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pacing the void

By Greg Clark
Arizona Summer Wildcat
August 6, 1997

Gazing at the glow


Photo courtesy A. Lyle Broadfoot
Arizona Summer Wildcat

UVSTAR, a spectrographic telescope built at the UA, is examined by a technician in the clean room at Kennedy Space Center prior to a 1995 launch aboard the Space Shuttle Endeavor. UVSTAR is now installed on Space Shuttle Discovery awaiting tomorrow's laun ch.

When Space Shuttle Discovery lifts off tomorrow for a scheduled 11-day mission, it will be carrying two UA instruments designed to observe the activity of Earth's upper atmosphere, and one, designed jointly with an Italian group, that will gaze at hot, far-away stars, and a plasma ring around Jupiter.

Assembled on a frame spanning the 15-foot width of the shuttle's cargo bay, the instruments are part of an array designed to collect and measure radiation at the extreme ultraviolet end of the spectrum.

The spectrographic telescopes are connected to Discovery's electrical and computer systems and will be controlled from the Goddard Space Flight Center, Greenbelt, Md. by a team of scientists that includes a dozen University of Arizona researchers.

The two Arizona Airglow instruments, GLO 5 and GLO 6, were designed and built entirely at the University of Arizona under the direction of the Lunar and Planetary Laboratory's Lyle Broadfoot.

Broadfoot and his LPL team designed the instruments, which were fabricated at the University Research Instrumentation Center, a machining and electronics design facility on the ground floor of the Gould-Simpson Building.

Now, mounted side by side in Discovery's cargo bay, the two identical instruments will observe Earth's upper atmosphere to give scientists a picture of electromagnetic activity at very high altitudes.

A third instrument, called the UVSTAR, for Ultraviolet Spectrograph Telescope for Astronomical Research, was developed jointly between Broadfoot's team and astronomers at the University of Trieste in Italy. It was built at the UA, and will be used to study the extreme ultraviolet emissions (EUV) of stars.

Demystifying "shuttle glow"

The Airglow instruments were initially developed to study the phenomenon of shuttle glow, under a six-year, $4.2 million grant from the U.S. Air Force Phillips Laboratory.

During early shuttle missions, which were mostly lower-altitude flights orbiting near 100 kilometers, astronauts noticed a white glow, or halo around the shuttle's tail.

To understand this phenomenon, Broadfoot and his team built the imaging spectrographs to collect light over the entire spectrum, from the extreme ultraviolet to the infrared, including the visible light in between.

The glow is now fairly well understood to be caused when the shuttle is bombarded by charged atoms of oxygen traveling through the atmosphere, said LPL senior research scientist Jay Holberg, who has worked closely with Broadfoot.

Having flown instruments on five previous shuttle missions, Broadfoot has now turned his attention to observing Earth's atmosphere.

By aiming the imagers through the atmosphere, and focusing primarily on extreme ultraviolet (EUV) emissions, Broadfoot's research is changing the way scientists think about the upper atmosphere.

The atmosphere was previously believed to be a fairly uniform, layered structure, but, Broadfoot said, evidence from last summer's shuttle flight of a GLO instrument contradicts that view.

"When we look at the atmosphere edge-on we see it is extremely chaotic," he said.

The research team hopes to learn something about the circulation of the upper atmosphere by looking at the motion of magnesium ions, which can be observed in the EUV bands.

Magnesium enters the atmosphere in comets that burn up about 100 kilometers above the surface, Broadfoot said.

However, magnesium atoms remain, becoming ionized, or positively charged, and then circulate throughout the upper atmosphere to altitudes of up to 300 kilometers.

"Magnesium becomes a very interesting tracer in viewing how ions are transmitted through Earth's magnetic and electrical field lines," Broadfoot said. He hopes to study magnesium ion clouds and columns of gas during Discovery's flight.

Data from the GLO instruments have been extremely valuable resources, according to Air Force research chemist Rainer Dressler at Hanscom Electronic Systems Command in Lexington, Mass.

"Broadfoot has the highest resolution and the largest spectral range of atmospheric emissions available," said Dressler, who has used GLO data of emissions of metals in the ionosphere extensively in his research.

Dressler said the vast amount of observational data collected by the GLO experiments will provide invaluable foundation from which to base models of the atmosphere.

Getting closer to the stuff of stars

Observations from the UVSTAR are also expected to fill in gaps in current scientific knowledge about stars and planets.

LPL senior research scientist Jay Holberg believes UVSTAR is a pioneering instrument for observing the extreme ultraviolet emissions of stars. EUV radiation does not penetrate Earth's atmosphere and is not detected by the Hubble Space Telescope.

But many high-energy stars, those about 5000 times hotter than the sun, Holberg said, emit most of their energy in the EUV.

"There is a very significant piece missing," said Holberg, "Because there is a lot going on in those wavelengths in terms of emissions from stellar gasses such as Hydrogen, Helium and Oxygen that we don't understand."

The shuttle as a telescope platform

Broadfoot believes the shuttle's potential for sending telescopes and other imaging systems into space is great, but it has, so far, been ignored.

"My thrust in this whole program is being able to use the shuttle as a viewing platform for astronomical instruments," Broadfoot said.

He said many in the scientific community doubt whether the shuttle can be effectively used for this purpose because of several long-standing misconceptions.

For example, Broadfoot said, one popular belief is that the shuttle environment is too dirty a place from which to make observations.

This is based on the idea that objects from Earth "out-gas" for a certain period of time when taken into space.

Some believe dirt on the shuttle, which becomes a cloud of debris in space, would interfere with observations.

Broadfoot said, the idea" is a fable generated long ago by people who don't have our best interests at hand."

Contrary to that belief, he said, there are actually several advantages to shuttle-based observation, the first being the sheer amount of data available.

With instruments being directly interfaced with Discovery's computers, and the shuttle in constant contact with mission control, data is transmitted continuously.

According to Broadfoot, an instrument aboard the shuttle will collect as much data in one flight as a satellite-based instrument collects in a year. This is because data from a satellite is only transmitted twice each day, during two 10-minute windows when the satellite passes over a ground-based receiving station.

Secondly, instruments flown aboard the shuttle can be operationally tested. After a flight, designers can make adjustments, upgrade, or even overhaul instruments as needed, a major advantage over a satellite, Broadfoot said.

After the current Discovery mission these instruments will fly again on a shuttle late next year. Broadfoot hopes to send instruments on shuttle flights twice a year.

"The shuttle and space stations are going to play a bigger role in astronomy than they have in the past," he said.

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