Small Astronomy Satellite 3
Mission type Astronomy
Operator NASA
COSPAR ID 1975-037A
SATCAT № 07788
Mission duration 4 years
Spacecraft properties
Manufacturer APL · Johns Hopkins University
Launch mass 196.7 kilograms (434 lb)
Power 65.0 watts
Start of mission
Launch date 7 May 1975, 22:45:01 (1975-05-07T22:45:01Z) UTC
Rocket Scout F-1 S194C
Launch site San Marco
End of mission
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Orbital parameters
Reference system Geocentric
Regime LEO
Eccentricity 0.0000313
Perigee 509.0 kilometers (316.3 mi)
Apogee 516.0 kilometers (320.6 mi)
Inclination 3.0033°
Period 94.90 minutes
RAAN 13.5403 degrees
Argument of perigee 37.2127 degrees
Mean anomaly 322.7960 degrees
Mean motion 16.22945651
Epoch 8 April 1979
Revolution number 21935
SAS 3 spacecraft as it might have appeared deployed on orbit. The nominal spin axis, or +z axis, points to the upper right, with the RMC and one star tracker for attitude determination. The remaining instruments and a second star tracker point out of the image towards the viewer. The four solar panels charged batteries during orbit day.

The Small Astronomy Satellite 3 (SAS 3, also known as SAS-C before launch) was a NASA X-ray astronomy space telescope.[1] It functioned from May 7, 1975 to April 1979. It covered the X-ray range with four experiments on board. The satellite, built by the Johns Hopkins University Applied Physics Laboratory (APL), was proposed and operated by MIT's Center for Space Research (CSR). It was launched on a Scout vehicle from the Italian San Marco launch platform near Mombasa, Kenya, into a low-Earth, nearly equatorial orbit. It was also known as Explorer 53, as part of NASA's Explorer program.[2]

The spacecraft was 3-axis stabilized with a momentum wheel that was used to establish stability about the nominal rotation, or z-axis. The orientation of the z-axis could be altered over a period of hours using magnetic torque coils that interacted with the Earth's magnetic field. Solar panels charged batteries during the daylight portion of each orbit, so that SAS 3 had essentially no expendables to limit its lifetime beyond the life of the tape recorders, batteries, and orbital drag. The spacecraft typically operated in a rotating mode, spinning at one revolution per 95-min orbit, so that the LEDs, tube and slat collimator experiments, which looked out along the y-axis, could view and scan the sky almost continuously. The rotation could also be stopped, allowing extended (up to 30 min) pointed observations of selected sources by the y-axis instruments. Data were recorded on board by magnetic tape recorders, and played back during station passes every orbit.[3]

SAS 3 was commanded from the NASA Goddard Space Flight Center (GSFC) in Greenbelt MD, but data were transmitted by modem to MIT for scientific analysis, where scientific and technical staff were on-duty 24 hours a day. The data from each orbit were subjected to quick-look scientific analysis at MIT before the next orbital station pass, so the science operational plan could be altered by telephoned instruction from MIT to GSFC in order to study targets in near real-time.


The major scientific objectives of the mission were:

  1. Determine bright X-ray source locations to an accuracy of 15 arcseconds
  2. Study selected sources over the energy range 0.1-55 keV
  3. Continuously search the sky for X-ray novae, flares, and other transient phenomena


SAS 3 carried four experiments:

  1. Rotating Modulation Collimator (RMC) Experiment, which observed along the rotation (Z) axis of the spacecraft, covering the 2–11 keV energy range, and providing high-precision locations of X-ray sources to an accuracy of up ~15 arcseconds.
  2. Slat collimated proportional counter instrument, covering 1–60 keV, looking out perpendicular to the spacecraft Z-axis, and providing coarse positions of unknown and transient sources.
  3. Tube collimated proportional counter instrument, also covering 1–60 keV and also looking out perpendicular to the spacecraft Z-axis, for detailed study of the spectral behavior and time-variability of sources observed during pointed or dithered observations.
  4. Low-Energy Detector (LED) system, covering 0.1-1 keV with a 2.9° FOV along the y-axis.

Research results

SAS 3 was especially productive due to its flexibility and rapid responsiveness. Among its most important results were:

  • Shortly after the discovery of the first X-ray burster by the ANS, an intense period of burst source discovery by SAS 3 quickly led to the discovery and characterization of about a dozen additional objects, including the famous Rapid Burster,[4] MXB1730-335.[5][6] These observations established the identification of bursting X-ray sources with neutron star binary systems.
  • The RMC was the first instrument to routinely provide X-ray positions that were sufficiently precise to allow followup by optical observatories to establish X-ray/optical counterparts, even in crowded regions near the galactic plane. Roughly 60 positions were obtained with accuracies on the order of 1 arcminute or less. The resulting source identifications helped to connect X-ray astronomy to the main body of stellar astrophysics.
  • Discovery of the 3.6 s pulsations of the transient neutron star/Be star binary 4U 0115+63.,[7] leading to determination of its orbit and observation of a cyclotron absorption line in its strong magnetic field. Many Be star/neutron star binaries were subsequently discovered as a class of X-ray emitters.
  • Discovery of X-ray emission from HZ 43 (an isolated white dwarf),[8] Algol, and from AM Her,[9] the first highly magnetic white dwarf binary system seen in X rays.
  • Established the frequent location of X-ray sources near the centers of globular clusters.
  • First identification of a QSO through its X-ray emission.
  • The soft X-ray instrument established that the 0.10-28 keV diffuse intensity is generally inversely correlated with the neutral H column density, indicating absorption of external diffuse sources by the foreground galactic interstellar medium.[10]

Lead investigators on SAS 3 were MIT professors Rodger E. Doxsey, Garrett Jernigan, John Doty, and many others, including numerous graduate students.


  1. ^ Annual Reviews of Astronomy and Astrophysics "X-ray Astronomy Missions", H. Bradt, T. .Ohashi,. and K. Pound., Vol. 30, p. 391 ff (1992)
  2. ^ HEASARC GSFC, retrieved Oct 17, 2009 Mission Overview
  3. ^ W. Mayer 1975, APL Tech Digest, 14, 14.
  4. ^ HEASARC Rapid burster Light curve of the Rapid Burster
  5. ^ Lewin, W. H. G. et al. Astrophys. J. Lett. 209, L95−L99 (1976)
  6. ^ H. L. Marshall et al., "Further analysis of SAS 3 observations of the rapid burster /MXB 1730-335",Astrophysical Journal, Part 1, vol. 227, Jan. 15, 1979, p. 555-562.
  7. ^ L. Cominsky et al., "Discovery of 3.6-s X-ray pulsations from 4U0115+63", Nature 273, 367 - 369 (01 June 1978); doi:10.1038/273367a0
  8. ^ Hearn, D. R. et al. 1976, Astrophys. Journal(Letters), Vol 203, L21
  9. ^ Hearn, Richarson, & Clarke, 1976, "SAS-3 Observations of AM Her = 3U1809+50", BAAS, Vol. 8, p.512
  10. ^ "SAS 3 survey of the soft X-ray background", F. J. Marshall and G. W. Clark, Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 287, Dec. 15, 1984, p. 633-652.