Even during the lowest ebb of solar activity, you can usually find one or two spots on the sun. But when Hathaway looked on Jan. 28, 2004, there were none. The sun was utterly blank.

It happened again last week, twice, on Oct. 11th and 12th. There were no sunspots.

"This is a sign," says Hathaway, "that the solar minimum is coming, and it's coming sooner than we expected."

Right: The blank sun on Oct. 11, 2004, photographed by the ESA/NASA Solar and Heliospheric Observatory.

Solar minimum and solar maximum--"Solar Min" and "Solar Max" for short--are two extremes of the sun's 11-year activity cycle. At maximum, the sun is peppered with spots, solar flares erupt, and the sun hurls billion-ton clouds of electrified gas toward Earth. It's a good time for sky watchers who enjoy auroras, but not so good for astronauts who have to be wary of radiation storms. Power outages, zapped satellites, malfunctioning GPS receivers--these are just a few of the things that can happen during Solar Max.

Solar minimum is different. Sunspots are fewer--sometimes days or weeks go by without a spot. Solar flares subside. It's a safer time to travel through space, and a less interesting time to watch polar skies.

Hathaway is an expert forecaster of the solar cycle. He keeps track of sunspot numbers (the best known indicator of solar activity) and predicts years in advance when the next peaks and valleys will come. It's not easy:

"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.

Above: Astronomers have been counting sunspots for centuries. This plot shows sunspot numbers from 1610 to 2000. Data are also available for the current cycle (1996-2004): click here.

But researchers are making progress. Hathaway and colleague Bob Wilson, both working at NASA's Marshall Space Flight Center, 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.

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.

Right: Robot moonship: an artist's concept. Credit: Pat Rawlings. [More]

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.

Info Taken from NASA