Fancy yourself one among billions of interstellar dust grains. You're cruising through the vast black at about 58,000 m.p.h. (93,340 kilometers per hour). Around you are your brethren -- some bigger, some smaller. Otherwise, there's not much. You are not even much -- far smaller than the width of a human hair.
Eons after your journey began, you all start to feel an attraction. It builds gradually, almost imperceptibly. There, far off in the distance, is a point of light -- another star. But this one is growing. It's altering your course, pulling you in.
Passing through the orbit of a tiny, rocky planet (hardly a planet at all, you think) you note that the main attraction around here is a central star, and you foresee spending the next 20 years or so moving through this system before zipping out the other side. So you settle in and enjoy the view, passing a couple of big, blue-green ice planets, another with some cool rings and a really huge one that seems to be the grand daddy of the nine planets you've noticed.
Presently, with a red planet looming in the distance, everything seems to go wrong. Some of your buddies -- the small and the large -- are breaking away from the pack, leaving you and the rest of your mid-sized companions behind. You feel something repulsive -- literally -- like an unseen force trying to push you back in the direction you just came from.
Photons, you guess. Those uncountable, invisible masses of elementary light particles, streaming toward you at the incredible speed of, well -- the speed of light.
Just beyond the ruddy planet, one step closer to the central star, is a pale blue dot, from which Markus Landgraf is monitoring all this activity. After measuring the results of dust-catching missions from two spacecraft, Landgraf (himself composed of so many of your friends and cousins) reaches the conclusion that sunlight is selectively thwarting your travels, while not affecting the movement of the largest and smallest dust grains. His study also reveals what you are made of, and so his report ends up in this Friday's issue of the journal Science.
What is missing
Interstellar dust is pretty much everywhere. Under good viewing conditions, it can be seen from Earth as dark bands that obscure central portions of our Milky Way galaxy. In 1992, researchers first detected a tenuous cloud of interstellar dust even closer to home -- one that envelops our solar system.
But in a swath cut through this local cloud -- two to four times as farther from the sun than Earth is -- Landgraf found a void of dust grains of a certain mass. Two spacecraft, Ulysses and Galileo, both equipped with dust-gathering devices, sampled the region. As it turns out, the missing particles had properties that caused them to react differently to the constant push of photons rushing outward from the sun.
"Photons carry much less momentum than dust grains, but they hit the grains in large number," explains Landgraf, who did the work as a research associate with the National Research Council at NASA's Johnson Space Center. "The grains feel a continuous pressure, much like you would feel if you were standing in a hail storm."
Landgraf, now back at his post at the European Space Agency, said interstellar dust grains come in many sizes -- from smaller than a tenth of a micron to many microns wide (a micron is one-millionth of a meter). The tiniest specimens are too small to effectively absorb light. The bigger ones are too big to be affected by this photon radiation. One group, however, is just right:
"For intermediate sizes (about one-half of a micron in diameter), the effect of radiation pressure is stronger than the gravitational pull of the sun," Landgraf said. "These particles are therefore effectively repelled by the sun."
The result, Landgraf and his colleagues learned, is that these mid-sized particles are slowed until they cannot get any closer to the sun. This finding, in turn, has provided clues to the composition of the particles, which are thought to be part of the basic ingredients of everything in the universe.
The researcher explained that photons would pass right through a dust grain made of glass, whereas they would be absorbed by a particle of carbon.
"For the carbon grain the radiation pressure is therefore much stronger than for the glass grain, even if they were identical otherwise," Landgraf said. "Since we can estimate the strength of radiation pressure from our observation, we can draw conclusions about the grains' composition. We found that either a material called 'astronomical silicates' or another material called 'organic refractories,' that has been produced in a lab, have the right optical properties to explain our observation."
The findings help paint a picture of how the sun moves through and interacts with the local interstellar environment, and how that might change if the sun moved through a region of dust that varied in density.
A mission called Galactic DUNE (Galactic DUst Near Earth), which would orbit the Earth and analyze the chemical composition of interstellar dust grains, has been proposed to the European Space Agency.
"The ultimate goal is to collect interstellar dust and to return it to a lab on Earth," Landgraf said.
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