On any given night, the naked eye can see a dark sky peppered with stars as the moon beams brightly back at Earth. Look closer and it is easy to pick out constellations -- the Little Dipper and nearby, the Big Dipper -- and perhaps, the international space station taking a sweep across the United States.

But as much as we can envision the night sky here on Earth, there is another unseen one in deep space -- the gamma ray version where no human eye can scan its contents.

"The sky wouldn't be dark, there would be no way of knowing if it was night or day," said David Thompson, GLAST deputy project scientist at NASA's Goddard Space Flight Center. "But what you would notice is the Milky Way, brilliant swaths of light across the sky. It is constantly changing, some brighter lights then dimmer lights, then blinding flashes of gamma-ray light and then a gamma-ray burst."

The newest kid on the cosmic block of observatories is GLAST, the Gamma-ray Large Area Space Telescope, being launched no earlier than Saturday from Canaveral Air Force Station in Cape Canaveral for a five-year mission. The launch window runs from 11:45 a.m. to 1:40 p.m. each day through Aug. 7.

NASA, along with its partners from five countries (United States, Italy, Sweden, Japan and France) designed and built a $690 million observatory to gather information about gamma rays, the highest energy form of light, from the extreme side of the universe.

Once that information is collected in the LAT instrument on GLAST, scientists from 18 institutions -- 270 in all -- will decipher it and be able to determine the origins of the energy and crack the mystery of these powerful explosions known as gamma ray bursts. The mission will also study supermassive black holes, pulsars and the origins of cosmic rays.

"We are excited about this mission. It's a great leap in capabilities for viewing the universe," said Steve Ritz, GLAST project scientist with Goddard Space Flight Center. "We are expecting to find surprises."

So what makes a gamma-ray sky different? It is extremely bright because gamma rays produce cosmic particles (pulsars) and rapidly spinning neutron stars that rotate and blink. Add black holes to the mix and there's a gamma-ray scenario, Thompson explained.

"It is like looking down the barrel of a gun filled with rapidly changing particles with gamma-ray bursts, extreme versions of exploding stars and black holes forming."

GLAST will launch on a Delta II heavy rocket with nine solid rocket motors. It will reside in a low-earth orbit at about 350 miles and orbit the Earth every 90 minutes. About 75 minutes after launch it will reach the orbit it needs to be in and within10 minutes the solar arrays will deploy and start capturing the sun.

"We designed (the mission) for five years, with a 10-year goal. There are no consumables that are being used up and it is based on solid state electronics so it will degrade gracefully (at the end)," Ritz said. "When the lifetime of GLAST is finished, it will re-enter the atmosphere and what doesn't burn up will fall into the Pacific Ocean."

Because it will not cruise to a planet, scientists can take their time to calibrate the instruments once the observatory gets into its spot in orbit. The first pictures are expected to beam back to Earth within three weeks.

Gamma ray bursts

Gamma ray bursts were discovered by American surveillance satellites in the late 1960s. These satellites were looking for gamma rays coming from possible clandestine Soviet nuclear tests, but instead found brief but intense flashes coming from random directions in space, according to NASA.

To this day gamma ray bursts remain one of the greatest mysteries of modern astronomy.

Despite lasting only a few milliseconds to several minutes, they are the brightest gamma-ray phenomena known, outshining all other sources of gamma rays combined, GLAST Deputy Project Scientist Neil Gehrels of NASA's Goddard Space Flight Center said.

"An individual gamma ray burst can release in a matter of seconds the same amount of energy that our sun will radiate over its 10-billion-year lifetime."

Things are changing in the universe all the time, Ritz said last week.

"The thing that is most exciting about this mission is the science that is not on anybody's list yet," he said.

Discoveries

The universe is home to numerous exotic and beautiful phenomena, some of which can generate almost inconceivable amounts of energy. Supermassive black holes, merging neutron stars, streams of hot gas moving close to the speed of light -- these are a few of the marvels that generate gamma-ray radiation, billions of times more energetic than the type of light visible to our eyes.

Astronomers have made considerable strides in recent years in understanding gamma rays. This is directly attributed to space observatories:
· Energetic Gamma Ray Experiment Telescope (EGRET) instrument on the Compton Gamma Ray Observatory (CGRO). Launched in 1991, EGRET made the first complete survey of the sky. EGRET showed the high-energy gamma-ray sky to be surprisingly dynamic and diverse, with sources ranging from the sun and moon to massive black holes.

· The Burst and Transient Source Experiment (BATSE) on Compton Gamma Ray Observatory detected several thousand gamma ray bursts and showed that they come from random directions in the sky, which strongly suggested that they are not of galactic origin and must occur at great distances.

· In the late 1990s, the Italian/Dutch BeppoSAX satellite was able to pinpoint the location of several gamma ray bursts, which enabled X-ray, optical and radio telescopes to monitor their afterglows. This was a crucial development, because it enabled astronomers for the first time to measure distances to bursts and observe how they interacted with their surrounding environments.

· The now-defunct HETE-2 satellite, launched in October 2000, and the currently operating NASA Swift satellite (2004) have significantly extended and improved these capabilities.