Stardust at Home

Scientists all along anticipated that tracing our primordial origins with interstellar dust would be like looking for a needle in a hay­stack (or a piece of grit in a gel block). They estimated that, while the side of the collector that faced the comet debris might collect a million particles, they’d be lucky to gather a few dozen from the side trawling for dust from interstellar space. Such was the case. One side of the aerogel was peppered with trails from comet particles, but interstellar dust was very rare, and difficult to spot among all the blemishes and markings that the other side of the aerogel had suffered after seven years in space. Imagine searching for a few dozen ants nestled deep in the grass of a football field. So the two hundred members of the international science team decided to get some help.

Stardust@home has engaged nearly 30,000 members of the public around the world in the search for interstellar dust. The archetypal citizen science project was SETI@home, where the “spare” CPU cycles of millions of PCs were harnessed to analyze chunks of radio data in order to search for transmissions from intelligent aliens.15 SETI@home had distributed computing as a model and no thought or intervention was required by people who participated. Stardust@home is more like Galaxy Zoo, where human eyes and brains are harnessed in pursuit of science goals and participants must undergo training.16 Citizen science is one of the exciting recent developments in outreach and the “democrati­zation” of research, where interested members of the public get on­line training in categorizing and sifting through large amounts of data, and then are able to contribute to the creation of new knowl­edge. Occasionally, these very attentive amateurs make important discoveries.17

The raw material for Stardust@home is a huge number of im­ages made with an optical microscope which can automatically focus at different depths in the aerogel. A set of forty images of a small area are taken with the focus ranging from just above the surface to 100 microns into the aerogel. These images are turned into an animated sequence or “movie” so the viewer seems to move through the aerogel. Altogether, 1.6 million movies were needed to cover the 1,000-square-centimeter surface of the collector. This huge number is part of the reason help was needed. Starting in Au­gust 2006, Stardust “movies” were made available to the general public. Each eager participant first had to undergo a short training session and take a test to show that they could indeed recognize particle tracks. Then they were unleashed on the “haystack.” The signature of a cosmic dust particle is a hollow wake that ends in a tiny particle, often no bigger than a micron in size (figure 6.2). A million such particles ploughed into the aerogel. Of these, only

ten were large enough to see by eye—a tenth of a millimeter or larger—and only one was as big as a millimeter across. Computer programs are unable to reliably identify telltale signs of a particle impact, and they can’t be trained since such detections haven’t yet been made! Additional information has come from the aluminum foil detectors, which were also peppered with dust impacts.18

Citizen scientists can’t get instant gratification from the proj­ect. They have to use the “Virtual Microscope” program in a web browser and report their results to Stardust @home headquarters in Berkeley. Each movie is sent to four users who each scan it in­dependently. Only if a majority of users claim a particle detection does it go to the Stardust science team for confirmation. What do the volunteers get in return for their labors? Mostly online certifi­cates, and the knowledge that they’re contributing directly to an important science mission. Bruce Hudson from Ontario in Canada did a bit better. He had suffered a stroke and turned to the Stardust mission as a good way to pass the large amount of time he had on his hands. Working up to fifteen hours a day for over a year, he not only found the first confirmed interstellar dust particle in the aerogel, he then found a second, named them (Orion and Sirius), and he’ll be a co-author on the paper that results. Hudson might be amused by the irony that astronomers rarely give names to as­teroids or craters less than a kilometer across, yet he put names on objects a billion times smaller. Interstellar dust is distinguished from comet dust by chemical analysis. Particles from deep space are glassy and contain lots of aluminum, along with manganese, nickel, chromium, iron, and gallium. Researchers take particular care not to drop or lose these particles—it would cost $300 million to replace them.

If that seems rather too high-tech and difficult, you can take on the somewhat easier task of gathering comet (and asteroid) dust in the comfort of your own home. Or at least on your roof. Each year 10,000 tons of micrometeorites and 100 tons of space dust land on Earth and a little of that material will also land on the roof of a house. (They’re not recoverable from the ground because they’re too similar to particles in the dirt.) The best scenario is a sloping metal, tin, or slate roof, with no overhanging trees. Col­lect the runoff from a day or more and filter it sequentially with a window screen and then a finer mesh, to remove all leaves, paint flakes, and other artificial materials. The next step involves using a very strong magnet (such as a Neodymium or rare earth magnet, easily obtained by mail order) to gather metallic morsels from the sludge that remains.19

This will isolate the primarily metallic particles, but many ter­restrial forms of debris can be magnetic so the last step involves a hand-held magnifier or cheap microscope. With a magnified view, the rounded, melted, and pitted shape of micrometeorites readily distinguishes them from more mundane terrestrial metal particles. Following this method patiently and carefully will net you a num­ber of particles from deep space, without leaving home, and for a much lower price tag than several hundred million dollars.