High anxiety: sudden solar flare highlights space risks.

Measurements of energetic particles from an unusually strong solar flare that pummeled Earth early this year suggest that astronauts traveling or working in space might sometimes need to reach shelter within minutes of a warning.

The Jan. 20 episode was the last and most powerful event in a 6-day series of flares (SN: 2/12/05, p. 109). Although the energy emitted by the final flare came nowhere near to setting a record, the cascade of particles that spewed from the sun took a path to strike Earth with almost unprecedented force and speed. That beam included the highest concentrations ever directly measured of protons packing more than 100 million electron-volts (MeV) of energy, says Richard A. Mewaldt of the California Institute of Technology in Pasadena. Such protons can bore through the human body, wreaking biochemical damage along the way.

The storm of charged particles temporarily knocked out more than a half-dozen satellites. When the ions slammed into Earth's upper atmosphere, they caused short-lived chemical changes and generated a plethora of neutrons.

The hail of neutrons measured by radiation detectors in Antarctica after the Jan. 20 event was the second-strongest such episode registered in the past 50 years, Mewaldt reported last week in New Orleans at a joint meeting of the American Geophysical Union and the Solar Physics Division of the American Astronomical Society. For such neutrons to reach Earth's surface, the flare-derived protons striking the atmosphere must carry more than 500 MeV of energy, Mewaldt noted.

What concerns scientists more than the strength of the Jan. 20 solar flare is the speed at which its charged particles reached Earth. Most proton storms arrive 2 or more hours after satellites visually detect the flare. In January's event, the quickest ions reached our planet about 15 minutes after light from the flare did.

Considering that the protons took a long, curving path along one of the sun's magnetic field lines, rather than the direct route that light travels, the particles must have been moving at more than 43 percent the speed of light, says Mewaldt. Astronauts outside Earth's magnetic field, which provides some protection against radiation, would have had little or no warning before the particle storm arrived, he says.

NASA is now developing standards for the time that it would take astronauts working on the surface of the moon to reach shelter in ease of dangerous solar storms. The current thinking is that lunar explorers should be able to take cover within 1 hour, says Francis A. Cucinotta of NASA'S Johnson Space Center in Houston. Even if there had been astronauts on the moon during January's flare, the first hour's dose of radiation wouldn't have been enough to make them sick, he says.