Third Small Astronomy Satellite
Encyclopedia
The Small Astronomy Satellite 3 (SAS 3, also known as SAS-C before launch) was a NASA X-ray astronomy
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...

 space telescope. It functioned from May 7, 1975 to April 1979. It covered the X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 range with four experiments on board. The satellite, built by the Johns Hopkins University
Johns Hopkins University
The Johns Hopkins University, commonly referred to as Johns Hopkins, JHU, or simply Hopkins, is a private research university based in Baltimore, Maryland, United States...

 Applied Physics Laboratory
Applied Physics Laboratory
The Johns Hopkins University Applied Physics Laboratory , located in Howard County, Maryland near Laurel and Columbia, is a not-for-profit, university-affiliated research center employing 4,500 people. APL is primarily a defense contractor. It serves as a technical resource for the Department of...

 (APL), was proposed and operated by MIT's Center for Space Research (CSR). It was launched on a Scout
Scout (rocket)
The Scout family of rockets were launch vehicles designed to place small satellites into orbit around the Earth. The Scout multistage rocket was the first orbital launch vehicle to be entirely composed of solid fuel stages....

 vehicle from the Italian/Kenyan San Marco launch platform
San Marco platform
The Luigi Broglio Space Centre is an Italian-owned spaceport near Malindi, Kenya, named after its founder and Italian space pioneer Luigi Broglio. Developed in the 1960s through a partnership between the University of Rome La Sapienza's Aerospace Research Centre and NASA, the BSC served as a...

 near Mombasa, Kenya, into a low-Earth, nearly equatorial orbit. It was also known as Explorer 53, as part of NASA's Explorer program
Explorer program
The Explorer program is a United States space exploration program that provides flight opportunities for physics, heliophysics, and astrophysics investigations from space. Over 90 space missions have been launched from 1958 to 2011, and it is still active...

.

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.

SAS 3 was commanded from the NASA Goddard Space Flight Center
Goddard Space Flight Center
The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...

 (GSFC) in Greenbelt MD, but data were transmitted by modem to MIT for scientific analysis, where scientific and technical staff were on-duty 24 hr per 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.

Objectives

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

Instrumentation

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 asec.
  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, which productivity was greatly augmented by the abundance of talented MIT scientific staff available, at the professorial, post-doctoral, and student levels. Among its most important results were:
  • Shortly after the discovery of the first X-ray burster
    X-ray burster
    X-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity peaked in the X-ray regime of the electromagnetic spectrum...

     by the ANS
    Astronomical Netherlands Satellite
    The Astronomical Netherlands Satellite was a space-based X-ray and ultraviolet telescope. It was launched into Earth orbit on 30 August 1974 at 14:07:39 UTC in a Scout rocket from Vandenberg Air Force Base, United States...

    , 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, MXB1730-335. 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
    Neutron star
    A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...

    /Be star
    Be star
    A Be star is a B-type star with prominent emission lines of hydrogen in its spectrum. The designation is combined by the spectral class, B, and the lowercase e denoting emission in the spectral classification system. Line emission from other atomic ions might be present as well, but is typically...

     binary 4U 0115+63., 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), Algol, and from AM Her
    Polar (cataclysmic variable)
    A Polar is a type of cataclysmic variable binary star system with a very strong magnetic field.In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf star in the form of an accretion disk. In polar systems, the magnetic field of the...

    , 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
    Hydrogen
    Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

     column density, indicating absorption of external diffuse sources by the foreground galactic interstellar medium
    Interstellar medium
    In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

    .


Lead investigators on SAS 3 were MIT professors George W. Clark
George W. Clark
George Whipple Clark is an American astronomer and professor emeritus at the Massachusetts Institute of Technology. When he retired, M.I.T. described him as "a central figure in the development of high-energy astrophysics, particularly in the design, analysis, and interpretation of experiments for...

, Hale V. Bradt, and Walter H. G. Lewin. Other major contributors were Profs Claude Canizares and Saul A. Rappaport, and Drs Jeffrey A. Hoffman
Jeffrey A. Hoffman
Jeffrey Alan Hoffman, Ph.D. is an American former NASA astronaut and currently a professor of aeronautics and astronautics at MIT....

, George Ricker, Jeff McClintock, Rodger E. Doxsey, Garrett Jernigan
Garrett Jernigan
J. Garrett Jernigan is an American physicist and astronomer who has made notable contributions to space astronomy, particularly in the areas of X-ray and infrared instrumentation....

, John Doty, and many others, including numerous highly talented graduate students.
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