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 prowgram. 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.
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.