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Flown to an altitude of around 40,000 ft (12,000 meters) by a Lockheed L-1011 carrier aircraft and then dropped, the Pegasus XL rocket is capable of carrying small satellites to orbital altitudes under its own power, resembling the flight of a cruise missile.
When in orbit, the IRIS observatory will observe a very small portion of the sun's lower atmosphere, known as the corona. Unlike NASA's Solar Dynamics Observatory (SDO), which is capable of imaging the whole of the solar disk in high-definition, IRIS will focus in on only one percent of the solar disk, examining a scientifically important region of the lower corona.
The "coronal heating mystery" has fascinated solar physicists for decades and many solar observatories have been tasked to understand why solar corona plasma is over one million Kelvin (Celsius) when the photosphere (colloquially known as the "solar surface") is only thousands of degrees Kelvin. An analogy of this would be the air around a hot light bulb getting hotter than the bulb's surface - physics simply doesn't work that way, hence the "mystery." There is some kind of heating mechanism at work that is somehow transporting energy deep within the sun, passing through the cooler photosphere, and then re-energizing the coronal plasma.
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Some theories exist as to the heating mechanism - the leading mechanism being waves that are transmitted along solar magnetic fields that inject energy into coronal plasma - but to prove that this mechanism (or others like it) exist, we need observatories capable of resolving very small structures deep inside the solar atmosphere.
Building on the efforts of other observatories such as NASA's SDO, the Japanese Transition Region and Coronal Explorer (TRACE) and the joint NASA/ESA Solar and Heliospheric Observatory (SoHO), IRIS will take a very small-scale view of the corona, resolving features traveling through the magnetically-dominated lower atmosphere of the sun to a resolution of 150 miles.
"This region is crucial for understanding how the corona gets so hot," said Joe Davila, IRIS project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "For the first time, we will have the capability to observe it at fundamental physical scale sizes and see details that have previously been hidden."
IRIS is equipped with a camera capable of imaging ultraviolet wavelengths and a spectrometer that will analyze the elements populating the lower corona.
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"By looking at spectra of material in these temperature ranges, we can also diagnose velocity and perhaps density of the material, too," said Bart DePontieu, the IRIS science lead at Lockheed Martin in Palo Alto, Calif.
Understanding why the corona is so hot isn't simply an academic pursuit.
The lower corona is the region of the sun that drives space weather. The magnetically-dominated lower corona often erupts with flares and coronal mass ejections. The solar wind is also rapidly accelerated through the sun's chromosphere and transition region, two layers of particular interest to IRIS.
Space weather can impact our everyday lives on Earth, from communications blackouts to power grid outages, so it's hoped that IRIS may be able to contribute to helping us not only understand the sun's atmosphere, but also add to the growing wealth of knowledge we have for space weather prediction models.
The launch of NASA's IRIS mission onboard its Pegasus XL launcher is scheduled for 10:27 p.m. EDT Thursday, June 27. Live NASA Television launch coverage begins at 9 p.m.
Image: IRIS being prepared in the clean room before launch. Credit: NASA