If astronauts had been on the Moon in early August, 1972, they would have been at best terribly queasy, and at worst — dead. A burst of charged particles that suddenly erupted from the Sun would have caused severe radiation sickness, dosing any unlucky Moonwalkers with about 400 rem — almost 50,000 times the radiation dose you get from a chest X-ray. Fortunately, the burst, known as a coronal mass ejection (CME), erupted during the months between Apollo 16 and Apollo 17, so no astronauts were harmed.

But as NASA looks to return astronauts to the Moon and send them beyond to Mars, the health hazards posed by these solar explosions are a serious concern. CMEs hurl billions of tons of charged particles into space at hundreds of kilometers per second (millions of miles per hour). The extremely energetic clouds pose little danger to us, since Earth's magnetosphere channels most of the particles to the North and South poles, producing aurora. But astronauts outside Earth's protective magnetosphere and atmosphere would have to shield themselves from the radiation, a problem complicated by the fact that no one knows when CMEs will strike.

"Currently, there is no model that predicts real CMEs," says Janet Luhmann (University of California, Berkeley), who models the effects of space weather. "There are models for what makes the corona erupt, but the CMEs in them are in effect physics cartoons."

Now, a team led by Zoran Mikic (Science Applications International Corporation, San Diego) has developed a model that can accurately predict the behavior of the corona, the Sun's hot outer atmosphere. At the June 26th meeting of the American Astronomical Society's Solar Physics Division, Mikic and his colleagues reported that their model had accurately predicted the structure of the corona during the March 29th solar eclipse. And though the model still cannot predict CMEs, researchers think that predicting the corona's structure is the first step.

"It's a real breakthrough," says Luhmann, who was not involved in the study. "I do not know of another coronal model with this level of realism. From now on, the corona can routinely be described more accurately — which is just what we need to go to the next step of modeling CMEs."

Currently, sunspots are the best clues that CMEs might be coming. These dark splotches are regions where magnetic-field lines close over the corona like restraining ropes. When the lines snap to new shapes, the region of suddenly freed coronal gas erupts. But CMEs sometimes unpredictably erupt nowhere near sunspots, when solar filaments flare up and then collapse back onto the corona.

Researchers have been trying to model the corona for decades, but no models have been able to take into account how energy is transferred within the atmosphere. The new model allows researchers to map pressure and density changes, and to predict the large-scale magnetic structure of the corona. Such maps are the first step toward predicting how magnetic-field lines will behave and when potentially dangerous CMEs will erupt.

"We must know the background corona to make accurate space weather predictions," says Joseph Davila (NASA/Goddard Space Flight Center), executive director of the International Heliophysical Year Secretariat. "The Mikic model is a big step in developing this background model."