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Last Updated on Wednesday, 05 May 2010 20:08
Published on Thursday, 24 February 2005 00:00

A small NASA spacecraft, dedicated solely to the study of high energy processes in gigantic explosions in the atmosphere of the Sun, is scheduled to be launched early next week.  Although the 293 kg (645 lb) satellite, known as the High Energy Solar Spectroscopic Imager (HESSI), carries only one scientific instrument, it will make a major contribution to studies of our nearest star.


HESSI will capture the first pictures and high resolution spectra of high-energy  ("hard") X-ray and gamma- ray emissions from solar flares - the signatures of  electrons and ions accelerated during the flare process. Sequences of such  images will show in unprecedented detail what happens at high energies when  flares erupt from the surface of the Sun.

By analysing HESSI's images and spectra, scientists hope to discover more  precisely where flares are born, what triggers them, and how they generate  such huge power in the form of fast particles as well as heat and bulk motion.

Previous observations indicate that the explosions are created when energy  stored in intense solar magnetic fields is suddenly released. As a result, large  amounts of thermal radiation, including lower energy "soft" X-rays, are created,  as the gases in the Sun's corona (outer atmosphere) are heated to tens of  millions of degrees.

At the same time, the electrons and ions in the searingly hot gas (known to  physicists as "plasma") are accelerated, resulting in the production of other  particles (neutrons and pions), hard X-rays and gamma-rays.  

HESSI will be the first spacecraft to simultaneously trace the different kinds of  particles being accelerated, locate the regions where the acceleration occurs  and determine when the particles get accelerated. The most direct tracers of  these accelerated particles will be the X-ray and gamma-ray emissions that they  produce as they travel through the solar atmosphere.

Among those looking forward to studying and analysing the images and spectral  data from the spacecraft is the Astronomer Royal for Scotland, Professor John  Brown of Glasgow University, where the tradition of solar activity studies dates  back to Alexander Wilson (1760).

Professor Brown and his team are the sole UK HESSI Co-Investigators. They  will be analysing the data using image/spectral processing codes and models  of particle acceleration and propagation which they have developed, in order  to improve current theories about how our violent Sun works, particularly at high  energies.

"HESSI will help us to understand the ultra-high temperature and non-thermal  processes taking place in the Sun and the Universe as a whole," he said. "It is

important to understand how such high temperatures (possibly up to one billion  degrees), and accelerated particle beam energies and fluxes, are generated in  the Sun's outer atmosphere."

 "It is also important to understand how high energy particles are created and accelerated to near light speed – about 300,000 km per second," he added.  "As well as being a key problem in plasma astrophysics, such particles and  their radiation pose a hazard to personnel in space and high altitude aircraft,  and have been implicated in upper atmospheric chemistry which may affect  climate."

"The Sun is a perfect natural laboratory for exploring plasmas and high energy  processes," he said. "Because of their large scale and the relative proximity of the Sun, solar flares enable us to study energy release in magnetised plasmas  that cannot be studied in such detail anywhere else. Indeed, the Universe is  teeming with accelerated particles from explosive events such as supernovae,  and the Sun gives us a nearby test bed to study these particles."

"The timing of this mission is particularly significant since the Sun has just passed the peak in its 11-year cycle of activity, when the largest explosive events are  expected to be most frequent," said Brown.

NOTES FOR EDITORS.  The HESSI mission is scheduled for launch on 5 February, some 18 months later  than originally planned. The satellite had to be rebuilt after it was severely damaged  during vibration testing in March 2000. The launch was rescheduled for March 2001,  but problems with the Pegasus launcher pushed it back to February 2002.

The rocket and satellite will be launched from beneath a Stargazer L-1011 aircraft flying over the Atlantic Ocean. After being dropped from the aircraft, the engine of  the Pegasus rocket will fire to deliver the spacecraft to a circular orbit 600 km  (373 miles) above the Earth, inclined at 38 degrees to the equator.

Solar flares occur when magnetic fields twist, snap and recombine, accelerating  particles to near light speed (about one billion kilometres per hour), and heating  gas in the corona – the Sun’s outer atmosphere which appears as a halo of light  during a total solar eclipse - to tens of millions of degrees. As a result, the solar atmosphere teems with high energy X-rays (20-100 keV) and gamma-rays (0.51  MeV and above), while accelerated proton and electron particles zoom into the  Solar System, sometimes impacting Earth's upper atmosphere. Radiation and  particles from solar flares sometimes affect orbiting spacecraft, interfering with  communications and threatening astronauts.

HESSI has the finest angular and spectral resolution of any hard X-ray or  gamma-ray instrument ever flown in space. Powerful X-rays and gamma-rays  penetrate all materials, to some extent, and cannot be easily focused, so  researchers are using another technique to form images. HESSI's sole instrument  - an imaging spectrometer - will construct a flare image from patterns of light and  shadows produced by high-energy radiation that passes through the telescope's  grids while the spacecraft rotates. Using this new method, HESSI is expected to  gather data on hundreds of flares during its two-to three-year mission.

By working together with ground-based flare observatories and with other solar  spacecraft - the Solar and Heliospheric Observatory (SOHO), Geostationary  Operational Environmental Satellites (GOES) and the Transitional Regional and  Coronal Explorer (TRACE) for flare radiation, and Wind, the Advanced Composition  Explorer (ACE), Ulysses, and Voyager for particle detection - HESSI will provide key  insights into the processes of energy release and particle acceleration at the Sun.

HESSI is the sixth Small Explorer spacecraft built by NASA. The HESSI mission  costs about 85 million dollars, which includes the spacecraft, launch vehicle, mission operations and data analysis.

Principal Investigator for the mission is Robert Lin of the Space Sciences  Laboratory at the University of California, Berkeley.  ( Tel: +1510-642-1149, e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it. )


Professor John Brown,  Astronomer Royal for Scotland,  Dept. of Physics and Astronomy,
University of Glasgow,  Glasgow,  G12 8QW  Tel: +44 (0)141-330-5182
Fax: +44 (0)141-330-3183  E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
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