(Updates with launch)
* IRIS to study how sun heats up its atmosphere
* Surface of the sun is much cooler than the corona
By Irene Klotz
CAPE CANAVERAL, Fla., June 27 (Reuters) - A small NASA telescope was launched into orbit on Thursday on a mission to determine how the sun heats its atmosphere to millions of degrees, sending off rivers of particles that define the boundaries of the solar system.
The study is far from academic. Solar activity directly impacts Earth's climate and the space environment beyond the planet's atmosphere. Solar storms can knock out power grids, disrupt radio signals and interfere with communications, navigation and other satellites in orbit.
"We live in a very complex society and the sun has a role to play in it," said physicist Alan Title, with Lockheed Martin Space Systems Advanced Technology Center in Palo Alto, California, which designed and built the telescope.
Scientists have been trying to unravel the mechanisms that drive the sun for decades but one fundamental mystery endures: How it manages to release energy from its relatively cool, 10,000 degree Fahrenheit (5,500 degree Celsius) surface into an atmosphere that can reach up to 5 million degrees Fahrenheit (2.8 million Celsius).
At its core, the sun is essentially a giant fusion engine that melds hydrogen atoms into helium. As expected, temperatures cool as energy travels outward through the layers. But then in the lower atmosphere, known as the chromosphere, temperatures heat up again.
Pictures and data relayed by the Interface Region Imaging Spectrograph, or IRIS, telescope may finally provide some answers about how that happens.
The 4-foot (1.2-meter) long, 450-pound (204-kg) observatory will be watching the sun from a vantage point about 400 miles (643 km) above Earth. It is designed to capture detailed images of light moving from the sun's surface, known as the photosphere, into the chromosphere. Temperatures peak in the sun's outer atmosphere, the corona.
All that energy fuels a continuous release of charged particles from the sun into what is known as the solar wind, a pressure bubble that fills and defines the boundaries of the solar system.
"Every time we look at the sun in more detail, it opens up a new window for us," said Jeffrey Newmark, IRIS program scientist at NASA headquarters in Washington, D.C.
The telescope was launched aboard an Orbital Sciences Corp Pegasus rocket at 10:27 p.m. EDT Thursday (0227 GMT Friday). Pegasus is an air-launched system that is carried aloft by a modified L-1011 aircraft that took off from Vandenberg Air Force Base in California about 57 minutes before launch.
The rocket was released from beneath the belly of the plane at an altitude of about 39,000 feet (11,900 meters) before it ignited to carry the telescope into orbit.
IRIS, which cost about $145 million including the launch service, is designed to last for two years. (Editing by Kevin Gray and Eric Walsh)