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Since man first turned his eyes upwards to the night sky, he has
wondered at the heavens. As eons rolled by and he sought to develop
means of studying the celestial sprinkle of lights that winked at him,
his knowledge of the stars would increase through his development of
optical telescopes. Unknown to the earliest astronomers, of course,
was that there existed radiations other than visible light that would
help him to study the heavens. Only in the 20th century would there
finally come a time when astronomers could look at our universe in
ways other than with instruments that saw light as they did. A major
milestone finally occurred in the last decade.
For a number of years in the late 1970s and early '80s, NASA's
Marshall Space Flight Center in Huntsville, Alabama, managed a famed
series of orbiting instruments known as the High Energy Astronomy
Observatories. The entire family of three HEAO satellites, designed
to study high energy radiation in the universe such as X-rays and
cosmic rays, returned significant data to scientists over a period of
several years. All three of the High Energy Astronomy Observatories
were regarded as highly successful.
Each of the observatories was about 18 feet in length, weighed
between 6,000 and 7,000 pounds, and carried about one and a half tons
of instruments. Preceding the era of the Space Shuttle, all three
HEAOs were lifted into orbit by Atlas-Centaur rockets.
HEAO-1, launched in August 1977, scanned the heavens in a general
survey and mapped X-ray sources throughout the celestial sphere. The
initial survey was completed in February 1978, the design lifetime of
the satellite, but it continued to study the skies until its control
gas was exhausted in January 1979. During that 17-month survey,
HEAO-1 increased the number of known celestial X-ray sources from 350
to 1,500. It discovered a new black hole candidate and indicated the
possible existence of a universal hot plasma which would constitute a
major fraction of the mass of the universe.
HEAO-1 also discovered a superhot halo of gas, 1,200 light-years
in diameter, surrounding the celestial Northern Cross. The halo, or
"superbubble," is about 6,000 light-years from Earth in the next
spiral arm of our galaxy. Its discovery has formed the basis of a new
theory of star formation. Long after successfully completing its
mission, HEAO-1 re-entered the Earth's atmosphere and disintegrated in
1979.
The second high energy observatory, HEAO-2, focused upon specific
observation partially guided by the general celestial survey of its
predecessor. Called the "Einstein observatory" by astronomers because
it was launched close to the date of the late scientist's 100th
birthday, HEAO-2 was rocketed into orbit in November 1978. Although
the observatory was designed for a mission life of 12 months, it
operated for nearly two and a half years.
Carrying the largest X-ray telescope ever built and a variety of
sensitive astronomy instruments, HEAO-2 conducted the detailed imaging
and spectroscopic observations of approximately 300 known bright X-ray
sources and discovered thousands of new faint X-ray sources. Almost
every type of star in the galaxy was shown to have an X-ray-emitting
atmosphere. HEAO-2 obtained the first X-ray images of supernova
remnants, pulsars, star clusters, galactic diffuse X-ray sources,
bright sources in other galaxies, and of diffuse emission from
clusters of galaxies. Hundreds of active galaxies and quasars were
detected, some of which may be the most distant objects ever observed
at any wavelength. The Einstein observatories have already had a
significant impact upon most fields of astronomy and astrophysics.
In April 1981, the observatory expended its control gas supply
and could no longer maintain its pointing attitude, and the spacecraft
was powered down. HEAO-2 fell from orbit in March 1982, and burned up
harmlessly in the atmosphere. It was the last of the HEAO series to
re-enter.
The final observatory in the HEAO series was launched in
September 1979. Unlike its predecessors, which were designed to study
X-rays, HEAO-3 furthered knowledge about cosmic ray particles and
gamma-ray photons, the two highest energy radiations in the universe.
Through the study of cosmic rays, which are the nuclei of elements and
are the highest energy particles known, HEAO-3 measured the relative
abundance of elements in the galaxy. The observatory found
significant differences between the abundances of certain nuclei in
the galaxy and the abundance of these same elements in the solar
system.
Gamma rays, higher on the energy spectrum than X-rays, are
emitted from galactic sources which may be unobservable to
conventional telescopes. HEAO-3 detectors achieved precise,
high-resolution measurements of gamma-ray "line emission" from solar
flares and from positive and negative electrons combining near the
galactic center. The data sent from HEAO-3 are expected to further
the understanding of the strength and extent of interstellar magnetic
fields, the distribution of interstellar matter, and - most
importantly - the stellar nucleosynthesis process which has created
the heavy elements contained in most ordinary matter. According to
theory, the "big bang" formed hydrogen and helium and led to star
formation. All heavier elements are produced in normal and explosive
stellar processes. HEAO-3 finally re-entered the Earth's atmosphere
in December 1981 and burned up.
Although the HEAO series has already expanded man's knowledge of
the universe, scientists say that they will require years to complete
the analysis of the massive amount of data returned from the three
observatories. In 1982, scientists at Marshall discovered the third
fastest pulsar known to exist, based on data obtained from HEAO-1 and
HEAO-2.
Discoveries such as this indicate that, for all of humankind, the
great adventure of astronomy has just begun.