October 19, 1998
The shuttle mission that will be next week's historic return to space for pioneer astronaut John Glenn will be a nine-day, non-stop observing marathon for teams of planetary scientists from The University of Arizona in Tucson.
Discovery will carry the 77-year-old Ohio senator on his first space mission since he made the first U.S. orbital flight in 1962. It also will carry UA- developed telescopes called UVSTAR and STAR-LITE, two of six astronomy experiments that make up the Hitchhiker payload, on the more than 3.5 million mile journey. Launch is set during a 2+ hour window that begins at 2 p.m. EST (noon MST) Thursday, Oct. 29.
A. Lyle Broadfoot, senior research scientist at the UA Lunar and Planetary Laboratory (LPL), heads the UA team that jointly developed the UVSTAR with a group headed by Robert Stalio of the Center for Advanced Research in Space Optics at the University of Trieste, Italy. The Arizona and Italian scientists first flew their instrument on the Endeavor space shuttle in August 1995.
The UVSTAR, or Ultraviolet Spectrograph Telescope for Astronomical Research, is mounted on a pallet on top of the Hitchhiker cross-bay bridge in Discovery's cargo bay. It consists of two telescopes with imaging spectrographs that cover overlapping spectral regions of 500 to 900 angstroms and 850 to 1,250 angstroms, or wavelengths far shorter than those visible to the human eye. (Visible light is between 4,000 and 7,000 angstroms. An angstrom is one ten-billionth of a meter.)
UVSTAR will be used in an ambitious schedule of solar system and stellar astronomy. It will measure the response of the solar system to the sun's changing energy output, a task that will complement results from another telescope on the Hitchhiker cross-bridge, a telescope that directly measures changes in solar energy. Extreme ultraviolet radiation cannot penetrate Earth's atmosphere, so those studying Earth's ultimate energy source -- the sun -- at these wavelengths must conduct their experiments from space.
UVSTAR scientists also will observe the Io plasma torus around Jupiter, supernovae remnants and the hot blue stars within globular clusters, or the very dense clusters of stars that hold important clues about stellar evolution. And should a comet or other special sudden event ("target of opportunity") occur in the sky, the scientists will observe these also.
Broadfoot led the Voyager Ultraviolet Spectrometer team that discovered the Io plasma torus just before the Jupiter flyby in 1979. The torus is an extensive, doughnut-shaped cloud of ionized atoms surrounding Jupiter at the orbit of its moon, Io. Jupiter's magnetic field traps ions formed from sulfur and oxygen that spew from Io, the most volcanically active object in the solar system. There are a million tons of material in the torus, which radiates as much power as all the electrical power plants on Earth. The ions emit their strongest radiation at extreme ultraviolet wavelengths.
Until UVSTAR, there was no instrument capable of forming images of the torus at such high resolution in each of its brightest emission lines. The UVSTAR spectral images are sharper than those Voyager could produce and those Galileo can produce. Also, Galileo is too close to the torus to image the entire structure, as does UVSTAR. The Hubble Space Telescope operates at wavelengths longer than those radiating from the torus.
For this mission, UVSTAR is carrying an instrument called the Extreme Ultraviolet Imager, which will measure Earth's atmosphere. It has two devices that will map the intensity of helium and oxygen ions in the atmosphere by scanning along the Earth's shadow line, allowing scientists to obtain precise measurements of Earth's ionosphere and plasmasphere.
The ionosphere, which begins about 30 miles (50 km) above the Earth's surface but is most distinct at altitudes above 50 miles (80 km), is produced primarily by solar radiation on neutral atoms and air molecules in the atmosphere. It is important in the propagation of radio waves. The plasmasphere is a dense, cool plasma within the ionosphere that rotates with Earth just as the atmosphere rotates with our planet.
LPL senior research scientist Jay B. Holberg heads research with STAR-LITE, unique as the first all-metal matrix telescope ever flown in space. Holberg and Broadfoot successfully proposed the idea for using extremely rugged aluminum-silicon carbide for a space shuttle telescope and began developing it 18 months ago.
The metal composite material is very tough -- so tough that grinding tools are made of the stuff, Holberg said. This strength assures that STAR-LITE's 16-inch (40 cm) primary mirror will withstand the forces of launch much better than would a primary mirror of glass. The composite material also is lightweight -- a necessary feature for a space payload. And because all parts of the telescope are made of the same material, the telescope uniformly expands and contracts in changing thermal environment, relieving astronomers of some frustrating focusing problems, Holberg said.
Researchers directed by Martin Valente of the UA Optical Sciences Center, a group experienced in making metal matrix mirrors for the U.S. Army, designed and built the mirror for the novel imaging spectrometer telescope. The UA's University Research Instrumentation Center built the roughly 3-foot telescope to fit on a scan platform that Broadfoot and others in the "LPL-West" group have used in several earlier space shuttle experiments. (The LPL West group is so named because it is housed in the Gould-Simpson Science Building, west of the LPL in the Kuiper Space Sciences Building on the UA campus.)
Like UVSTAR, STAR-LITE works at the very short ultraviolet wavelengths (900 to 1150 angstroms), a part of the electromagnetic spectrum that astronomers have only begun to explore. There are formidable technical challenges to working at these wavelengths, Holberg said: Extreme ultraviolet wavelengths are detectable only from space, detectors are exposed because they cannot be protected with optical windows, all optics have to be reflecting optics rather than lenses, and only 30 percent to 40 percent of the light at these wavelengths is reflected, compared to the 98 percent reflectivity of optical telescope mirrors.
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. STAR-LITE has a larger aperture than UVSTAR and a much larger field of view, so that it is better suited to study extended structures, such as galaxies, supernova remnants and globular clusters.
Holberg's group will use STAR-LITE (the Spectrograph/Telescope for Astronomical Research) to view the pattern of shock waves in supernova remnants, or the clouds of dust and gas blown out by the explosion of a star, for a better idea of how fast shock waves travel and what the clouds are made of. Other science targets include nearby star-forming regions and galaxies. These results, at the shortest ultraviolet wavelengths, can be compared with data on red-shifted light from distant star-forming regions gathered by the Hubble Space Telescope to see how galaxies have changed over several billion years.
Discovery carries another payload of special interest to Arizonans and infrared astronomers. One of four experiments aboard the Hubble Space Telescope Orbiting Systems (HOST) platform will test a new technology cryogenic cooler for the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). The new, advanced cryogenics are being tested in the zero-gravity of space as a system to replace the current cold-storage bottle, or "dewar," that will keep NICMOS' heat-sensitive detectors cool only until about Jan. 1, 1999. Space-walking astronauts would install NICMOS' new cooling system in a servicing mission scheduled for launch May 2000.
NICMOS was conceived and built by a team headed by Professor Rodger I. Thompson of the UA Steward Observatory, principal investigator, and UA astronomy Professor Marcia J. Rieke, deputy principal investigator.