HESSI will be carried aloft inside a Pegasus XL rocket under the belly of Orbital Science Corporation's Stargazer L-1011 aircraft. The L-1011 is scheduled to lift off from Cape Canaveral Air Force Station, Fla., at 3:21 p.m. EST. After the aircraft is about 40,000 feet over the Atlantic Ocean, it will drop the Pegasus rocket. Following a free fall and a series of short rocket motor burns, the rocket will deliver HESSI to its 373-mile (600-kilometer) circular orbit above the Earth, inclined at 38 degrees to the equator.

In order to understand what triggers a solar flare and how it explosively releases energy, scientists need to identify the kinds of particles being accelerated, locate the regions where the acceleration occurs, and determine when the particles get accelerated. The most direct tracer of these accelerated particles is the X-ray and gamma ray radiation they produce as they travel through the solar atmosphere.

The spacecraft's sole instrument, an imaging spectrometer, will construct flare images from patterns of light and shadows, that are produced by high-energy radiation as it passes through the instrument's grids while the spacecraft rotates.

Inside solar flare explosions, magnetic fields twist, snap and recombine, blasting particles to almost the speed of light and firing solar gas to tens of millions of degrees. This intense action causes the solar atmosphere to sizzle with high-energy X-rays and gamma rays and drives proton and electron particles into the solar system. Radiation and particles from solar flares can sometimes affect orbiting spacecraft.

HESSI was originally scheduled for launch in July 2000, but was postponed after the satellite suffered damage during vibration testing. Since then, flight delays due to launch vehicle failures have affected the launch date. However, officials have since cleared the way for next Tuesday's scheduled launch.

In order for scientists to understand the physical processes and conditions within flares, they will use the spectrometer aboard HESSI to create images of the gamma rays and highest energy X-rays emitted by each flare. These images will be the first to simultaneously measure the location and energy content of radiation from the flare material and should improve predictability of flare occurrences at the Sun and the subsequent consequences we experience here on Earth.

Working together with several other solar spacecraft such as the Solar and Heliospheric Observatory (SOHO), Geostationary Operational Environmental Satellites (GOES), and Transition Region and Coronal Explorer (TRACE) for flare radiation, as well as Wind, Advanced Composition Explorer (ACE), Ulysses, and Voyager for particle detection, HESSI will provide scientists with vital insight into the impulsive energy release and particle acceleration processes at the Sun.

The HESSI mission costs about $85 million, which includes the spacecraft, launch vehicle, mission operations and data analysis. The Explorers Program Office at NASA Goddard Space Flight Center, Greenbelt, Md., will provide mission management and technical oversight under the auspices of NASA's Office of Space Science in Washington.

For detailed information about HESSI and its science mission, go to:

http://hesperia.gsfc.nasa.gov/hessi

http://hessi.ssl.berkeley.edu/