News Release

Ultraviolet light lies between visible light and X-rays on the electromagnetic spectrum, with shorter wavelengths and higher frequency than visible light.

Most ultraviolet light is absorbed by Earth's ozone layer in the stratosphere. That's why instruments designed to observe UV emissions must get above the atmosphere, either by using rocket-borne instruments for brief observations or by using satellites and spacecraft for longer observations.

The Astro observatory is a package of three instruments, the Hopkins Ultraviolet Telescope (HUT), developed at The Johns Hopkins University, the Ultraviolet Imaging Telescope (UIT), developed by NASA's Goddard Space Flight Center and the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), developed at the University of Wisconsin.

Astro-1 orbited the Earth aboard the space shuttle Columbia for nine days in December 1990. An X-ray telescope, operating on a separate pointing system, was also included on the Astro-1 mission but is not part of Astro-2.

Astro-2 is scheduled to be launched from the Kennedy Space Center in Florida aboard the space shuttle Endeavour in March 1995. The mission is planned to last up to 16 days with a landing planned at KSC.

Like Astro-1, the Astro-2 observatory will be housed inside the shuttle's payload bay, with astronomers serving as payload specialists and mission specialists operating the telescopes from the aft flight deck of the shuttle. As the shuttle orbits 220 miles above Earth, a large contingent of scientists and engineers will guide the mission from the Spacelab Mission Operations Control Center at Marshall Space Flight Center in Huntsville, Ala., while NASA controls shuttle operations from Johnson Space Center in Houston, Texas.

Each component of the observatory has a specific role. HUT conducts spectroscopy in the far ultraviolet portion of the spectrum. Its observation range encompasses the wavelengths from 900 to 1,800 angstroms, but its greatest sensitivity is in the range of 900 to 1,200 angstroms. The portion of the spectrum observed by HUT, coupled with the instrument's sensitivity, enables it to see a slice of the ultraviolet universe other observatories are unable to detect. The Hubble Space Telescope, for example, can observe only at wavelengths longer than about 1,200 angstroms.

WUPPE can determine the polarization of ultraviolet light, measurements that help determine the composition and structure of dust in interstellar space and clouds of dust and gas that surround stars. It explores an area that has not been readily studied, especially for stars that are too bright for Hubble's Faint Object Spectrograph and for nebulae too large for Hubble's smaller spectrograph openings.

UIT takes photographs of objects in ultraviolet light, recording the images on film for processing back on Earth. Before Astro-1, very few ultraviolet images had been taken and those that were available were taken during brief rocket flights. UIT observes a field of view two-thirds of a degree across, an area larger than the full moon. This is considered "wide field" for astronomers; each UIT photo covers an area more than 200 times the size of the Hubble Space Telescope's Wide Field/Planetary Camera (but at lower resolution). For many galaxies or star clusters, this is large enough to encompass the entire object in a single UIT frame.

Before the advent of UV space telescopes, scientists had to be satisfied with rocket-borne telescopes. In fact, the Astro-2 telescopes were built after prototypes were designed and flown separately on sounding rockets. But a typical rocket flight might gather 300 seconds of data on a single object. On Astro-2, scientists expect the three telescopes to gather a total of nearly 600 hours of data on as many as 250 targets.

Astro-2 complements other ultraviolet instruments. Each Astro telescope has capabilities that make it unique. In addition, since the telescopes are aligned, joint observations of many objects add a new dimension to the payload's capabilities. Several guest investigator programs make explicit use of more than one telescope simultaneously.

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