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Discovery is carrying a battery of telescopes to study the sun, planets, and supernovae
29 October, 1998: Ultraviolet rays from the sun and the cosmos are almost completely blocked by Earth's atmosphere. For those of us who disdain sunburns the protection afforded by the ozone layer is a good thing. For some astronomers it's a nuisance. The sun and planets, supernovae, and interstellar clouds of gas are all copious sources of ultraviolet radiation that simply can't be seen from Earth's surface. To study the cosmos at UV wavelengths it is necessary to go into orbit above the atmosphere, and that's what will happen when Discovery takes off later today. The payload bay of Discovery is packed with no fewer than six telescopes. Most are mounted on a platform called the International Extreme Ultraviolet Hitchhiker. A primary mission of the Ultraviolet Hitchhiker is to study UV rays from the sun and how they affect planets throughout the solar system.
Different telescopes will have different jobs to do during the mission. The Solar Extreme Ultraviolet Hitchhiker (SEH), developed at the University of Southern California, will keep an eye on the sun. SEH is able to photograph the sun at ultraviolet wavelengths between 250 and 1,700 angstroms. UV images like the one below trace hot gas and magnetic structures in the sun's atmosphere. Solar activity is on the rise as the sun heads for a sunspot maximum in the year 2000. By operating ultraviolet telescopes during the Sun's increasingly active phase, scientists hope to explore the connections between complex solar magnetic fields and potentially hazardous solar eruptions.
A Peek Inside
Shuttle Discovery
On Launch Complex 39B, the open payload bay doors on the orbiter Discovery reveal (bottom) International Extreme Ultraviolet Hitchhiker (IEH-3) and (above it) Hubble Space Telescope Orbital Systems Test Platform (HOST), two of the payloads for mission STS-95. The other payloads include the Spartan solar-observing deployable spacecraft and SPACEHAB, a single module containing experiments on space flight and the aging process.
This ultraviolet image of a sunspot group was taken by a telescope aboard NASA's Transition Region and Coronal Explorer (TRACE) spacecraft. The bright areas are million degree hot plasma loops trapped in magnetic bottles above sunspots. SEH will also measure changes in the Earth's atmosphere caused by solar EUV and daytime temperatures. Its observations will be coordinated with identical instruments on two other spacecraft--a payload to be launched on a sounding rocket and the ESA/NASA Solar and Heliospheric Observatory (SOHO) --to provide tight cross-calibration of the three instruments.
This ultraviolet image of the sun in the light of ionized Helium atoms was recorded by SOHO on 13 Oct 1998.
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While SEH watches the sun, another instrument called UVSTAR will focus on the planets. UVSTAR actually consists of two telescopes that cover overlapping regions of of the UV spectrum, from 500 to 900 angstroms and from 850 to 1,250 angstroms. For comparison, visible light is between 4,000 and 7,000 angstroms. An angstrom is one ten-billionth of a meter. One of UVSTAR's telescopes will target Jupiter and the Io plasma torus. Io, one of Jupiter's four Galilean moons, is the most volcanically active body in the solar system. It spews sulfur and oxygen ions into space where they are trapped by Jupiter's magnetic field to form a giant donut-shaped torus encircling Jupiter. Jupiter and Io are electrically connected along magnetic field lines that arc from the torus to Jupiter's north and south poles. Electrical currents that flow in this system are enormous, carrying more than 10
12 Watts of power (10
12 is a "1" followed by twelve "0's"). That's greater than the output of all power plants on Earth combined. These tremendous currents give rise to magnificent aurora on both Jupiter and Io, and extremely powerful radio emissions that can be heard on the loudspeakers of simple ham radios here on Earth. Observations of Jupiter and the Io torus by UVSTAR will help scientists understand the complex Jupiter-Io interaction and how that system responds to events on the Sun.
A Hubble Space Telescope image of Jupiter's aurora, seen in ultraviolet light, overlaid on an optical picture of Jupiter. Aurora on Earth occur when electrons from the solar wind strike the upper atmosphere, but Jupiter's auroras are caused by particles spewed out by volcanoes on Io. Another telescope on UVSTAR, the Extreme Ultraviolet Imager, will keep an eye on Earth. The EUI will map the intensity of spectral lines emitted by helium and oxygen ions in the atmosphere, allowing scientists to monitor conditions in the ionosphere and plasmosphere in concert with the solar observations.
Pictured above is a Hubble image of the Cygnus Loop, an expanding cloud of gas from a supernova explosion 15,000 years ago. While UVSTAR and SEH watch the sun and planets, a unique all-metal telescope called STAR-LITE (Spectrograph/Telescope for Astronomical Research) will look to the stars. Scientists will use STAR-LITE to study supernova remnants like the Cygnus Loop, pictured left, to learn more about what happens after a star explodes. Other science targets include nearby star-forming regions and galaxies.
STAR-LITE was made by University of Arizona scientists from rugged aluminum-silicon carbide. The strong metal composite material assures that STAR-LITE's 16-inch primary mirror can withstand the forces of launch better than a primary made of glass. The composite material also is lightweight, making it an ideal space payload. Unlike UVSTAR, which depends on shuttle orientation to see its science target, STAR-LITE can point anywhere in the shuttle bay, cradled on its rotating platform.
Future stories in this series will cover more science on STS-95, including human protein experiments, attempts to measure the solar constant, and microgravity fluid research.
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Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: John M. Horack
