The newest spacecraft to launch in NASA's oldest program -- the Explorer project that dates back to America's first satellite -- was propelled into Earth orbit Thursday night by an air-launched rocket off the coast of California.

Image of the Pegasus XL's launch with the IRIS satellite off the coast of California at 7:27 p.m. PDT Thursday. Credit: NASA TV/Spaceflight Now

The Interface Region Imaging Spectrograph spacecraft, or IRIS, is embarking on a $181 million mission to peer into the mysterious region around the sun where the temperatures dramatically escalate from the surface to the tumultuous atmosphere.

"IRIS will fill crucial gaps in our understanding over the role the interface region plays in powering its dynamic million-degree atmosphere, called the corona," said Jeffrey Newmark, IRIS program scientist at NASA Headquarters.

Fitted with a 20-centimeter ultraviolet telescope and a multi-channel imaging spectrograph, IRIS will scan across the sun to construct data over a range of heights, temperatures and densities in the solar atmosphere.

"What we want to discover is what the basic physical processes are that transfer energy and material from the surface of the sun to the outer atmosphere of the sun -- the corona," said Alan Title, IRIS principal investigator at Lockheed Martin's Advanced Technology Center.

"The visible surface, the place where virtually all of the light that leaves the sun leaves from, its about 10,000 degrees Fahrenheit. Immediately above that the temperature rises to the million-degree corna. How that happens is a mystery. What are the processes that occur there?"

The data is considered a key ingredient in helping to understand the solar wind and coronal mass ejections that erupt from the sun and can impact the Earth, upsetting power grids, upsetting communications and navigation signals, and be harmful astronauts and satellites.

An Orbital Sciences Pegasus XL rocket, making its 42nd flight since 1990 and the last one currently scheduled, departed Vandenberg Air Force Base at 7:30 p.m. local time hooked to the belly of an L-1011 carrier jet under the controls of Don Walter.

Flying a pre-determined "race track pattern" northward over the Pacific, the aircraft flew by the drop zone to measure weather conditions while the launch team conducted countdown testing.

Making a U-turn offshore from Monterey, the plane dubbed "Stargazer" then headed back southward as the countdown neared drop time.

Achieving the precise heading and receiving a final "go" from the ground-based launch officials, co-pilot Ebb Harris pushed a button on the center console of the cockpit that opened hooks holding the rocket and Pegasus was away cleanly.

The 51,000-pound, 55-foot-long rocket free-fell for five seconds, dropping 300 feet below the aircraft while traveling at Mach 0.82. During the plunge, the onboard flight computer sensed the rocket's separation from the carrier jet and issued a command to release the safety inhibits in preparation for ignition.

At 7:27 p.m. local time, the first stage lit to power the 403-pound IRIS into a sun-synchronous polar orbit around Earth.

About 13 minutes after launch, the satellite was cast free from the spent third stage of Pegasus into a slightly elliptical orbit of 420 by 385 miles, as planned, at an inclination of 98 degrees to the equator, circling the planet every 97 minutes from pole to pole.

The orbit flies along the dawn-dusk line, provides eight months of continuous observations per year and maximizes eclipse-free viewing of the sun, officials said.

Photo of the IRIS spacecraft undergoing assembly and testing in a clean room. Credit: Lockheed Martin

In about 21 days, the telescope's door will be opened and the commissioning process should be completed to start the science mission in about 60 days.

"IRIS is about a factor of 10 higher resolution than any other instrument that has explored this region and, even more importantly, it's about a factor of 20 faster. So it can take images about once a second. This is critical because the processes that occur in this part of the atmosphere happen very, very fast," Title said.

The mission's goal is observing how solar material moves, gets heated and energized through this unexplored region around the sun.

"Previous observations suggest there are structures in this region of the solar atmosphere 100 to 150 miles wide, but 100,000 miles long," Title said. "Imagine giant jets like huge fountains that have a footprint the size of Los Angeles and are long enough and fast enough to circle Earth in 20 seconds. IRIS will provide our first high-resolution views of these structures along with information about their velocity, temperature and density."

At just 7 feet long and 4 feet in diameter at its rear, the satellite grew to 12 feet in width from tip-to-tip shortly after launch when the power-generating solar arrays were unfolded.

"IRIS is small, light-weight, low-power satellite designed to perform complex solar observations," said Gary Kushner, IRIS program manager at Lockheed Martin's Advanced Technology Center.

With no onboard fuel or consumables, designers expect IRIS to long surpass its two-year mission life, perhaps operating for a couple of decades, said John Marmie, IRIS assistant project manager at NASA's Ames Research Center.

The mission operations strategy calls for a morning science team meeting to make requests to flight controllers who write commands that are uplinked to the spacecraft to execute. One command load will be sent up IRIS every weekday and the resulting science data made available online for scientists and general public within hours.

"We have preplanned a large number of observing sequences that are targeted to seeing things like solar flares," Title said.

IRIS will work in concert with NASA's Solar Dynamics Observatory that monitors the sun's surface and the joint Japanese-U.S. Hinode spacecraft studying the sun's outer atmosphere.

"For the first time we will have the necessary observations for understanding how energy is delivered to the million-degree outer solar corona and how the base of the solar wind is driven," Newmark said.