.5 keV.
  • Bragg crystal spectrometer (FPCS).
  • Monitor Proportional Counter (MPC), which monitored the 1-20 keV.X-ray flux of the source(s) being observed by the telescope
  • Broad Band Filter (BBFS) spectrometer
  • Objective Grating Spectrometer (OGS).
Nation: USA
Type / Application: Astronomy, X-Ray, Gamma
Operator: NASA
Contractors: TRW
Equipment: X-ray Telescope, HRI, IPC, SSS, FPCS, MPC, BBFS, OGS
Configuration: HEAO Bus
Power: Solar cells, batteries
Mass: 3150 km
Orbit: 520 km × 541 km, 23.5
Satellite COSPAR Date LS Launch Vehicle Remarks
HEAO 2 (Einstein) 1978-103A 13.11.1978 CC LC-36B Atlas-SLV3D Centaur-D1AR


  • NSSDC Master Catalog: HEAO 2
Further HEAO missions: