The Great Conveyor Belt is a massive circulating current of hot plasma within the Sun. It has two branches, north and south, each taking about 40 years to perform one complete circuit. Researchers think the turning of the belt controls the sunspot cycle, which is why a slowdown is important.

"Normally, the conveyor belt moves about 1 meter per second � walking pace," Hathaway explained. "That's how it has been since the late 19th century." In recent years, however, the belt has decelerated to 0.75 meters per second s in the north and 0.35 meters per second s in the south. "We've never seen speeds so low," he added.

Observations and theoretical research have shown that the speed of the conveyor belt foretells the intensity of sunspot activity as far ahead as 20 years in the future. A slow belt means lower solar activity, while a fast belt means stronger activity.

"The slowdown we see now means that Solar Cycle 25, peaking around the year 2022, could be one of the weakest in centuries," Hathaway said.

If true, this could be mixed news for astronauts. Solar Cycle 25 is when activities related to NASA's Vision for Space Exploration could be peaking, with astronauts back on the Moon and preparing to go to Mars. A weak solar cycle means they will not have to worry as much about solar flares and radiation storms.

On the other hand, they will have to worry more about cosmic rays - high-energy particles from deep space that can penetrate metal, plastic, flesh and bone. Astronauts exposed to cosmic rays develop an increased risk of cancer, cataracts and other maladies. Ironic, but solar flares, which produce their own deadly radiation, sweep away the even deadlier cosmic rays. As flares subside, cosmic rays intensify.

Hathaway's prediction differs somewhat from another recent forecast. A team led by physicist Mausumi Dikpata of the National Center for Atmospheric Research in Boulder, Colo., has predicted that Cycle 24, peaking in 2011 or 2012, will be intense.

Hathaway agrees. "Cycle 24 will be strong," he said. "Cycle 25 will be weak. Both of these predictions are based on the observed behavior of the conveyor belt."

Both researchers based their predictions on sun spot data. Sunspots are magnetic knots that bubble up from the base of the conveyor belt, eventually popping through the surface of the sun.

Astronomers have long known that sunspots have a tendency to drift�from mid solar latitudes toward the sun's equator. According to current thinking, this drift is caused by the motion of the conveyor belt. "By measuring the drift of sunspot groups," Hathaway said, "we indirectly measure the speed of the belt."

Using historical sunspot records, Hathaway has clocked the conveyor belt as far back as 1890. The results: For more than a century, "the speed of the belt has been a good predictor of future solar activity."

If the trend holds, he said, Solar Cycle 25 in 2022 could be "off the bottom of the charts."

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