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Jerry Berg
Marshall Space Flight Center
Huntsville, AL 35812
Later in this decade, NASA will launch a unique orbiting observatory which will
open a window for more detailed views than ever before possible of the universe
as revealed through X-rays. The Advanced X-ray Astrophysics Facility is
expected to yield data that will improve knowledge and understanding in some of
the most fundamental areas of scientific investigation.
Astronomers know whenever they observe X-ray emissions, they are viewing the
most violent processes in the universe, since a tremendous amount of energy is
required to generate X-rays. Such X-ray sources include neutron stars,
suspected black holes, debris from supernova explosions, quasars, the centers
of active galaxies, and hot gas in individual galaxies and galaxy clusters.
However, these phenomena and objects -- among the most interesting and puzzling
in the universe -- are usually hidden from human observation, obscured by vast
clouds of interstellar or intergalactic dust.
While clouds of matter in space hinder such observations with optical, or
light-gathering telescopes, X-rays are able to penetrate dust-shrouded areas of
the universe. But, X-rays and other types of high-energy radiation are
absorbed by the Earth's atmosphere. Therefore, it was only in the 1970s, with
placement into Earth orbit of specialized X-ray instruments, that the first
significant surveys of space for X-ray sources were made. The High Energy
Astronomy Observatory (HEAO-2, widely known as the Einstein X-ray Observatory),
provided such exceptional results that astrophysicists, working with NASA,
proposed development of a much more capable X-ray observatory. The result was
the Advanced X-ray Astrophysics Facility or AXAF.
Originally planned to be accomplished as a single comprehensive mission,
AXAF was restructured in mid-1992. The program restructuring was necessary to
stay within budget constraints, while preserving a high percentage of the
originally planned AXAF scientific capabilities. The restructured program
consisted of two smaller, more specialized missions, AXAF-I, specializing in
imaging observations at high-energy wavelengths, and AXAF-S, designed for X-ray
spectroscopy studies. As a result of congressional reductions in NASA's fiscal
1994 budget, the AXAF-S project had to be terminated in late 1993.
AXAF-I
The AXAF-I mission will be especially suited for imaging investigations.
It will develop and place into orbit a telescope for capturing high resolution
images and spectra of X-ray sources. That is, its observations will produce
"picture-like" images analogous to those made in visible light with an optical
telescope. However, it will use X-rays to create them, not visible-energy
wavelengths, so that the images will reveal otherwise-hidden features, just as
a medical X-ray does. AXAF-I's capabilities in the area of X-ray spectroscopy
will complement its imaging function. Spectra, or spectrographs, reveal the
"chemical fingerprint" of an object by separating the radiation received from
it according to wavelengths, much as a prism splits visible light into
constituent colors. Again, AXAF performs these specialized functions in the
X-ray region rather than in visible wavelengths.
Key Features
To provide these capabilities, AXAF-I will have a unique mirror
assembly. In an X-ray instrument, the mirrors themselves are much different
from those used in optical telescopes. With very short wavelengths and high
energy levels, X-rays are absorbed rather than reflected from the surface of a
conventional mirror. Therefore, X-ray telescopes use "grazing-incidence"
mirrors -- finely polished cylinders of glass that look something like ordinary
drinking glasses with no bottom. The walls of the cylinders are not straight,
however, but are very slightly angled so that X-rays graze off the surfaces,
like a stone skipping on a pond. This configuration, coupled with a slight
curvature of the surfaces, funnels the rays toward a point of focus behind the
mirrors, in the focal plane. AXAF-I's mirror assembly will consist of four
sets of these cylindrical mirrors, mounted concentrically in a nested array.
The use of more than one set of mirrors increases the area for collecting X-ray
energy, just as does increasing the mirror diameter in a visible-light
telescope. The aperture diameter of the largest mirror set will be 47.2 in.
(1.2 m), making it the largest X-ray grazing incidence telescope to date. The
focal length of the mirror assembly will be 32.8 ft. (10 m).
The specialized types of scientific information to be collected by
AXAF-I require that its design incorporate several sophisticated data
collection elements: a charge-coupled device imaging spectrometer and a
high-resolution camera in the observatory's focal plane; and two grating
spectrometers behind the high-resolution mirror assembly. During observation
periods, data being collected ill be stored on-board and periodically
transmitted to the ground for analysis.
Prior to launch, testing and calibration of the AXAF mirrors and science
instruments will be performed at the Marshall Space Flight Center in
Huntsville, Ala., in the unique, world-class X-ray Calibration Facility.
Weighing roughly 11,500 pounds (5,200 kg), the observatory will be
approximately 39 feet (11.9 m) long by 14 feet (4.2 m) in diameter. It is
planned to be launched in late 1998, into a high elliptical orbit of 5,400 by
54,000 nautical miles (10,000 by 100,000 km). As a goal, AXAF-I is designed to
have an operational life of five years.
Development
Following a competitive procurement, TRW Inc. of Redondo Beach, Calif., was
selected in August 1988 as the prime contractor to develop what was then
planned as the single-mission Advanced X-Ray Astrophysics Facility. TRW's major
subcontractors are Hughes Danbury Optical Systems of Danbury, Conn., and Kodak
Federal Systems Division of Rochester, N.Y. Following the 1992 restructuring
which created a two-mission AXAF program, the prime contractor's role was
modified to reflect those changes. TRW's effort was specified as encompassing
development and initial operation of AXAF-I.
Management of the overall AXAF program, including supervision of design,
development, assembly and testing activities and the establishment of the
Operations Control Center and the Science Center is performed for NASA by the
Marshall Center. The Office of Space Science at NASA Headquarters in
Washington, D.C. is responsible for the overall direction of the program.
AXAF OPERATIONS CONTROL CENTER AND SCIENCE CENTER
Once AXAF-I is in orbit, the focal points for the mission will be the
AXAF Operations Control Center at the Marshall Center and the AXAF Science
Center at Cambridge, Mass. The two facilities will be electronically linked for
the most efficient interaction in mission planning and data transfer. The
facilities will each serve specialized functions:
The AXAF Science Center will assist the international science community
in preparing to use AXAF-I's capabilities, specifically in areas such as
preparation of research proposals, planning of science observations, and
supporting the calibration of themirrors and instruments prior to their
installation into the observatory. The Science Center will also manage
the receipt, calibration, distribution and support the analysis of data
from observations. In March 1991, a contract was awarded to the
Smithsonian Astrophysical Observatory for design, development,
management and operation of the Science Center. The contract is managed
by the Marshall Center.
The AXAF Operations Control Center will receive observation requests
from the Science Center and integrate them into a detailed schedule for
transmission to the spacecraft. In addition, the Control Center will
provide capabilities for mission planning and scheduling, processing of
telemetry, command processing and management, attitude determination and
from observations. In March 1991, a contract was awarded to the
Smithsonian Astrophysical Observatory for design, development,
management and operation of the Science Center. The contract is managed
by the Marshall Center.
The AXAF Operations Control Center will receive observation requests
from the Science Center and integrate them into a detailed schedule for
transmission to the spacecraft. In addition, the Control Center will
provide capabilities for mission planning and scheduling, processing of
telemetry, command processing and management, attitude determination and
sensor calibration. It will interface with NASA's Deep Space Network
and Space Network for spacecraft communications.
During the operational lifetime of AXAF-I, it will enable scientists to
view the universe in the widest possible scope. The AXAF program will bridge
the region of energy wavelengths between those covered by the Hubble Space
Telescope and by the Compton Gamma Ray Observatory. The program represents a
major stride toward advancing the new age of astronomy and astrophysics.
November 1993