Spacecraft measures record amount of stellar debris.

Intruder alert! Stardust is sneaking into our solar system at three times the rate that it was just 6 years ago. Moreover, the influx of dust could triple again by the end of 2010.

These findings are based on the latest measurements by the Ulysses spacecraft, launched in 1990 to become the first observatory to explore the sun's poles. Before Ulysses' tour, which takes it from Jupiter's neighborhood to far above the solar poles, most astronomers didn't think much dust from other stars could penetrate the solar system. Sunlight rapidly charges dust attempting to enter the solar system, and, according to conventional theory, the sun's magnetic field then ejects the particles.

But in 1993, the space probe found that some particles barrel through the sun's magnetic shield. During solar maximum, when activity on the sun is most intense, even more stardust penetrates. That's because the sun's magnetic field begins flipping polarity, and the disordered field can't efficiently deflect dust particles.

Yet even though solar maximum ended in 2001, the rate at which stardust enters the solar system has remained high, report Markus Landgraf of the European Space Agency in Darmstadt, Germany, and his colleagues in the October Journal of Geophysical Research.

The explanation lies in the gradual way that the magnetic field changes polarity, Landgraf says. The field is now pointing nearly sideways, with its north and south magnetic poles lying along the sun's equator. In this configuration, the direction of the magnetic field rotates along with the sun every 28 days. Because the dust takes several months to react to the magnetic field, it experiences an average magnetic force of nearly zero. As a result, the magnetic field is now ineffective as a gatekeeper.

When the magnetic field finishes the reversal it began in 2001, it will focus stardust into the plane in which the planets orbit rather than expel the material. Landgraf says that he expects that the rate at which stardust enters the solar system will increase through about 2010.

About 0.6 micrometer in diameter, the particles are several times larger, and hence more massive, than astronomers had predicted. Greater mass may help the dust particles infiltrate the solar system.

Although the particles are still too tiny to affect the planets, their abundance probably leads to more collisions with comets and asteroids than usual, says Landgraf. That would result in more asteroid and comet fragments being generated.

These findings bode well for the Stardust space probe, which is scheduled to bring samples of cometary and stardust particles to Earth in 2006. The increased amount of stardust and the particles' larger-than-expected size should make for greater, easier-to-detect samples, says Stardust investigator Donald E. Brownlee of the University of Washington in Seattle.

"People often think of dust as trivial, but it's a major character in the solar system and throughout the galaxy," he says. Not only does dust absorb starlight, reradiating it as heat, it ferries such elements as iron, carbon, and silicon into the solar system, Brownlee notes.

Studying stardust near our sun, adds Landgraf, offers clues to how other stars interact with their surroundings.