Michael Mecham San Francisco

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.