Something strange happened on the sun last week: all the sunspots
vanished. This is a sign, say scientists, that solar minimum is coming
sooner than expected.

October 18, 2004: Six … long … years.

Solar physicist David Hathaway has been checking the sun every day since
1998, and every day for six years there have been sunspots. Sunspots are
planet-sized "islands" on the surface of the sun. They are dark, cool,
powerfully magnetized, and fleeting: a typical sunspot lasts only a few
days or weeks before it breaks up. As soon as one disappears, however,
another emerges to take its place.

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."

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.

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.

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.