The scientific importance of these first samples from the Galaxy can’t be overstated. The major form of heavy elements in interstellar space is in dust. This interstellar dust plays a major role in the formation of new stars and planetary systems. Our own Solar System formed from gas and dust in the interstellar medium 4.6 billion years ago. The heavy elements making up Earth and our bodies were once interstellar dust. In the words of Joni Mitchell, “We are Stardust.” But we don’t even know what the typical interstellar dust grain looks like! We are extremely excited about the prospect of studying directly the first contemporary interstellar dust.
Interstellar dust was first discovered flowing across the Solar System by dust detectors aboard the Ulysses spacecraft in 1993 and was later confirmed by the Galileo mission to Jupiter. The particles were identified as coming from a location in the sky in the Constellation Ophiuchus, looking toward the center of the Milky Way Galaxy.
What we have learned about interstellar dust comes from remote observations of how the dust absorbs, scatters, polarizes, and even emits light. Also, some ancient interstellar dust has been identified in meteorites found on Earth. Interstellar dust is small, ranging in size from 0.01 microns all the way up to 20 microns. They are made of different minerals such as silicates, graphitic carbon, hydrogenated amorphous carbon, alumina, and even diamond carbon.
Interstellar dust grains form by condensation in the regions around stars that are coming to the end of their life cycle: red giants, planetary nebulae, white dwarfs, novae, and supernovae. The dust grains mix with the interstellar medium (the stuff between the stars) and slowly experience chemical and isotopic changes from interactions with the gas and radiation in interstellar space. Dust grains do not last for very long in the interstellar medium before being dissociated by radiation, maybe a few hundreds of millions of years. This is why we say that the dust collected by the Stardust mission is contemporary dust, it must be only a few hundred million years old at most, whereas dust found recovered in meteorites would have been incorporated into them at the time of the formation of the Solar System (4.6 billion years ago).
While finding the interstellar dust grains captured in Stardusts aerogel collectors is the goal of the Stardust@home project, the identification of these grains is only the first step. The next step is the analysis. Once we have a few examples to examine, a committee of experts will decide on the next steps. Because they are so small and so precious, each track is worth about a million dollars if there turn out to be 100 of them! The analysis of these particles will have to be done extremely carefully and will take many years. Many types of analysis destroy the samples, so we will have to start with the gentlest techniques and proceed very carefully. The great advantage of this type of sample-return mission is that one can take advantage of the improvements in analytical techniques for years or even decades to come. Analytical techniques improved dramatically even during the seven years between the launch and the return of Stardust, and there is no sign of a slowdown in progress. So no matter what, some of these interstellar dust particles will be set aside for our great-grandchildren to analyze.
More on interstellar dust from the JPL Stardust website:
Aerogel is one of the strangest materials ever developed. It is a solid, yet is only a few times as dense as air. If you hold it in your hand, you can only barely feel its weight, and it looks bluish and ghostly like solid smoke. While it looks blue, it casts an orange shadow. It does this for the same reason that the sky is blue and sunsets are red!
Aerogel has extremely bizarre properties. It is a solid, glassy nanofoam, yet weighs next to nothing. Aerogel has the almost magical property that it can capture particles moving at very high speeds (several miles per second or more) better than any other material. In some cases, particles can be captured in a nearly pristine state. Particles moving at these speeds vaporize if they hit any other material.