Despite being the closest star to Earth, the sun still has its secrets. What drives the powerful eruptions of gas known as coronal mass ejections? How does the sun regulate Earth's climate? Why are the upper layers of the sun's atmosphere hotter than those next to the surface? Last week's successful launch of the IRIS (Interface Region Imaging Spectrograph) spacecraft will help fill in the missing pieces.

On June 28, at 7:27 PDT, IRIS hitched a ride on a L-1011 cargo airplane from California’s Vandenberg Air Force Base. 100 miles northwest and 39,000 feet over the Pacific Ocean, a Pegasus rocket carrying the spacecraft launched from the belly of the plane. IRIS successfully achieved Earth orbit thirteen minutes later. From an altitude of 280 miles, the spacecraft will spend the next two years studying a little-understood part of the sun called the interface region, which stretches a few thousand miles above the sun's surface to just below the solar corona, the tenuous and very hot gas that surrounds our star.

About the size of a Mini Cooper, and weighing 440 lbs, the satellite carries an ultraviolet spectrometer: a device similar to a prism that breaks incoming light up into its component frequencies or colors. The spectrometer will let astronomers measure the temperature in the interface region once every few seconds. The high frequency of image collection, coupled with the ability to see features as small as 150 miles across, will provide astronomers with unprecedented detail about the sun's atmosphere.

Astronomers still don't understand why the temperature of the sun's atmosphere is backwards. Rather than cooling off at higher altitudes, the gas around the sun climbs from 6000 degrees near the surface to millions of degrees in the corona. Because the interface region is where material and energy gets shuttled to the corona from the surface, IRIS may be able to help solve that mystery. The interface region might also hold clues about what drives solar flares. The most powerful of these eruptions can knock out power grids and communication networks. If space physicists could predict the next flare before it blacks out Quebec (again), engineers might be able to protect critical infrastructure with a controlled shutdown.

IRIS is the twelfth spacecraft in the NASA Small Mission Explorer (SMEX) program. Started in 1998, SMEX uses small, low-cost missions to further our understanding of the universe.