Dubbed the "Cluster" because they will fly in formation like Star Wars spaceships, the four identical pilotless craft will be trying to detect billions of invisible solar particles that constantly bombard our planet.

Most are deflected by Earth's huge protective magnetic field. But some manage to penetrate, and that spells trouble for Earthlings. Understanding their interaction is the main job for Cluster 2.

"It's like a never-ending football game," said Philippe Escoubet, project scientist for the European Space Agency's Cluster 2. "The sun is kicking particles towards us, like footballs. The Earth is the goal and its magnetic field is the goalkeeper. It's always trying to push the 'balls' away, but some get past. When particles score goals they disrupt the Earth. Sometimes the sun is very quiet, but when it's very active we get a lot of 'balls' coming through."

Because the sun is at the peak of its 11-year cycle, detecting the "invaders" is high priority this year. The satellites are scheduled to be launched on a two-year mission by two Russian Soyuz rockets. The launch of the first two on July 15 is to be officially confirmed on Monday. The second set is scheduled for August 9.

A team of 40 space engineers from Europe's new aerospace giant Astrium, which built the spacecraft, and from Starsem, the French-Russian commercial launch company that commercializes the Soyuz rocket for the Western marketplace, is preparing to launch the Cluster 2 satellite fleet from Russia's Baikonur Cosmodrome in Kazakhstan.

The Cluster mission will investigate for the first time the invisible solar phenomena that affect and sometimes seriously disturb our planet, causing power blackouts, satellite disruptions and other problems. Don't expect to see any photos taken by Cluster 2; its main objective is fundamental physics.

Contrary to the common belief that space is pure vacuum where nothing happens, "every second, millions of tons of material are blasted out from the sun into space in the form of charged particles -- mainly electrons and protons," said Alberto Gianolio, European Space Agency (ESA) deputy project manager for Cluster 2. This stream of particles creates a solar wind traveling at supersonic speeds toward Earth.

While most of the charged particles from the sun are not dangerous to human's health, they have a variety of effects, some of which are still unknown.

In March 1989, for example, 6 million people in Quebec were left freezing in harsh winter without electricity due to a huge solar-induced magnetic storm. Earth had been hit by a rare violent particle storm which caused the Earth's magnetic field to swing, releasing intense electric currents. In turn, they induced high electric voltages in power lines.

Even minor gusts of solar wind can also affect space technology. In 1997, the unexpected failure of the TV satellite Telstar was attributed to a solar eruption. Solar storms have even been blamed for increased corrosion in steel oil pipelines. "With the increasing use of new technologies, computers on satellites, for example, can be easily disturbed by solar activities," said Escoubet.

During solar peaks, like the one occurring this year, radiation levels can become hazardous to astronauts and frequent flyers of high-altitude aircraft.

But fortunately, most of the time Earth's magnetic field creates a giant, protective bubble in space -- the magnetosphere. Like an island in a stream, Earth is able to divert the flow of particles around it. As they sweep around the planet, the magnetosphere is molded into an aerodynamic shape, with a blunt head on the sunward side of Earth, and a long tail stretching millions of miles in a direction away from the sun.

However, Earth's magnetic defenses can be breached by high-energy solar particles. Two weak points above the planet's magnetic poles, known as cusps, allow the solar wind to leak into the magnetosphere and spiral down magnetic field lines into the thin upper atmosphere. Other particles, which are trapped in the magnetic bubble, can collect and then sweep down into the atmosphere. In either case, the particles collide with molecules of air, creating spectacular curtains of light known as auroras or "polar lights." Cluster 2's ultra-sensitive instruments will also be able to track auroras around Jupiter's poles.

Hundreds of scientists around the world are eager to get some Cluster data, hoping for help in solving unanswered questions, like: what is the long-term influence of the sun on Earth's climate? Without the magnetosphere to protect Earth, this planet's climate and its protective ozone layer would be very different.

Simultaneous measurements from the Cluster 2 quartet will provide invaluable snapshots of events taking place inside and outside the magnetosphere.

"Cluster 2 will give us the best information yet on how the sun affects the near-Earth environment. There are a lot of unknowns there," said Escoubet.

"The spacecraft will give us four viewpoints -- like having one camera behind the goal at a football match and three others at different angles. It will be the first time this has been done for the Earth's magnetic field. This is very exciting because such unprecedented detail will give us a much better opportunity to understand the space environment which surrounds our planet," he said.

This satellite squadron will investigate the changing "space weather" around our planet in unprecedented detail. In order to achieve this, each spacecraft carries an identical set of 11 instruments, provided by scientific institutions in different countries. For the first time, these will measure -- simultaneously in three dimensions -- charged particles, along with magnetic and electrical fields in near-Earth space, within 74,400 miles (120,000 kilometers) of Earth's surface.

For those working on the ground, this mission will prove challenging. They must receive and process a vast amount of data -- equivalent to 290 million printed pages -- that will be returned to Earth over the mission's two-year lifetime. The stream of information returned by the 44 instruments will be distributed to eight national data centers: six in Europe, one in the U.S. and one in China.

Each cylindrical Cluster 2 spacecraft looks like a giant drum -- 9.5 feet (2.9 meters) in diameter and 4.27 feet (1.3 meters) in height, weighing 1.2 tons. In orbit, the probe's most striking visual feature is its rotation with its spectacular booms. Each Cluster will cover the size of a soccer field when they spin at 15 r.p.m. with two 16.4 foot- (5 meter-) long experiment booms and four 164 foot- (50 meter-) long wire booms extended.

"Cluster 2 will give us the best information yet on how the sun affects the near-Earth environment. There are a lot of unknowns there."
ESA Project Scientist Philippe Escoubet

They will burn fuel equivalent to 54 percent of their mass in order to reach the correct polar orbits and to enable them to maneuver in space for two years.

The Cluster 2 scientific community includes more than 200 scientists from the ESA with its 14 member states, the United States, Canada, China, the Czech Republic, Hungary, India, Israel, Japan and Russia.

One of the key scientific tools on board is the U.S. Wide Band Data instrument (WBD) which was designed and built at The University of Iowa through funding provided by NASA's Goddard Space Flight Center. It's designed to provide high-resolution measurements of both electric and magnetic fields in selected frequency bands.

Cluster 2 is one of the ESA's most critical science missions because it's a replacement for the original Cluster mission. The first four automatic spacecraft were destroyed in June 1996 when an Ariane 5 exploded 30 seconds after liftoff.

The launch was free, but extremely costly in the end because it carried no insurance to cover the $508 million price tag it carried in original development costs. Under tight financial constraints, an identical replica of Cluster was built for half the price: $315 million, including operations.

Unable to afford another Ariane 5 launch, ESA decided to buy two Russian Soyuz rockets from Starsem launch services, costing 50-percent less.