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UC Berkeley/NASA satellite set to launch June 7 on two-year mission to study solar flares
01 June 2001

By Robert Sanders, UC Berkeley Media Relations and
Susan Hendrix, NASA's Goddard Space Flight Center

Berkeley - 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 space.

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 Earth.

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 years.

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 form.


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.