Final pre-launch tests are underway on a NASA Small Explorer mission that is expected to obtain ultraviolet spectra and images of the Sun's photosphere and corona with an unusual resolution, just 0.33 arcsec., helping physicists better understand how the Sun transports energy and, perhaps, helping advance fusion energy research.

After finishing the integration of the science instrument package for the Interface Regional Imaging Spectrograph (IRIS) satellite, Lockheed Martin Space Systems has begun the spacecraft's thermal vacuum testing. IRIS Program Manager Gary Kushner says the spacecraft is on schedule for a March 1 shipment to Vandenberg AFB, Calif., for an April 28 launch .

Launch will be by an Orbital Sciences Corp. Pegasus XL booster, air-dropped from an L-1011, which is to place the satellite into a 600 km (373 mi.) sunsynchronous orbit inclined 6 deg.

IRIS's $170 million budget assumes a nominal two-year mission, but there is every likelihood the spacecraft's 20-cm (8-in.) UV telescope and spectrograph can operate much longer, given an accurate orbital placement along the solar sunrise orbital line, says physicist and Principal Investigator Alan Title, a senior fellow at the Lockheed Martin Advanced Technology Center in Palo Alto, Calif. He is a veteran of the 1998 Transition Region and Coronal Explorer (Trace) mission, another study of the photosphere launched on a Pegasus.

Like Trace, IRIS has no station-keeping propellant system, so mission longevity depends on accurate orbital placement. Trace's placement was so good that it lasted until 2010's Solar Dynamics Observatory (SDO) launch.

The Pegasus XL uses the same fairing design as Orbital's Taurus XL launcher, which NASA is investigating after two failures (AW&ST Dec. 17, 2012, p. 24). NASA says there is no hold on the IRIS launch due to the inquiry.

IRIS is a 140-kg. (300-lb.) disk-shape spacecraft with a power rating of 200 watts. Protruding from the disk is the 3-meter-long (9.8-ft.) UV telescope built by the Smithsonian Astrophysical Observatory. Lockheed Martin's multichannel imaging spectrograph, designed with Montana State University, will observe in the extreme ultraviolet, between 1,200-3,000 angstroms, far higher than previous missions. The instrument's mirrors have a quality better than the Hubble Space Telescope's, Title says.

The mission slipped from 2011 for an unusual reason. IRIS uses the same momentum wheels, from Goodrich Aerospace, as NASA's 2011 Grail (Gravity Recovery and Interior Laboratory) mission. Grail's wheels have performed without incident. But when pushed to extremes in IRIS testing, the wheels blew a fuse. An investigation revealed that their logic control can draw excess current if they are accelerated too quickly. Lockheed Martin verified that if they are started “gently,” they will operate nominally, Title says. The issue mainly pertains to an unanticipated spacecraft shutdown and restart, he notes.

The dynamics of solar energy have been studied for decades, but the interface between the Sun's photosphere and corona remain a challenge in solar and heliospheric science. Where past instruments, such as the Atmospheric Imaging Assembly (AIA) on the SDO, view the solar surface and atmosphere broadly (2,000 arcsec.), IRIS must take a narrow view of just 50 arcsec, as if it were a P-3 Hurricane Hunter flying into a hurricane's eye instead of a weather satellite viewing the whole storm, as AIA does.

The big leap that distinguishes IRIS is advances in supercomputing. “We've finally reached the state where we can interpret the data that [IRIS's spectrograph] will capture,” Title says. “The region of the Sun that we're looking at is where temperatures go from 6,000K to several million degrees. There's a lot of complex physics and until quite recently the data were hard to interpret.”

The spacecraft will transmit enormous data sets—0.7 Mbps.—in X-band to Norway's Svalbard ground station. Number-crunching, chiefly by NASA Ames Research Center's Pleiades supercomputer, is so important to the mission's success that “Ames has had a presence from the beginning of the design,” Title says.

Understanding the energy transfers that take place in the corona may play a role for energy projects on Earth. The Sun is an excellent laboratory for understanding the electro-mechanical processes necessary to make fusion reactors practical, and for that reason IRIS's science collaborators include physicists at the Princeton University Plasma Physics Laboratory.