A satellite dedicated solely to the study of solar flares, designed, built
and operated by an international consortium led by scientists at the
University of California, Berkeley, is set for launch on Thursday, June 7,
by the National Aeronautics and Space Administration (NASA).

The High Energy Solar Spectroscopic Imager, or HESSI, will embark on a two-
to three-year mission to look at high-energy X-ray and gamma ray emissions
from solar flares — enormous explosions in the solar atmosphere. Though
various satellites have made X-ray and gamma ray observations of flares,
HESSI will be the first to snap pictures in gamma rays and the highest
energy X-rays.

“With intense flares, we can take X-ray images with very high resolution,
very fast, and create movies of flares lasting from 10 seconds to tens
of minutes,” said Robert P. Lin, professor of physics in the College of
Letters & Science at UC Berkeley and principal investigator for the mission.

Using these images, plus X-ray and gamma-ray spectra with unprecedented
energy resolution, the scientists hope to discover what triggers flares and
how energy stored in the solar magnetic fields is suddenly released to
accelerate particles to very high speeds and to heat the gases in the solar
atmosphere to tens of millions of degrees.

“From these hard X-ray and gamma-ray measurements, we can reconstruct the
energy distribution of the particles and trace back to where everything was
accelerated,” Lin said.

The mission begins near the peak of the sun’s 11-year cycle of activity,
providing an unprecedented opportunity for study of these explosive events.
What scientists learn will give insight into the processes that accelerate
other particles whizzing at nearly light-speed through the universe.

HESSI is the sixth Small Explorer (SMEX) spacecraft scheduled for launch
under NASA’s Explorers Program. Total cost for the mission, including the
spacecraft, launch vehicle and mission operations, is about $85 million.

Solar flares, along with the often associated explosions called coronal
mass ejections, are the solar events that most affect “space weather.” The
intense energy associated with these events — up to the equivalent of a
billion megatons of TNT — and the energetic particles they throw out
impact the Earth’s magnetic field, compressing it and interfering with
radio communications on Earth. Astronauts and cosmonauts aboard the
International Space Station or the Space Shuttle also can receive
dangerous doses of radiation from the high-energy particles.

“Coronal mass ejections sometimes have flares associated with them and
sometimes don’t,” said Brian Dennis, HESSI mission scientist at NASA’s
Goddard Space Flight Center in Greenbelt, Md. “We don’t understand why this
should be or what the relationship is between these two types of events.”

The 645-pound (293 kilograms) HESSI satellite will be launched atop a
Pegasus XL rocket dropped from the belly of an L-1011 aircraft flying out
of Cape Canaveral Air Force Station, Florida. After the plane reaches an
altitude of about 40,000 feet over the Atlantic Ocean, the rocket will be
released to free-fall in a horizontal position for about five seconds
before igniting its first stage motor. The three-stage rocket will place
the spacecraft into a circular orbit about 373 miles (600 kilometers)
above the Earth, inclined at 38 degrees to the equator.


Once in orbit, the satellite comes under UC Berkeley’s control, with
commands uplinked and data downlinked through a 36-foot (11 meters) radio
dish perched in the wooded hills above UC Berkeley. From mission control
in the nearby Space Sciences Laboratory, HESSI mission operators will
monitor the automatic pointing of the satellite toward the sun, deployment
of the four solar panels, and the spin-up of the satellite to about 15
revolutions per minute.

Once power is being supplied from the solar panels to onboard batteries,
the operators will start the onboard cooler to get the germanium detectors
down to 75 Kelvin (324 degrees below zero Fahrenheit or 198 degrees below
zero Celsius). This will take several days, but is necessary before any
flare observations can be made. The nine germanium detectors represent
the largest and most advanced array of germanium detectors ever flown in

Because hard X-rays and gamma rays cannot be focused like visible light —
gamma rays can pass right through the spacecraft — HESSI uses a novel
technique to produce pictures of flares. Pairs of grids, each etched from
dense materials that block gamma rays and hard X-rays, are aligned to
let radiation from only a small portion of the sun pass through to the
detector. As the satellite spins along an axis pointing at the sun, nine
of these grid pairs create a moving light show on the germanium detectors.

The information is sent back to Earth, where computers translate the data
into pictures of the flare every second or so. It is also possible to
obtain the energy spectrum of the hard X-rays and gamma-rays at each
location in the pictures.

Information is stored on the spacecraft and sent to the ground whenever
the satellite passes over Berkeley, about every 90 minutes for about a
third of the day. The HESSI team has enlisted the help of radio antennas
in Europe and the eastern United States to download data during very
active periods when several large flares may fill the onboard memory
before it can be emptied during the Berkeley passes. The team hopes to
detect upwards of a hundred gamma-ray flares, more than a thousand X-ray
flares, and perhaps 10,000 microflares during the two- to three-year
duration of the mission.

Lin and his scientific team will compare the X-ray and gamma-ray images
from HESSI with pictures from other solar observing satellites at different
wavelengths to probe the mechanism of flare formation. The data will be
made freely available online within hours of receipt at the ground station.


Solar flares, the most powerful explosions in the solar system, typically
are associated with sunspots in “active regions” of strong magnetic field
in the solar atmosphere. Sunspots form where the sun’s magnetic field
lines arc out of the surface in bright loops, and flare explosions seem
to emanate from these loops.

One possible explanation for solar flares dates from the 1950s and involves
magnetic reconnection. As the sun’s strong magnetic field lines reach out
into space they sometimes cross or reconnect within the corona or atmosphere
of the sun. In seconds, the short circuit heats the gas to tens of millions
of degrees, and perhaps as high as 100 million Kelvin, accelerating
electrons and protons to speeds approaching the speed of light. The
electrons and protons slamming into gas particles, mostly hydrogen, in the
lower corona and chromosphere produce X-rays and gamma-rays, respectively.

While microflares last for seconds, larger flares may emit X-rays for tens
of minutes and remain visible for hours. The large ones extend for as much
as 100,000 km above the solar surface, nearly 10 times the diameter of the

Most of what scientists know about flares has come from ground-based
observations at visible and radio wavelengths and from instruments aboard
Skylab, the Solar Maximum Mission, the Japanese/U.S. Yohkoh spacecraft and
other spacecraft. X-rays have been recorded from flares for more than 30

HESSI will have the finest angular and spectral resolution of any hard
X-ray or gamma-ray instrument flown in space, providing scientists with
the first high fidelity color movies of flares in their highest energy
emissions. The data will help scientists pinpoint where and how flares


HESSI is the first Small Explorer (SMEX) spacecraft to be managed in a
way that gives the principal investigator — in this case, Robert P. Lin,
director of UC Berkeley’s Space Sciences Laboratory — responsibility
for most aspects of the mission. This includes not only the scientific
instrument but also the spacecraft, integration, all environmental
testing, and operations and data analysis after launch. The Explorers
Program Office at Goddard provides management and technical oversight
for the HESSI mission under the direction of the Office of Space Science
at NASA headquarters in Washington, D.C.

Other mission facts:

* UC Berkeley scientists built the spectrometer with its germanium
detectors and data processing electronics.

* The Paul Scherrer Institut in Switzerland provided the imaging telescope
and optical aspect system.

* Goddard Space Flight Center provided the grids and the cryocooler and
supported the alignment of the imaging telescope.

* Spectrum Astro Inc., of Phoenix, Ariz., provided the spacecraft
electronics, the satellite skeleton (called the spacecraft bus) and
integration support.

* Tecomet, a subsidiary of Thermo Electron, Inc., Waltham, Mass., and van
Beek Consultancy of the Netherlands, supplied the tungsten and molybdenum
imaging grids for the instrument. Tecomet used new microfabrication
techniques to create slits in the grids as narrow as 20 microns — less
than one thousandth of an inch.

* The ORTEC division of PerkinElmer Instruments provided the largest and
most advanced array of germanium detectors ever flown in space. The nine
germanium crystals, one under each pair of grids, were artificially grown
to be pure to over one part in a trillion. They are maintained at a
temperature of -324 degrees Fahrenheit (-198 Celsius) using a new type
of mechanical cooler manufactured by Sunpower, Inc., that has never
before been flown in space.

NOTE: Robert Lin can be reached at (510) 642-1149 or
boblin@ssl.berkeley.edu . Brian Dennis can be reached at the Laboratory
for Astronomy and Solar Physics, Goddard Space Flight Center, (301)286-7983
or at Brian.R.Dennis.1@gsfc.nasa.gov .

Video is available both from NASA and UC Berkeley. The NASA video file
includes HESSI spacecraft animation, footage of spacecraft testing at JPL
and the imager assembly at the Paul Scherrer Institut, plus interviews with
Lin and Dennis. Also included are solar max and solar flare images captured
by the Solar and Heliospheric Observatory (SOHO) spacecraft. UC Berkeley
has video interviews of Lin and fellow scientists Peter Harvey and Manfred

Images of a flare that occurred last year on Bastille Day (7/14/00) are at