DENVER -- A strong geomagnetic storm was expected to hit Earth on Friday with the potential to affect electrical grids and satellite communications.

SOHO coronagraphs captured this movie of a coronal mass ejection (CME) heading toward Earth Wednesday. NOAA forecasters expect the CME to cause a geomagnetic storm when it reaches Earth Friday.

"People that have cell phones or TV connections or anything with a satellite hookup could see problems from these storms," said Larry Combs, a forecaster for Space Weather in Boulder, Colo.

One of the largest sunspot clusters in years developed over the past three days and produced a coronal mass ejection, similar to a solar flare, at 1 a.m. Wednesday, forecasters at the National Oceanic and Atmospheric Administration in Boulder, Colo., said.

The disturbance was expected to produce a geomagnetic storm rated G3. A G5 storm is the strongest.

The storm could make the aurora visible as far south as Oregon and Illinois.

A coronal mass ejection is an explosion of gas and charged particles into space from the corona, the outermost layer of the sun's atmosphere.

Solar storms can damage to satellites, power grids and pipelines, as well as other electrical disturbances that can affect cable TV, pager and cellular telephone service.

Satellites in orbit high above the Earth's protective atmosphere are particularly susceptible to solar radiation. High-energy electrons from the storm can penetrate spacecraft, zapping the electronics and turning data bits from "0" to "1" or vice versa, causing the satellites to go into unexpected modes and maneuvers.

The list of major satellites knocked out by solar storms is long and costly. A recent example is the $200 million AT&T Telstar 401 satellite that experienced a massive power failure in 1997 only days after a solar storm arrived at Earth. Among networks affected were ABC, Fox and PBS, which used the satellite to beam programming to affiliate stations. The satellite also carried pager service, which was knocked out to 45 million people.

Telstar 401 was built by Lockheed Martin at its space facility in Jefferson County, Colo.

Three earlier-model satellites were also disabled in 1994 by a solar storm which triggered electrical failures in them.

The approaching solar storm is not the end of the latest alert. A second sunspot cluster, not yet visible from Earth, could produce more geomagnetic storms in the next two weeks, NOAA said.

At over 869,919 miles wide, the sun contains 99.86 percent of the mass of the entire solar system: well 1 million Earths could fit inside it. The total energy radiated by the sun averages 383 billion trillion kilowatts, the equivalent of the energy generated by 100 billion tons of TNT exploding every second.

Additional Resources:

• Scientists Track The Solar Flare
• Space.com: Sun Cam
• SOHO Satellite Watches The Sun