An artist's concept of the Stardust spacecraft during interstellar dust collection. Photo: NASA

The first in-situ chemical analysis of interstellar dust particles produces a puzzling result: These cosmic particles consist mostly of 3-dimensionally cross-linked organic macro-molecules, so-called polymeric-heterocyclic- aromates. "They rather resemble tar-like substances than minerals" say Dr. Franz R. Krueger (contractor) and Dr.Jochen Kissel, Max-Planck-Institut fur extraterrestrische Physik (for extraterrestrial Physics), Garching near Munich, Germany, in the latest issue of 'Sterne und Weltraum' a monthly, German language Astronomy magazine in Heidelberg, Germany.

So far, 5 interstellar dust particles (dust between the stars) have hit the Garching built dust impact mass spectrometer CIDA (Cometary and Interstellar Dust Analyzer) onboard the NASA spacecraft STARDUST. Launched on Feb 7th 1999 STARDUST will visit comet Wild-2 (pronounce Vild-2) in 2004.

To reach the comet, STARDUST has to perform three orbits about the sun. At the close fly-by (miss-distance 500 km/300 miles) another instrument will collect cometary dust and return it, well packed, to earth in January of 2006. During its 7 year mission, STARDUST will face the stream of interstellar dust several times. This dust is part of the local environment in the Milky Way which the solar system currently passes through at high speed. It has recently be seen by dust instruments of the Heidelberg-based Max-Planck-Institut fur Kernphysik (for Nuclear Physics) on both NASA's Galileo and ESA's Ulysses spacecrafts. The first measuring campaign for CIDA from February through December 1999 has produced the new results.

During this time STARDUST was at a distance of about 240 million kilometers (150 million miles) from the earth when the first impact occurred. Just before the campaign the spacecraft pointed the instrument into the direction of the interstellar dust, so that it would not measure the more frequent interplanetary dust particles, which are parts of our solar system.

Microscopic photo of aerogel with a dust particle. Photo: NASA

At an impact speed of about 30 kilometers/second (18 miles/second) these interstellar dust particles are vaporized immediately and broken up into molecular fragments. A fraction of those carries a positive or negative electronic charge. By its electric field in front of the target CIDA pulls the positive ions into the instrument to the detector. Depending on their mass it takes the ions different times to travel the 1.5 meters (5 feet) distance (heavier ions travel longer). This way they are detected mass after mass with in some 200 millionth of a second, and a mass spectrum is generated.

"It is the size of these molecular fragments with nuclear masses of up to 2000 (water e.g. has 18 such units) which surprised us as much as the seemingly absence of any mineral constituents", explains Dr. Kissel of the Garching-based Max-Planck-Institut für extraterrestrische Physik. "Only organic molecules can reach those sizes". The largest molecules found in space so far are the polycyclic aromatic hydrocarbons (PAH) which reach masses of a few hundred mass units.

The details of the mass spectra measured with CIDA show that the molecules of the interstellar dust must have about 10 percent of nitrogen and/or oxygen in addition to hydrogen and carbon. This means that these cannot be pure PAHs, which are planar, but are especially due to the nitrogen extend into all three spacial directions.

Such three dimensional molecules can form links to their neighbors and reach a thermal stability necessary to survive the trip into the inner solar system with 300 to 350 Kelvin (70 to 180 degrees Fahrenheit). "The organic material analyzed with CIDA in the interstellar dust particles is another type of reactive molecules which we found in the dust of comet Halley 14 years ago" says Dr. Kissel. "When they got in contact with liquid water on the young earth, they could have triggered the type of chemical reactions which are a prerequisite for the origin of life."