Solar Cycle 2004

"Contrary to popular belief," says Hathaway, "the solar cycle is not precisely 11 years long." Its length, measured from minimum to minimum, varies: "The shortest cycles are 9 years, and the longest ones are about 14 years." What makes a cycle long or short? Researchers aren't sure. "We won't even know if the current cycle is long or short - until it's over," he says.

But researchers are making progress. Hathaway and colleague Bob Wilson, both working at NASA's Marshall Space Flight Centre, believe they've found a simple way to predict the date of the next solar minimum. "We examined data from the last 8 solar cycles and discovered that Solar Min follows the first spotless day after Solar Max by 34 months," explains Hathaway.

The most recent solar maximum was in late 2000. The first spotless day after that was Jan 28, 2004. So, using Hathaway and Wilson's simple rule, solar minimum should arrive in late 2006. That's about a year earlier than previously thought.

Credit Pat Rawlings Robot moonship: an artist's concept.

The next solar maximum might come early, too, says Hathaway. "Solar activity intensifies rapidly after solar minimum. In recent cycles, Solar Max has followed Solar Min by just 4 years." Do the math: 2006 + 4 years = 2010.

By that time, according to NASA's new vision for space exploration, robot ships will be heading for the moon in advance of human explorers. If Hathaway and Wilson's prediction is correct, those robots will need good shields. Solar flares and radiation storms can damage silicon brains and electronic guts almost as badly as their organic counterparts.

For now, says Hathaway, we're about to experience "the calm before the storm." And although he's a fan of solar activity - what solar physicist isn't? - he's looking forward to the lull. "It'll give us a chance to see if our 'spotless sun' method for predicting solar minimum really works."

Solar Max will be back soon enough.