They can shut down satellites or disable aircraft navigation devices. They frequently interrupt high-frequency radio communications and also expose orbiting astronauts - particularly spacewalkers - to high doses of deadly cosmic radiation.

What's more, the unpredictable outbursts often are accompanied by coronal mass ejections -- billion-ton clouds of gas that billow away from the sun, triggering geomagnetic storms that can knock out power grids that transmit electricity to homes and businesses.

Yet even the most erudite solar physicists have no idea what sparks solar flares, or how they unleash so much energy so quickly. And scientists would like to figure out how to forecast impending solar eruptions so that more accurate space weather warnings can be issued.

"These are gigantic explosions. Just one of them can release the equivalent of one billion megatons of TNT," said Robert Lin, a solar physicist at the University of California at Berkeley.

"We want to understand how these explosions actually work. How do they happen? Why do they explode? And how can they release so much energy?" he said.

"If we knew what the conditions were that proceeded these explosions, then we could look for them and perhaps be able to predict when they might happen. And that, of course, would be of great benefit for our advanced technological society in general."

Enter NASA's High Energy Solar Spectroscopic Imager.

More commonly called HESSI, the spacecraft will be carried aloft inside an Orbital Sciences Corp. Pegasus XL rocket mounted to the belly of a Lockheed-built L-1011 aircraft.

The aircraft is scheduled to take off from Cape Canaveral Air Force Station about 2:30 p.m. EST (1930 GMT), heading for a point 40,000 feet (12,133 meters) above the Atlantic Ocean.

Once there, the Pegasus and its 645-pound (290-kilogram) payload will be dropped from the belly of the aircraft so that the air-launched rocket can be ignited, hauling HESSI into an orbit 373 miles (597 kilometers) above the planet.

Circling high above Earth's obscuring atmosphere, the spacecraft's sole scientific instrument will employ nine specially designed detectors to gather high-energy X-ray and gamma ray photons emitted by solar flares.

A sophisticated processing technique then will be used to create images, and the resulting pictures will form the cinematic basis for the first high-fidelity color movies of solar flares.

The movies, in turn, are expected to help scientists define the origins of the energetic eruptions -- a mystery ever since English astronomer R.C. Carrington first spotted one of the outbursts when he was charting sunspots on Sept. 1, 1859.

Principal investigator Lin and other HESSI researchers had hoped to begin unraveling the scientific puzzler in July 2001, but the planned launch of their spacecraft has been repeatedly delayed.

First, the HESSI satellite was accidentally shaken apart during a March 2000 vibration test at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Designed to withstand double the force of gravity, the spacecraft was exposed to 10 times that limit when a testing table malfunctioned.

"It broke the spacecraft, the bus and smashed our solar panels," Lin said. "We spent nine months rebuilding the whole thing."

A March 2001 launch then was scheduled, but the mission was pushed back by another three months when problems cropped up during a separate launch of the same type of Pegasus rocket slated to carry HESSI into space.

Things then went from bad to worse.

A Pegasus rocket and its payload - NASA's experimental X-43 hypersonic test vehicle - were destroyed during a June 2001 launch over the Pacific Ocean, casting new doubt on HESSI's launcher and indefinitely delaying its mission.

And then a planned Jan. 24 launch was postponed - this time to give engineers time to look into a national missile defense system test failure that took place in last December.

Launched from Vandenberg Air Force Base in California, that prototype missile defense booster was equipped with a first stage similar to the one which will propel HESSI's Pegasus XL launcher, and engineers wanted to wrap up a review of common components and systems to guard against a like failure.

Said Lin: "We've been through a lot with this spacecraft."

The original $75 million cost estimate for the project grew by $10 million -- the price of rebuilding the spacecraft and absorbing the initial launch delays. And there also is an economic upshot to the more recent postponements.

Intended for launch at the peak of the most recent 11-year solar cycle, the spacecraft now will be orbiting Earth during its wane, so the eruption of solar flares probably won't be as frequent as they would have been otherwise.

"Certainly we won't get as many flares as we would have if we had launched on time," Lin said.

Consequently, project officials are vying for enough extra money to continue HESSI's planned two-year mission long enough to achieve their original scientific objectives. Current estimates indicate the spacecraft would have to operate an extra six months. The exta cost involved: about $2 million.

The repeated launch delays have had another adverse impact.

Project scientists had planned to conduct complimentary studies with researchers working with several other solar spacecraft over the course of the HESSI mission.

One of those -- the Japanese Yohkoh solar probe -- was knocked out of commission last December, and engineers have yet to revive it. Others -- such as NASA's Ulysses solar probe and the Solar and Heliospheric Observatory, or SOHO spacecraft -- have been in orbit for years and there's no telling exactly how long they'll continue chugging along.

"We're worried that some of some of these spacecraft that will add complimentary data to ours will fail," Lin said. "HESSI can do a lot of great science by itself, but to really maximize the science, you want to get these other observations."

Project scientists, as a result, are anxious to get the mission under way.

After all, the data from HESSI the companion craft also are expected to generate new insights into more far-flung celestial eruptions, such as the type of supernovae that generally represent the volatile death throws of stars similar to our own sun.

"I would like to know how solar flares - which, of course, are smaller versions of explosions that happen all over the galaxy and all over the universe -- start with magnetic fields and plasma and end up with explosions that can be so enormous," Lin said.

"So the sun is a great laboratory to study these processes. It's close enough that we can see the whole thing, and we can see it in detail," he added. "And that's something we can't see anywhere else."