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Korean Satellite To Measure Galaxy's Ultraviolet Glow

September 16, 2003
University Of California Berkeley
An ultraviolet telescope designed and built in a collaboration between University of California, Berkeley, and South Korean scientists is scheduled for a Sept. 26 launch from Russia as the main payload of South Korea's first scientific satellite.

An ultraviolet telescope designed and built in a collaboration between University of California, Berkeley, and South Korean scientists is scheduled for a Sept. 26 launch from Russia as the main payload of South Korea's first scientific satellite.

According to UC Berkeley astrophysicist Jerry Edelstein, the primary payload, called Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR), will capture the first far ultraviolet pictures of hot and glowing gas in the Milky Way galaxy, providing clues to the course of the galaxy's evolution.

SPEAR will measure the glow over the whole sky from gas between the stars that has been heated by supernova blast waves, blazing hot stars and colliding interstellar clouds.

The experiment will fly aboard the Korean STSAT-1 (Space and Technology Satellite 1), which will carry other experiments to measure energetic particles bombarding the Earth's atmosphere and causing auroras. SPEAR also will work in conjunction with these experiments by looking downward to capture the auroral ultraviolet emissions.

In the violent ebb and flow of gas within the Milky Way galaxy, the death of stars is marked by supernova explosions belching out 1,000,000 degrees Kelvin gas bubbles moving in blast waves at 500,000 miles per hour (200 km/sec). The hot gas smashes into surrounding galactic matter, slowing and somehow cooling into wispy interstellar clouds of a mere 10,000 degrees Kelvin. Completing the galactic life cycle, the gas finally condenses into massive, thick and frigid 10 degree Kelvin clouds that ultimately collapse with gravity, birthing new stars.

While satellites such as Chandra have seen the X-ray glow from the million-degree Kelvin gas, and the Hubble Space Telescope has produced beautiful pictures of the cooler 10,000 degrees gas glowing in visible light, the telltale ultraviolet glow from galactic gas at the "warm" in-between temperatures - from 30,000 to 500,000 degrees Kelvin - is barely understood, Edelstein said.

"Supernovas inject energy into the galaxy, and we're looking at what happens to that energy over the life cycle of the galaxy," said Edelstein, a senior fellow at UC Berkeley's Space Sciences Laboratory. "We know where the hot and cold stuff are, but how you get from hot to cold has been a mystery for 40 years."

These data will help astronomers determine what the Milky Way galaxy looks like. Is it filled with bubbles of hot gas created by supernovas, much like a Swiss cheese, or do the supernovas blow hot gas out of the disk of the galaxy which then cools and falls back into the disk like a fountain? Alternatively, does the hot gas mingle into the galactic disk, creating a glowing plasma that permeates the middle of the galaxy?

"We know that warm gas exists in the galaxies, but we don't really understand where. The only way to find this stuff is to look in the ultraviolet. SPEAR will map the ultraviolet sky glow from atoms of hot oxygen, carbon and nitrogen gas, showing us both where this gas is and how it cools off," Edelstein said.

Because cooling plays a role on a larger scale in the evolution of galaxies, SPEAR may also provide clues to the origins of large-scale structure in the universe. It may be that the "voids" found in the distant universe are actually filled with gas at a temperature of around 100,000 Kelvin and will glow in the ultraviolet.

In January, NASA launched a small UC Berkeley satellite called CHIPS (Cosmic Hot Interstellar Plasma Spectrometer) to measure the temperature of the low-density interstellar bubble of hot gas surrounding the sun. SPEAR will complement CHIPS by completing the thermal picture of the local region and extend this to the entire galaxy.

The Korean STSAT-1 is one of six satellites to be launched aboard a liquid-fueled Russian Cosmos rocket from Plesetsk cosmodrome, Russia's busiest launch site, located 400 miles northeast of St. Petersburg. The Korean satellite, about the size of a washing machine, was put together by the Korea Advanced Institute of Science & Technology (KAIST), South Korea's leading engineering university.

From its orbit 440 miles (700 kilometers) above the Earth, SPEAR will gaze away from the sun and, during the course of a year, steadily scan a thin wedge of the sky from north to south, moving one degree each day to complete a full-sky map in the far ultraviolet. For the second year, SPEAR will stare at particularly fascinating features in the sky that were identified in the all-sky survey.

SPEAR was designed by Edelstein and his Space Sciences Laboratory colleagues, including project scientist Eric Korpela, in collaboration with scientists at the KAIST's Satellite Technology Research Center (SaTReC) and the Korea Astronomical Observatory. The instrument carries two photon-counting spectroscopes, each sensitive to a different range of wavelengths in the far ultraviolet. Edelstein's Korean colleagues include Professor Kyoung Wook Min of KAIST and Dr. H. W. Han of the Korea Astronomical Observatory.

The UC Berkeley SPEAR effort is funded by $1.5 million from the U.S. National Aeronautics and Space Administration (NASA). The mission is funded by additional monies from the Korean Ministry of Science and Technology. The entire $13 million cost for payloads, satellite and launch is about a quarter of the cost for a comparable U.S.-built mission, Edelstein said. NASA funds for the SPEAR payload came through the "research carriers" program that traditionally supports the training of scientists and engineers using sounding rocket and ballooning payloads.

The low cost of the mission means that the small UC Berkeley and SaTReC teams often used off-the-shelf instead of custom-made parts, but Korpela said that hard work and built-in redundancy will hopefully make SPEAR reliable beyond its planned two-year mission life.

Edelstein has studied ultraviolet emissions from the sky for more than 20 years, having participated in sounding rocket, space shuttle and satellite missions such as that of UC Berkeley's Extreme Ultraviolet Explorer satellite. That satellite was launched in 1992 and, after a successful mission mapping the extreme ultraviolet emissions from nearby hot gas, fell to Earth in 2002.

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University Of California Berkeley. "Korean Satellite To Measure Galaxy's Ultraviolet Glow." ScienceDaily. ScienceDaily, 16 September 2003. <>.
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