To the naked eye, the sun appears to be a benign, peaceful star that provides the energy for life. But the sun is really a violent body spitting out streams of hot, electrically charged atomic particles that can disrupt life on earth. The U.S. space agency NASA is launching a spacecraft to investigate this phenomenon, known as solar flares. The craft will enter [has entered] Earth orbit after being launched from a small rocket released from the wing of an airplane over the Atlantic Ocean.
Imagine an unbelievably powerful explosion that shuts down electrical generating systems over a wide territory. Is this the scenario for a terrorist attack? No, just our sun acting up again.
The sun's atmosphere periodically emits gigantic explosions called flares. "These are the largest explosions in the solar system," says Robert Lin, a solar physicist at the University of California at Berkeley. He says the most powerful solar flares can throw tons of charged atomic particles at us at nearly the speed of light and wreak havoc on communications and electrical power systems. "When these flares first go off, the x-rays and extreme ultraviolet radiation that come from the sun ionize the upper atmosphere of the Earth and change the conditions for radio propagation. So you can get problems with communication," he says. "Related to flares are coronal mass ejections where the sun throws off a lot of material. These produce magnetic storms on the Earth that can affect spacecraft, can even affect the power-carrying grid on the surface of the Earth."
This is more than just a theoretical possibility. In March 1989, for example, a massive solar explosion overwhelmed electrical circuits in Quebec, Canada, blacking out the province for nine hours.
Previous observations indicate that solar flares occur when energy stored within solar magnetic fields is suddenly released. The magnetic fields twist, snap, and recombine, heating solar gas to tens of millions of degrees. This causes the solar atmosphere, or corona, to sizzle with high-energy x-rays and gamma rays and eject proton and electron particles out toward the planets.
But knowing what occurs is not enough. Robert Lin says scientists want to know what triggers the energy release. "Our main objective is to understand how all the energy can be released from the sun in such a short time and how it can accelerate those particles to such high energies," he says.
To understand what causes solar flares, scientists must identify the different kinds of particles the sun spews out, locate the regions where this occurs, and determine when the particles get accelerated.
That is where the NASA spacecraft called HESSI comes in. HESSI is the English acronym for High Energy Solar Spectroscopic Imager. Mr. Lin, the principal investigator for it, says the craft will trace the origin and movement of the particles. "HESSI is going to look at these particles by looking at the x-rays and gamma rays that these particles produce," he says. "These are the very highest energy light that one can see."
Although several U.S., European, and Japanese satellites are observing atmospheric process on the sun, HESSI will be the first to trace the atomic particles from their origin. NASA solar physicist Brian Dennis says the spacecraft will do it with high-resolution color motion pictures. "The one thing that's always impressed me about HESSI is that when I go to a hospital and have an x-ray picture taken of me, it's always in black and white," he says. "I never understood why they don't make color x-ray pictures of people. But in fact HESSI will make color x-ray pictures of solar flares."
NASA says that HESSI, working with the other solar observatories, will provide scientists with vital insight into the sun's impulsive energy release.