The next sunspot cycle will be a year late and as much as 50 percent stronger than the last one, according to a new forecast released Monday by scientists from NASA and the National Science Foundation.

Such predictions are vital because the solar storms associated with the sunspots not only endanger humans in space, but also can slow satellites in orbit, disrupt communications, interfere with GPS positioning and bring down power systems.

The most recent cycle, which peaked in 2001, was relatively weak with few significant disruptions reported. A major solar storm associated with the sunspot cycle on March 13, 1989, brought down the power grid in Quebec, darkening much of the province for nine hours.

The latest forecast was made using a sophisticated computer program developed by solar scientist Mausumi Dikpati of the National Center for Atmospheric Research in Boulder, Colo., and her colleagues.

They predict that the next sunspot cycle, called cycle 24, will begin in late 2007 or early 2008 and will produce sunspots across an area slightly larger than 2.5 percent of the sun’s surface.

The cycle is likely to reach its peak about 2012 before subsiding.

The computer program is based on mapping of subsurface plasma flows discovered by the National Aeronautics and Space Administration’s Solar and Heliospheric Observatory, which was launched in 2005. The satellite observatory uses sound waves in the sun’s interior to reveal the details of its structure, much as a doctor uses ultrasound to view internal organs.

Sunspots – first observed by Galileo in 1610 – are dark areas on the sun’s surface that are about 2,700 degrees Fahrenheit cooler than surrounding areas. They are known to be caused by the eruption of magnetic fields through the sun’s surface, disrupting plasma and creating cool spots.

The magnetic eddies that break through the surface release enormous amounts of energy, sending sheets of ionized particle and ultraviolet radiation toward Earth. The particles can be dangerous to humans exposed to them in space and can disrupt not only electronic devices on satellites but even power grids on the ground.

Heat from the ultraviolet emissions causes the Earth’s atmosphere to balloon slightly, increasing the drag on sat- ellites in low-Earth orbit, including the international space station.