Space Dust Flooding Our Solar System

John Roach
for National Geographic News
August 27, 2003
A flood of interstellar dust is breaching the sun's weakened magnetic shield and drifting into the solar system, according to European astronomers.

The interstellar dust particles measure about one-hundredth the diameter of a human hair. The bits are thought to supply the building blocks of all solid bodies in the galaxy, including the planets and humans.

"All atoms in Earth were in interstellar grains before the solar system formed," said Donald Brownlee, an astronomer at the University of Washington in Seattle. The dust is believed to be composed of heavy elements such as carbon, magnesium, iron, and calcium.

The dust grains pose no serious threat to the planets. But they could chip away at the solar panels on spacecraft, causing a gradual loss of power, and knock particles off asteroids, filling the solar system with even more dust. On Earth, stargazers may observe a greater number of shooting stars.

"All these effects are not yet observed…but they are expected," said Markus Landgraf, an astronomer with the European Space Agency in Darmstadt, Germany. Landgraf discovered the influx of dust using data from the agency's Ulysses spacecraft.

Since its launch in 1990, Ulysses has monitored how much dust enters the solar system from the interstellar space around it. Until ten years ago, astronomers believed that bits of interstellar dust could not penetrate the sun's magnetic field.

Weakening Field

Using data gathered by Ulysses, astronomers learned that stardust can enter the solar system. However, the flow of stardust is regulated by the sun's magnetic field, which is drawn out by the solar wind—a flow of ionized gas that expands away from the sun's surface and extends out beyond the edge of the solar system.

The field was thought to be strong enough to prevent the tiny interstellar dust particles from entering the solar system.

"The dust grains are however about five times larger and one hundred times more massive than was thought before," said Landgraf. "That's why the force of gravity is about the same or even a little less than the solar radiation pressure."

When the magnetic field weakens, more grains of dust are able to leak into the solar system. The field weakens periodically during phases of intense sunspot activity as part of the sun's 22-year cycle.

These phases of intense activity are called solar maximums. The intense activity causes the magnetic field to become disordered as its polarity reverses, rendering it less effective as a shield against tiny dust particles floating around in interstellar space.

What surprised Landgraf and his colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg is that the influx of dust has continued to increase since activity on the sun calmed following the 2001 solar maximum.

The scientists believe the continued influx is due to the way in which the polarity changed. Instead of reversing completely, flipping north to south, the sun's magnetic poles have only rotated halfway and are now more or less lying sideways along the sun's equator.

"Before the current solar maximum, the grains were deflected out of the vicinity of the sun. Now with a global solar magnetic field weakened during maximum conditions, we see more grains," said Landgraf.

This weaker configuration of the magnetic field is allowing two to three times more stardust to enter the solar system than at the end of the 1990s. This influx of stardust could continue to increase as the field further weakens until the end of the current solar cycle in 2012.

Independent of the variations in the solar magnetic field, astronomers expect the influx of interstellar dust to increase sometime in the next 10,000 years when the solar system drifts into a galactic cloud known as the G-cloud.

Measurements of the G-cloud indicate it is full of gas. "And where there's more gas in the galaxy, there's more dust, normally," said Landgraf.

Building Blocks

Brownlee, the University of Washington astronomer, serves as the principal investigator for NASA's Stardust spacecraft. Stardust was launched in 1999 to collect bits of interstellar dust and comet fragments and return them to Earth for analysis. Brownlee said the discovery of increased interstellar particles makes his job easier.

"In terms of Stardust collecting interstellar grains as part of its cruise, it makes us happier that the flux has increased recently and the particle sizes are bigger than estimated earlier," he said.

Stardust will deploy material called "aerogel" to capture the particles, which move at about 16 miles (26 kilometers) per second relative to the sun. When the particles hit the aerogel—a block of silicon-based substance that is 99.8 percent air—they slow down as they penetrate the material, creating carrot-shaped tracks.

A paper by Landgraf and colleagues on the influx of interstellar dust will appear in the October 2003 issue of the Journal of Geophysical Research.

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