The spacecraft in this case is one funded by NASA and its mission is to collect tiny particles of dust that enter our solar system from the far reaches of space. It began the first part of its mission in February, when it unfolded collectors equipped with aerogel, the lightest material ever made by humans. The collectors will try to snag interstellar dust over a three month period, then be stowed again until mid-2002 and another stint at dust collecting. If all goes at planned, it will be back by 2006 and land by parachute in a desert region in Utah. Different sides of the collectors will be used, with one side dedicated to sweeping up interstellar dust and the other, the dusty debris from Comet Wild-2's tail.
Interstellar dust is an unknown.
But empty is a relative term here. In our part of the galaxy, the spaces between stars holds about one atom of interstellar gas per cubic centimeter. There are only 25 to 30 microscopic bits of solid particles per cubic kilometer. To contrast, air at sea level on Earth contains about 10 to the 19th molecules of gas per cubic centimeter - that a 10 followed by 19 zeros. The other 99 percent is gas.