Astronomy and Space Physics
During this first complete operational year of NASA's refurbished Hubble Space Telescope (HST), astronomers made many dramatic discoveries stretching to the edge of the universe from neighboring planets in our solar system. (HST was launched in April 1990 and serviced in December 1993.) HST continues to be one of the most widely used observatories in history, as at least 60 percent of all astronomers in the United States are HST investigators who work through the Space Telescope Science Institute to accomplish their observations.
A team that included scientists from the Smithsonian Astrophysical Observatory (SAO) and other institutions used HST to derive a new, higher value for the universe's expansion rate, thus implying an unexpectedly young age for the universe. By accurately determining the distance to a galaxy in the Virgo cluster and calculating for the local effects of the universe's expansion rate, scientists were able to make this precise measurement. Astronomers determined that the universe is smaller and younger than previously thought, about 10 billion years oldonly twice the age of the planet Earth.
Another team used HST to gather evidence that the clouds of hydrogen gas found between galaxies at distances of billions of light-years from Earth are at least 1 million light-years in diameter, or about 10 times larger than previously thought, and may have a remarkable sheet-like structure. These results shed new light on the properties of hydrogen gas clouds, whose nature has been a mystery since their discovery a quarter of a century ago, and may provide clues to understanding the evolution of galaxies in the early universe.
HST observations by SAO astronomers of faint stars deep inside a globular cluster provided strong evidence for the existence of cataclysmic variables. These are violently interacting double-star systems that may hold clues to the evolution of the clusters, which contain some of the oldest stars in the universe.
HST images of the most distant galaxies yet seen also showed how the structure of galaxies evolved over most of the history of the universe. Dramatically detailed images of energetic stars in our own galaxy showed the process whereby material is ejected from new stars in one direction while disks of dust, similar in size to our solar system, accumulate around the star. Scientists from the National Institute of Standards and Technology (NIST) calibrated benchmark oscillator strengths for a number of atoms calculated from the state-of-the-art atomic structure theory; this helped NASA judge the reliability of atomic data against the high accuracy of observed data such as that from HST.
In other astronomical news, the Astro-2 observatory achieved exceptional results with three telescopes observing ultraviolet light in a record-setting 16-day flight on the Space Shuttle Endeavour in March 1995. The most important result was a definitive measurement of the amount of helium spread throughout intergalactic space, measured to be the amount predicted by the Big Bang hypothesis. This states that the element helium was created during a hot phase of the primordial universe, only a few minutes after the Big Bang itself. The Astro-2 mission was also the first Shuttle mission with live Internet access, with more than 2 million requests logged in for mission information.
The Compton Gamma Ray Observatory (CGRO), launched in April 1991, continued a variety of observations of gamma rays, the most energetic form of light. By the end of FY 1995, scientists had recorded more than 1,400 of the mysterious gamma ray bursts, spread evenly over the entire sky. CGRO gives astrophysicists their only tool for continuing observations of this most dramatic celestial mystery. Scientists do not know whether these gamma ray explosions, lasting a few seconds, come from mysterious objects surrounding our own galaxy or whether they arise in other galaxies near the outer edges of the universe. A public debate by astrophysicists did not resolve the question. CGRO also completed a new survey of the highest energy gamma ray sources, demonstrating that about half of them are quasars with beams of energy pointed directly toward us but leaving the other half as yet unidentified.
Following CGRO and HST, the next Great Observatory will be the Advanced X-Ray Astrophysics Facility (AXAF). Figuring and polishing of the eight x-ray reflecting mirrors for AXAF were completed during FY 1995, in preparation for mirror coating. These are by far the most precise optics ever developed for imaging x-rays, and the completed mirrors significantly exceed performance requirements. In addition, the high-resolution camera being constructed at the SAO passed its critical design review.
The Ulysses spacecraft, launched in October 1990, successfully completed its passage over the northern pole of the Sun, completing the first ever exploration of the solar wind above its polar regions. Ulysses is now moving away from the Sun and will return again to pass over the Sun's poles in the years 2000 and 2001. The polar passages occurred during solar minimum, when activity on the Sun is at its lowest and when the polar regions are dominated by high-speed solar wind flows. Ulysses found that the solar wind was dominated by high-speed flow for latitudes above about 30 degrees north or south. Scientists working on the Ulysses and Voyager spacecraft projects detected oscillations in solar wind measurements, providing clues about the deep interior of the Sun. SPARTAN 201, a small satellite deployed and retrieved by the Space Shuttle in September 1994 and September 1995 during the polar passages by Ulysses, discovered the presence of unexpectedly hot (about 10 million degrees Centigrade) gas above the Sun's poles. This may explain why the solar wind speed is so high (500 miles per second) in the solar polar regions.
Voyagers 1 and 2, launched in 1977, and Pioneers 10 and 11, launched in 1972 and 1973, respectively, continued their exploration of the outer frontiers of the solar system. Now nearly twice as far from the Sun as Pluto, these spacecraft are approaching the boundary between the solar system and interstellar space. The Voyagers, which have sufficient power reserves to operate until 2015, could reach that boundary by the end of this century and become the first interstellar probes.
NASA was also active in several international programs devoted to space physics research. Yohkoh, a joint Japanese/U.S. mission launched in August 1991, continued its measurements of the solar corona and observation of the change in its x-ray brightness, activity, and structural complexity as it evolves from solar maximum to solar minimum. In November 1994, the Global Geospace Science (GGS) Wind spacecraft was launched successfully into a path upstream of the Earth's magnetosphere, where it has been providing information on the solar wind that determines conditions in the magnetosphere, including the downstream tail region. The Geotail spacecraft discovered particles from the Earth's ionosphere in the distant magnetic tail region at distances (210 Earth radii) beyond that of the Moon. Yohkoh, Wind, and Geotail are key elements of the International Solar-Terrestrial Physics (ISTP) Program for studying solar variability and its effects on the near-Earth space environment. This program involves spacecraft from the United States, Japan, Europe, and Russia. Several other ISTP spacecraft were scheduled to be launched by 1996.
In the suborbital area, NASA scientists made new discoveries about upper atmospheric flashes during aircraft and ground campaigns. These flashes, called sprites (red flashes) and jets (blue fountains), appear over intense thunderstorms and extend as high as 60 miles into the ionosphere. Scientists obtained the first spectra of a sprite in summer 1995 and also obtained high-resolution images showing a new type of sprite and its complex structuring. Another flight campaign over intense thunderstorms in Central America detected far fewer events than were seen over similarly intense storms over the Great Plains last year. Scientists believe that the study of these dramatic upper atmospheric flashes, and the thunderstorms with which they are associated, can lead to increases in airplane safety. In another element of the suborbital program, NASA's sounding rocket program had 30 consecutive launch successes in FY 1995.
NASA's Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) spacecraft continued its study of the energetic electrons and atomic ions from the Sun, as well as interplanetary, interstellar, and magnetospheric space. SAMPEX data are providing critical insights into how cosmic rays are accelerated out at the heliospheric shock, caused by the collision of the solar wind with the interstellar gas. SAMPEX measurements have provided data on energetic particles in the trapped radiation belts, which can affect electronic systems in spacecraft such as communication satellites.
Using the Very Long Baseline Array (VLBA) of the National Science Foundation (NSF), a team of scientists from the SAO and Japan were able to show compelling evidence for the existence of a black hole in the center of the galaxy NGC 4258. The latest telescope of the NSF-sponsored National Radio Astronomy Observatory, the VLBA's 10 antennas simulate the magnifying power of a radio telescope more than 5,000 miles in diameter. Astronomers from Japan's National Astronomical Observatory and the Harvard-Smithsonian Center for Astrophysics pointed the VLBA to the center of NGC 4258 to make images of such high resolution that the individual positions of water vapor masers could be measured as well as the speed of their motion along the line of sight to the galaxy. This is equivalent to reading a sign on a truck in Los Angeles from New York and measuring the speed of the truck as well. A simple calculation yielded the mass of the central object, 30 million solar masses, much too large a mass to be contained in the available volume except by a black hole. The massive black hole in the center of NGC 4258 has presumably grown to its huge size through years of accretion of matter in the densely populated region of the galaxy's center.
Astronomers at the NSF-sponsored Kitt Peak National Observatory investigated the formation of the Milky Way galaxy's Galactic Halo, an enigmatic distribution of older stars that appears key to understanding the formation of our galaxy. The challenge in studying the Halo came in isolating a sample of stars guaranteed to be members of that population. The team of astronomers was surprised to observe that the flattened population of "young" stars seems to be older than the spherical population of "old" stars. Was this subsystem of galactic stars formed during the collapse of the protogalaxy from an initial spherical distribution of gas, or was it formed by the aggregation of smaller dwarf galaxies trapped in the gravitational field of the larger Milky Way? One explanation is that both scenarios for formation of the Milky Way contributed, with the flatter distribution of stars coming from the collapse phase of the proto-galaxy and the younger spherical population resulting from the capture and shredding of neighboring dwarf galaxies. This challenging topic remained the focus of intense observational effort.
Scientists used the NSF-sponsored Cerro-Tololo Inter-American Observatory's Blanco Telescope to observe and analyze the Magellanic Stream, a large filament of neutral hydrogen gas from the Milky Way's radio emission that originates at the Small Magellanic Cloud (a Milky Way satellite dwarf galaxy) and extends almost one-third of the way around the sky. The astronomical team discovered unexpectedly strong optical line emission from hydrogen at the leading edge of the stream, where the density of neutral hydrogen detected in radio increases steeply. What is the source of this energetic emission? The association with the cloud's leading edges suggests that as the stream moves through high-pressure, low-density gas, shock waves propagating into the gas of the stream produce this emission. While radio, optical, and x-ray observations have long shown that there is diffuse gas associated with the Milky Way up to 150,000 light-years away from our galaxy, the origin and distribution of this hot gas remained controversial.
Astronomers at the NSF-sponsored vacuum tower telescope on Sacramento Peak used a new technique called phase-diverse speckle imaging to take time-series images of hot gaseous bubbles rising to the surface of the Sun. The resulting time series of a magnetic region without sunspots showed the highly dynamic visible layer of the solar atmosphere at scales of less than 200 kilometers. This new imaging technique depicted the detailed evolution of the bright edges of granules (the convective cells on the solar surface) for the first time. Scientists hoped that further study of these granules will yield useful information about the hot gaseous layers of chromosphere and corona above the Sun's surface.
The Center for Astrophysical Research in Antarctica (CARA), one of NSF's 25 Science and Technology Centers, completed its second year of year-round operations. The cold dry atmosphere and lack of diurnal variation make the South Pole the best site on Earth for many radio and infrared measurements. Site measurements by the South Pole InfraRed Explorer telescope have now indicated that the sky at the South Pole is darker by a factor of at least 20 than any other site previously surveyed. PYTHON, one of two telescopes comprising the Cosmic Background Radiation Anisotrophy, made measurements at the South Pole during the past two austral summers and operated for the first time during the winter. PYTHON confirmed the Cosmic Microwave Background (CMB) anisotropy, first measured by the Cosmic Background Explorer (COBE) satellite, and began to make a finer scale map of the CMB than COBE could. The 1.7-meter Antarctic Submillimeter Telescope and Remote Observatory, built by the SAO, was installed at the South Pole during the austral summer of 1994-1995. It quickly produced more than 10,000 spectra of neutral carbon lines in the galaxy and the Large Magellanic Cloud and also made measurements of atmospheric trace gases, such as ozone and carbon monoxide. Australian and NASA investigators working with CARA have undertaken a survey of the South Pole atmospheric transparency in the mid-infrared (5 to 40 millimeters). Measurements made during the 1994-1995 summer were encouraging enough for Goddard Space Flight Center (GSFC) scientists to propose that monitoring equipment be wintered during 1996.
During the austral summer of 1994-1995 in Antarctica, NSF deployed a new Automatic Geophysical Observatory (AGO), bringing to four the number now operating in the field. The AGO's, which were built by Lockheed, provide heat, power, and data storage for a suite of several remote-sensing instruments for years of unattended operation. When all the AGO's are deployed, they will provide, in conjunction with a few manned stations, uninterrupted and overlapping observations of the very high magnetic latitude ionosphere with a number of instruments. Following the lead of NSF, the British and Japanese Antarctic programs began developing their own AGO's, which will provide additional data in the lower latitude auroral zone. The AGO network will complement significantly the ISTP Program, especially the NASA Polar satellite.
Also during the austral summer of 1994-1995, NASA and NSF continued their joint program of long-duration ballooning in Antarctica. NASA launched two joint payloads during FY 1995 one carried emulsion track chambers to study the composition of heavy cosmic ray particles, and the second payload was a very large, high-energy gamma ray detector. Both payloads were recovered after flights of more than 10 days.
A scientist at the Laboratory for Astrophysics at the National Air and Space Museum (part of the Smithsonian Institution) in Washington, DC detected the first "natural" laser in space. Aboard NASA's Kuiper Airborne Observatory (KAO), the scientist used the aircraft's infrared telescope to observe a young, very hot, luminous star in the constellation Cygnus. Such lasers are created as intense ultraviolet light from the star "pumps," or excites, densely packed hydrogen atoms in the gaseous, dusty disk surrounding a young star, causing the atoms to emit an intense beam of infrared light. The discovery of this naturally occurring laser has given astronomers a powerful tool for probing the conditions in circumstellar disks where planets are thought to form.
Curator: Lillian Gipson|
Last Updated: September 5, 1996
For more information contact Steve Garber, NASA History Office,