Send in the Brooms: A Dust Cloud Draws Nigh

Astronomers scanning the solar system's route though space have spotted possible trouble ahead. An unexpected cloud of dust and gas is heading our way. If we run into it, the dusty stuff could penetrate far enough into the solar system to reach Earth.

University of Chicago astronomer Priscilla Frish, one of the cloud's discoverers, says "nobody really knows" what will happen. Nevertheless, the prospect is a reminder that the environment in which we live extends into interstellar space.

When Dr. Frish first reported this during the June meeting of the American Astronomical Society in Madison, Wisconsin, she said more data are needed to know exactly what she is seeing. Now her Chicago colleague Daniel Welty is preparing to get some of those data during an August session at the Kitt Peak observatory complex in Arizona.

Meanwhile, Frish explains that the solar system probably has already entered a new space environment. It had been drifting through a relatively empty region between the spiral arms of our Milky Way galaxy. But during the past 10,000 years, it seems to have entered a diffuse bubble of material thrown into space from a region of active star formation called the Scorpius-Centaurus Association. As Frish explains, "we have data in the direction in which the sun is heading that show very different characteristics from data in the [opposite] direction." The new-found cloud is a million times denser than average within the Scorpius-Centaurus bubble.

Astronomer David Meyer of Northwestern University in Evanston, Ill., says that such a concentration fits in with what he and his colleague John Watson are discovering about the interstellar medium. He explains that this medium is very uneven on large and medium scales. On the small scale of less than one light year, astronomers had expected features to be fairly smooth. That turns out not to be the case.

Drs. Meyer and Watson have looked at 17 binary star systems located hundreds of light years away. In all cases, they see evidence that clumps of gas located between us and the stars are absorbing some of the star light. Some of the clumps are the size of our solar system.

"This raises some interesting issues about passage of the solar system through such a clump," Meyer says. Right now, the sun's outward flowing solar wind of gas laced with magnetic fields keeps most interstellar material at bay far beyond the planets. An encounter with one of the newly discovered dense interstellar clouds could compress that protective solar shield to within Earth's orbit. That, in turn, might affect Earth's own magnetic field and even the climate. On the other hand, Meyer notes, it may not do much of anything. He cautions, as does Frish, that nobody will know what might happen until detailed theoretical studies are made.

In any case, Frish says the cloud would not be a long-term threat even if we did enter it sometime during the next 50,000 years. She explains that "Our observations [so far] don't allow for a large dense cloud." The solar system probably would pass through the dense part in about 10 years.

These small-scale clouds are a tiny part of the interstellar medium that permeates our galaxy. About half of the material is in the form of a diffuse gas - mainly hydrogen - with a density of about 0.1 atoms per cubic centimeter and temperatures about 5,000 degrees C. It's mixed with dust grains. Much material also is grouped into large clouds typically 30 light years in size with densities of 10 to 20 atoms per cc and temperatures around 220 degrees below zero C. Such a cloud may have the mass of a thousand suns. About half the volume of the interstellar medium, but only a tiny fraction of its mass, is occupied by a gas at temperatures near 1 million degrees and with densities of a mere 0.001 hydrogen atoms per cc.

The result of this complexity is an overall Swiss-cheese-like structure with numerous holes of low-density, high-temperature gas. Regions with supernova star explosions and active star formation may superpose their own gas bubbles, as in the case of the bubble our solar system now seems to have entered.

Then there are the molecular clouds where stars are born. They account for about half of the interstellar mass. Measuring 50 to 100 light years across, they may have the mass of 1 million suns at densities of 1,000 to 10,000 hydrogen molecules per cc or more. Astronomers have located more than a hundred different types of molecules - including complex organic molecules - in such clouds.

All of this has been complex enough for astronomers. Now they are finding that, even on the small scale where they had expected some uniformity, the interstellar world through which we are traveling has its surprises.