|Go, and catch a falling
Tell me, where all past years are...
--John Donne, 17th Century Poet
Every year, more than 10,000 tons of space dust enter the atmosphere and fall to earth. Particles the size of one-sixth the diameter of a human hair hit the earth at the rate of about one particle per square meter per day. That means if you spend a good deal of time outdoors, you will be hit by several cosmic particles per week. But you won't notice their impacts; with a mass of only one-billionth of a gram, the particles are moving at a speed of about a centimeter per second.
These micrometeorites, as they are called, are interesting to scientists because they are the ancient and primitive materials that formed during the early history of the solar system. They were stored inside comets and asteroids for billions of years before fate brought them to earth. These organic-rich particles may have played a role in the origin and early development of life.
UW astronomer Donald E. Brownlee, considered the "father of micrometeorites," has revolutionized the methods for collecting and studying these bits of primordial matter, known as Brownlee particles. Finding them is not exactly a simple task.
"The problem with the earth is that while it is a good cosmic dust catcher, the actual recovery of individual dust samples for laboratory study is very difficult," notes Brownlee. Although cosmic dust is all around us, it is almost impossible to find, since it is mixed in with dust of terrestrial origin.
It turns out that there are two good places to catch cosmic dust: the deep-ocean floor in regions far from land masses, and the upper atmosphere. Brownlee and colleagues have pioneered approaches for catching the particles in both locations.
A cubic meter of sea floor mud from the center of the Pacific Ocean contains all of the extraterrestrial particles that have accumulated over some 500,000 years. This long collection time allows rare particles to be captured. Most cosmic dust particles in the 0.1 to 1-millimeter range are magnetic, a property that allows them to be separated out from terrestrial matter. UW researchers developed the "Cosmic Muck Rake," a sled-like device that allows magnetic space dust to be collected from the ocean floor. The rake is towed through the water at depths of about 5 kilometers and picks up some tens of thousands of cosmic dust particles in a day.
The first particles collected in the atmosphere and positively identified as cosmic dust were collected by Brownlee and colleagues in 1970 by means of a high-altitude balloon experiment called the "Vacuum Monster." Named "for the creature in the Beatles movie Yellow Submarine that sucked up everything," the device used a large pump rigged underneath a balloon. Powered by 150 pounds of hydrazine rocket fuel, it could suck air at the rate of half the speed of sound through an orifice specially fitted with a grid of thin glass rods on which the dust particles could be snagged.
The Vacuum Monster succeeded in collecting one particle of cosmic dust on its first flight, and caught nine on its second. But the researchers needed a larger sample. So Brownlee and colleagues enlisted the help of one of NASA's U-2 aircraft—an ultra-high flying former "spy plane." It could soar high above the lower, more contaminated layers of the atmosphere, and could stay aloft for hours. In 1974, Brownlee's collecting plates—pieces of glass the size of playing cards, coated with a sticky oil—were mounted on the U-2 craft. The plates managed to nab particles at the rate of about one or two per hour.
When viewed under a powerful electron microscope, each bit of dust appears as a porous aggregate of tiny grains, not unlike a tiny bunch of grapes. The elemental composition of the grains is similar to that of the sun and most stars; and Brownlee believes many of the examples collected so far are from comets. These bits of star dust may have been formed in the far reaches of space before the solar system was formed, or perhaps during the solar system's earliest days. Either way, they have much to reveal about the processes that formed our cosmic neighborhood called the solar system, which condensed out of a cloud of dust and gas some 4.6 billion years ago.
Brownlee is leading the Stardust Mission, which will gather samples of comet dust and return it to earth. The project was selected by NASA in the fall of 1995 to become the fourth flight mission in its Discovery program. A spacecraft will fly by the comet called Wild-2 in January, 2004. On the way, it will also gather and return samples of interstellar dust.
The Discovery program at NASA features small planetary exploration spacecraft with focused scientific goals. They must be built quickly—in 36 months or less—for less than $150 million, excluding the cost of the launch vehicle. Edging out two other competing proposals, "Stardust was rated highest in terms of scientific content and, when combined with its low cost and high probability of success, this translates into the best return on investment for the nation," said Dr. Wesley T. Huntress Jr., NASA Associate Administrator for Space Science.
Stardust will be launched on an expendable launch vehicle in February 1999. The return capsule carrying the dust samples will parachute to earth in January 2006, landing on a dry Utah lake bed. The craft will use an advanced material called an aerogel to capture the dust. Aerogels are porous materials of extremely low density, made of silicaa pure form of sand. The material can absorb large amounts of gas or particulates, thanks to a remarkably large internal surface area.
Comet Wild-2 is called a "fresh comet" because its orbit around the sun was deflected from much farther out in the solar system by the gravitational attraction of Jupiter in 1974. Stardust will come as close as 62 miles to the comet's nucleus.