In 2003, three days before Halloween, one of the most powerful solar flares ever recorded exploded from the sun, hurling billions of tons of electrified particles toward Earth.
The National Oceanic and Atmospheric Administration quickly sounded the alarm. Forecasters warned that a geomagnetic storm with the power to cripple satellites and take down power grids would collide with the planet in 24 hours.
The solar squall showed up - but five hours earlier and with far less punch than predicted. "We were surprised," says Joseph Kunches, chief forecaster at NOAA's Space Environment Center in Boulder, Colo.
The incident is a sobering reminder that space weather forecasting - like its Earthly counterpart - is tricky business. But now a Maryland-built NASA observatory set to launch next week could give space weather forecasters a new edge.
The $520 million Solar Terrestrial Relations Observatory, or STEREO, is designed to capture the first 3-D snapshots of the sun, particularly the explosive events known as coronal mass ejections.
Coronal mass ejections, or CMEs, are the hurricanes of space weather - gigantic bubbles of charged gas that erupt from the sun's outer atmosphere on the heels of solar flares. They sweep through the solar system at lightning speed and can unleash violent geomagnetic storms on Earth.
While not as widely known as flares, CMEs are potentially more destructive. "Some of them are real whoppers," says Michael Kaiser of NASA's Goddard Space Flight Center in Greenbelt, which will oversee STEREO's two-year mission. One major concern is radiation. During one solar squall in January 2005, astronauts aboard the International Space Station were ordered to take cover from approaching blasts of solar radiation in the better-shielded Russian Zvezda service module.
Airlines, meanwhile, were forced to reroute flights away from the poles, where more potentially harmful radiation seeps through from space. United Airlines alone had to reroute 26 polar flights to lower latitudes.
But the impact of most space squalls has been economic.
The charged particles swept along within CMEs can cripple a satellite's sensitive electronics. A 1997 solar eruption, for example, is suspected of knocking out the television satellite Telstar 401. Japanese controllers lost contact with their $640 million ADEOS-2 climate satellite after a series of violent solar storms near Halloween 2003.
By heating the outer atmosphere with ultraviolet radiation, solar eruptions create drag that can rip low-flying satellites from orbit. A storm-swollen atmosphere, for example, is blamed for bringing down NASA's first space station, the 85-ton Skylab, in 1979.
Back on the ground, coronal mass ejections can short out power grids. A 1989 storm plunged 6 million people into darkness in Quebec, Canada, for nine hours.
(Not all the news about solar storms is gloomy: They are also responsible for the shimmering, otherworldly beauty of the aurora borealis, or northern lights.)
Scientists are just beginning to unravel the intricacies of space weather. "We're probably back where Earth weather forecasters were in the 1950s," says Goddard's Kaiser.
While much about CMEs remain a mystery, scientists know the secret to the sun's explosive outbursts boils down to a single force: magnetism.
Like Earth, the sun is surrounded by a magnetic field, whose lines resemble that of a bar magnet. But that's where the similarities end.
The sun is a swirling ball of scorching gas. These gases rotate faster at the equator than they do at higher latitudes, twisting the magnetic field lines "almost like rubber bands," says Craig DeForest, a solar physicist at the Southwest Research Institute in Boulder, Colo.
As tension builds, the magnetic field eventually bursts through the sun's surface. The result: sunspots, which are darker, heavily magnetized regions of cooler gasses.
When magnetic fields of opposite polarity intersect, they can trigger violent explosions. Scientists estimate that these explosions unleash as much energy as 10 billion hydrogen bombs.
One barrier to better space weather forecasts is the lack of space hardware designed to track the sun's behavior.
"It's like measuring ocean currents with only a few buoys. It can't be done," says Madhulika Guhathakurta, a NASA astrophysicist. "We're in the same situation."
Today, most detailed measurements of CMEs come from the 11-year-old Solar and Heliospheric Observatory, or SOHO. Circling a million miles sunward of Earth, SOHO has one major weakness: its two-dimensional camera can't determine whether a coronal mass ejection is headed toward Earth or away from it.
The spacecraft is also unable to measure the size of a coronal mass ejection. "We plain don't know if its going to be the mother of all magnetic storms or just a little punch in the chin," says NOAA's Kunches. "That's a huge challenge."
STEREO could change that by offering scientists their first insight into how a CME evolves as it plows through interplanetary space toward Earth.