[Fact Sheets]

ASTRO MISSION OBJECTIVES

The Astro Mission is the first of a series of attached payload missions for the Space Shuttle that is dedicated to perform experiment and observations from individual scientific disciplines. Astro was designed to take the next step forward with payloads originally developed as sounding rocket experiments to observe celestial objects in the ultraviolet and X-ray regions of the electromagnetic spectrum.

On a sounding rocket, such a payload can observe just a few selected targets during the brief minutes of the flight; while on the Shuttle, the Astro experiments can observe dozens of targets over a period of as many as ten days and nights. Ultraviolet light and X-rays are blocked by the Earth's atmosphere. Therefore, it is necessary to mount telescopes on an orbital or suborbital platform in order to observe those forms of radiation from the universe.

Astro consists of a Spacelab segment that is operated by onboard Payload Specialists and another segment, which is operated by astronomers in a control center at the Goddard Space Flight Center (GSFC), Greenbelt, Maryland. The Astro mission as a whole is controlled from the Marshall Space Flight Center in Huntsville, Alabama.

The Spacelab segment of Astro consists of three U.S.-made ultraviolet instruments that are jointly mounted and co-aligned on an Instrument Pointing System (IPS). The IPS was developed by the European Space Agency. The other Astro segment is an X-ray telescope, mounted on a Two Axis Pointing System (TAPS) that was developed at GSFC.

Scientific Instruments

The three ultraviolet instruments are the Ultraviolet Imaging Telescope (UIT), the Hopkins Ultraviolet Telescope (HUT), and the Wisconsin Ultraviolet Photopolarimetry Experiment (WUPPE). These devices observe celestial objects by taking photographs, obtaining spectrograms, and measuring the intensity and polarization of their light, respectively. The X-ray instrument of the Astro mission is the Broad Band X-Ray Telescope (BBXRT).

HUT

The HUT is a 0.9 meter (36-inch) telescope fitted with an electronic detector system. Its recordings are processed by an onboard computer system and are telemetered to the ground for later analysis. It will record spectra in the far ultraviolet region, including observations of distant quasars and active galaxies too faint to have been observed in this spectral region by previous, less-sensitive space experiments. Spectral observations reveal the chemical composition and physical conditions in astrophysical objects, meaning their motions in the direction toward or away from the Earth. The HUT was developed under the direction of Principal Investigator (PI) Professor Arthur Davidsen of the Johns Hopkins University, Baltimore, Maryland.

UIT

The UIT is a 38-centimeter (15-inch) telescope with a relatively wide field of view. It can photograph an area that is 40 arc minutes in diameter, about 25 percent wider than the full Moon. Its electronic sensors produce images that are permanently recorded on photographic film. Ultraviolet imagery is used to determine the locations and spatial relationships of hot stars and other sources of ultraviolet radiation. This information is used by astronomers to analyze the temperatures and other physical properties of the radiation sources and to identify such objects as young, massive stars and old and very dense objects called white dwarf stars.

Upon landing the Shuttle Columbia, the film will be processed at GSFC. The permanent archive for UIT data will be the National Space Science Data Center (NSSDC), located at GSFC. The UIT was developed and built at GSFC by a team led by Principal Investigator (PI), Theodore Stecher.

WUPPE

The WUPPE is a 50-centimeter (20-inch) telescope fitted with a spectropolarimeter, an instrument that records both the spectrum and the polarization of the ultraviolet light gathered by the telescope. Operating at ultraviolet wavelengths that are mostly longer than those observed by HUT (but with some useful overlap), WUPPE provides chemical composition and physical information on celestial targets that emit significantly in its region of observation. Additionally, it provides unique information from its measurements of the polarization light, meaning the tendency flight waves to oscillate preferentially in certain directions. This tendency reflects physical conditions in the source of the light or in the interstellar medium through which the light travels to the Earth. For example, polarization provides information on the shapes of stars that are distorted out of round by the gravity of nearby companion stars, on the magnetic fields present in stars, and on the properties of microscopic dust grains in interstellar space. The WUPPE was produced at the University of Wisconsin at Madison by a team led by Pi Professor Arthur D. Code.

BBXRT

BBXRT consists of two coaligned X-ray telescopes that focus X-rays by reflecting them at grazing angles (so that the X-rays are not absorbed in the gold-coated aluminum foil mirrors, as they would be if they struck the mirrors at or near right angles to the mirror surfaces). The X-rays are focused on solid-state detectors that are cooled to a temperature of about 100 kelvins (-148 degrees Fahrenheit) for maximum sensitivity. Observations of the X-ray spectrum of celestial targets reveal the physical properties and chemical compositions of gases at extremely high temperatures, ranging up to 100 million kelvins (180 million degrees Fahrenheit) and sometimes higher. BBXRT will be able to obtain X-ray spectra of much fainter and more distant targets than previous X-ray region over which detailed spectra can be obtained. The BBXRT was designed and built by GSFC by a team led by PI Dr. Peter Serlemitsos.

Although BBXRT was not part of the Astro payload, as originally selected, it was added to the mission after the appearance of Supernova 1987A in February 1987, in order to obtain vital scientific information about the supernova. In addition, data gathered by BBXRT on other objects will enhance studies that would otherwise be limited to data gathered with the three ultraviolet telescopes.

X-ray and ultraviolet observations complement each other in providing information on different aspects and component regions of the targets of study. For example, in the accretion disk, a swirling region of infalling material surrounding a black hole or other source of powerful gravity in space, ultraviolet and x-radiations are preferentially emitted at different distances from the center. In studies of the outer atmospheres of stars, ultraviolet spectra are especially useful in investigations of the layers called chromospheres, while X-ray spectra are a prime source of information about the regions known as coronas, which overlie the chromospheres. The ultraviolet and X-ray emissions of many celestial objects, such as supernovae, active galaxies, quasars, and close binary stars are known to vary with time. Therefore, the intent of the Astro mission to obtain simultaneous X-ray and ultraviolet observations with BBXRT, WUPPE, HUT, and UIT represents a major scientific capability (called co-observing) that will be used repeatedly during flight.

Astronomical Targets

The Astro mission will investigate a great variety of astronomical objects, ranging from Io, an actively volcanic moon of the planet Jupiter, to white dwarf stars, interacting binary stars (in which streams of gas pass from one star to another), globular star clusters consisting of hundreds of thousands of closely packed stars, to nearby galaxies such as the Large Magellanic Cloud and distant clusters of galaxies and quasars. More than 200 individual scientific investigations of these targets are planned for a single flight of the mission, based on priorities assigned according to estimated scientific importance. Here are some of the principal scientific goals of the individual Astro experiments:

HUT

Hut will extend sensitive, high-quality spectroscopy into the far ultraviolet, covering wavelength range from 450 angstroms to 1850 angstroms. It will observe wavelengths shortward of those that can be measured with the International Ultraviolet Explorer and the Hubble Space Telescope, breaking new ground in every object that it explores. Following are examples of scientific programs planned for HUT:

UIT

The UIT will photograph up to about 200 fields in the sky during the Astro 1 mission (some may be duplicates). These regions, each 40 arc minutes in diameter, will be surveyed down to ultraviolet magnitudes of about 20 to 21, corresponding to visible ("V") magnitudes of about 24 to 25 or hot, unreddened stars. The total equivalent sky area will be about 70 square degrees. Although the Hubble Space Telescope will take much sharper photographs, and record much fainter stars, the UIT will photograph much larger regions at once, and the UIT will suffer much less interference from visible light, since it is provided with "solar blind" detectors. For certain classes of targets, such as diffuse, ultraviolet-emitting or ultraviolet-scattering nebulae, UIT may be a more sensitive imager. Besides filters for various wavelength regions of ultraviolet light, UIT will be equipped with a diffraction grating. Photographing selected fields of view through the diffraction grating will provide ultraviolet spectra for many objects in the fields. Examples of specific studies with UIT are:

WUPPE

WUPPE will obtain high signal-to-noise measurements of polarization in the ultraviolet spectral range from wavelength 1300 angstroms to wavelength 3300 angstroms. Such observations have previously not been made. The advantage of making the measurements in the ultraviolet is that the polarization imposed on the light by interstellar dust in the foreground will be reduced compared to the corresponding effect in the visible light region, while increased polarization is expected in the ultraviolet from physical conditions that are intrinsic to the target under observation. Thus, a larger and more readily analyzed effect may be found. Examples of specific studies by WUPPE include the following:

A reflection nebula is one that is illuminated by nearby stars, rather than one that emits its own light. A plume is an elongated structure of gas or plasma, moving outward in a stellar wind. White dwarfs are known to have powerful magnetic fields. Disks of gas and (sometimes) dust are believed to surround certain stars, typically in their equatorial planes.

BBXRT

BBXRT will obtain the first high-quality X-ray spectra of many of the objects discovered by HEAO 2, the Einstein satellite. HEAO 2 was sensitive enough to detect these objects, but in most cases, was not sufficiently sensitive to record meaningful spectra of them. The high sensitivity, BBXRT will provide double the spectral resolution of HEAO 2 and will observe any targets at higher energies (shorter wavelengths) than previous X-ray telescopes. Examples of BBXRT science programs include the following: