The Galaxy and the Universe
What is the universe? How did it come into being? How does
it work? What is its ultimate fate? These are some of the fundamental
questions addressed by the galaxy/universe research segment of
NASA's space science program. The long term goal is to provide
the answers to those questions within 25 to 50 years. The near
term goals are to complete development of an initial observational
capability; complete the survey of cosmic rays and interstellar
gas (examples of extrasolar matter); and carry out basic new
tests of gravitational theory.
The Advanced X-ray Astrophysics Facility, third in NASA's
Great Observatories series, will make its service debut in 1998.
Astronomy has made giant strides since the advent of satellite-based
telescopes in the 1960s. Operating above the obscuring atmosphere,
these instruments provide undistorted views of the universe,
and they can observe in bands of the electromagnetic spectrum-
ultraviolet, infrared, x-ray and gamma ray, for example-to pick
up star radiations normally absorbed or blocked out by the atmosphere
and thus not detectable by ground telescopes. This latter capability
is particularly important to astronomical science, because each
band of the spectrum offers a different set of clues to the origin
and evolution of the universe.
Since the 1960s, NASA has orbited a series of orbiting observatories
of ever-increasing capability, culminating with the service debuts
of the Hubble Space Telescope (1990) and the Compton Gamma Ray
Observatory (1991). These two "Great Observatories" have been regularly
providing astronomers major new discoveries about the cosmos.
For example, in 1995-96, the Hubble Space Telescope (HST)
confirmed the presence of a second black hole in the universe;
recorded the emergence of infant stars from dense, compact pockets
of interstellar gas; and made the first unambiguous detection
and imaging of a "brown dwarf," an object too massive
and too hot to be a planet but too small and too cool to shine
like a star. The brown dwarf is the faintest object ever seen
orbiting a star.
In development for initial service in 2000 is the Stratospheric
Observatory For Infrared Astronomy, a modified Boeing 747 carrying
a 2.5 meter telescope.
The Compton Gamma Ray Observatory (CGRO), which had recorded
more than 1,400 mysterious gamma ray bursts by the end of 1995,
completed a survey of the highest energy gamma ray sources and
showed that about half of them were quasars with beams of energy
pointed directly at Earth; the remaining sources were not identifiable.
Designed for a 15-year lifetime, made possible by Space Shuttle
servicing/reequipment missions, HST will continue to operate
well into the 21st century. Since the telescope became operational,
Goddard Space Flight Center has had responsibility for controlling
the HST and processing its imagery and data; the data is collected
and distributed by the Space Telescope Science Institute in Baltimore,
Maryland.
Unlike Hubble, the CGRO is not designed for Shuttle servicing,
but its lifetime can be extended to 10 years through occasional
altitude reboosts by its on-board propulsion system. Goddard
Space Flight Center is NASA's CGRO manager; TRW Inc. is principal
contractor. International participation includes Germany, The
Netherlands, United Kingdom and the European Space Agency (ESA).
ESA and NASA are teaming in development of a CGRO successor,
the International Gamma Ray Astrophysics Laboratory (INTEGRAL),
planned for launch in 2001.
The third of the Great Observatories is the Advanced X-ray
Astrophysics Facility, in development and scheduled for launch
in September 1998. AXAF will address some fundamental science
questions by obtaining x-ray images of such objects as neutron
stars, black hole candidates, quasars and active galaxies. Project
manager is Marshall Space Flight Center and TRW Inc. is principal
contractor.
The fourth member of the Great Observatories family is the
Space Infrared Telescope Facility, which is intended to conduct
advanced investigations of prime interest targets developed by
earlier infrared observatories; managed by Jet Propulsion Laboratory,
it is in study status in anticipation of hardware development
in 1998 and orbital service in 2002.
Another infrared observatory development is the Stratospheric
Observatory For Infrared Astronomy (SOFIA), a joint project of
NASA and the German Space Agency DARA. SOFIA is an airborne rather
than an orbital observatory. It consists of a 2.5 meter telescope
mounted in a specially modified Boeing 747SP transport; it is
planned for initial operation in 2000. SOFIA will replace NASA's
aging Kuiper Airborne Observatory, a telescope-equipped C-141
transport that has been in service since 1974. SOFIA's telescope
will be three times the diameter and about 10 times more sensitive
than the Kuiper system.
One other major mission in the galaxy/universe research segment
is Gravity Probe B, being built by Lockheed Martin under the
management of Marshall Space Flight Center. Intended for launch
in December 1999, Gravity Probe B will perform tests of two fundamental
predictions of Albert Einstein's general theory of relativity.
The spacecraft will carry four precisely-manufactured, golf-ball-size
crystal spheres inside a large dewar (thermos container). The
levitated spheres, isolated from heat influence, will be spun
at a precise rate and trained on a reference star. The reference
axis will be compared with the gyro spin axes with high precision;
if Einstein's predictions relative to the warping of time and
space are correct, each sphere's axis should drift slightly from
that of the reference star. The relatively large spacecraft-3
tons-will operate in a 400-mile-high polar orbit for a design
lifetime of 1.6 years.
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