Update, 7:20 A.M. ET, 06 November 2003: NOAA's Space Environment Center (SEC) has classified this flare as an X28, making it in fact the strongest ever recorded. A source told SPACE.com that the SEC is aware other scientists still think the flare was even stronger. The article below remains as it originally appeared. - RRB

A flare released by the Sun on Tuesday could be the most powerful ever witnessed, a monster X-ray eruption twice as strong as anything detected since satellites were capable of spotting them starting in the mid-1970s

The strongest flares on record, in 1989 and 2001, were rated at X20. This one is at least that powerful, scientists say. But because it saturated the X-ray detector aboard NOAA's GOES satellite that monitors the Sun, a full analysis has not been done.

The satellite was blinded for 11 minutes.

Craig DeForest, a solar physicist at the Southwest Research Institute, said others in his field are discussing the possibility that Tuesday's flare was an X40.

"I'd take a stand and say it appears to be about X40 based on extrapolation of the X-ray flux into the saturated period," DeForest told SPACE.com.

That estimate may even be conservative, he said.

The flare leapt from a sunspot that is rotating off the visible face of the Sun, so its effects were not directed squarely at Earth. Nonetheless, a radio blackout occurred at many wavelengths as the storm's initial radiation arrived just minutes after the eruption. Radio blackouts are ranked from R1 to R5 by NOAA's Space Environment Center, the space counterpart to the National Weather Service.

"This is an R-5 extreme event," said SEC forecaster Bill Murtagh. "They don't get much bigger than this."

Paal Brekke, deputy project scientist for the SOHO spacecraft, which monitors the Sun, also told SPACE.com the outburst could be as strong as X20 "or much higher."

At least X20

The SEC is still evaluating the flare's ranking. For now, they are calling it an X20+, indicating that it is indeed the most powerful on record. The only known event that might outrank it is an 1859 solar storm that zapped telegraph lines in an era when solar monitoring could not provide an evaluation of a flare's strength.

The radiation flare was accompanied by a coronal mass ejection (CME), an expanding cloud of charged particles -- actual matter that moves at supersonic speeds but not as fast as light. Had this CME been aimed at Earth, scientists would have feared a potential space storm unlike anything seen in the Space Age.

As it is, the expanding cloud is expected to provide a glancing blow sometime Thursday.

The storm, if it arrives, will not likely be major, forecasters say. But as with all space weather, satellites and communication systems will be at risk of disruption or damage. Colorful sky lights called auroras may be active at high latitudes and possibly into northern U.S. states and Europe.

More to come?

Tuesday's flare was generated by Sunspot 486, which is about 15 times the size of Earth.

Sunspots are dark, cooler regions of the solar surface, areas of pent-up magnetic activity. They're a bit like caps on a shaken soda bottle, and upwelling matter and energy can blow at any moment. Scientists cannot predict when a flare will occur.

During the past two weeks, number 486 and two other large sunspots set off nine other major flares. It was one of the stormiest periods of activity ever witnessed, all experts agree. The number of intense flares, some shooting out within a day of another, was unprecedented. Auroras were seen as far south as Texas and Florida.

The second strongest flare in this historic two-week series was an X17 event on Oct. 28. It was aimed at Earth and generated severe geomagnetic storming when it blew past the planet less than 24 hours later.

A period of relative calm is now expected on the solar surface. But another round is possible.

The Sun spins once on its axis once every 25 days at its equator, carrying sunspots around. Sunspots can last days or weeks. Any of the three that have rotated off the right side of the Sun could return in about two weeks on the left side and, possibly, send more major storms toward Earth.