NASA’s High Energy Solar Spectroscopic Imager (HESSI)
remains on track for a Feb. 5 launch. HESSI will study solar
flares – gigantic explosions in the atmosphere of the Sun-
with a unique kind of X-ray vision, producing the very first
high-fidelity color movies of solar flares during their
highest energy emissions.

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/