Following a preliminary analysis of the dust grains that had been returned to Earth by NASA’s Stardust spacecraft in 2006, a research team identified seven particles that might have originated from the interstellar medium that permeates the entire Milky Way galaxy. If confirmed, this discovery will represent the first time ever that humanity has sampled the stardust itself—the stuff the Solar System and all life on Earth are made of.
Even though we may be tempted to think of the interstellar medium as being a vast expanse of empty space, in reality it is far from a perfect vacuum. Despite having an average density which is lower than that of any artificial vacuum created on Earth, the interstellar medium is consisted for the most part of a rarefied gas that permeates the entire galaxy and is made up of atomic and molecular hydrogen and helium as well as ionized particles. Besides these huge amounts of gas, the interstellar medium also contains trace amounts of solid microscopic particles which are believed to be a few molecules to a few tenths of a micrometer (millionth of a metre) in size and are composed of heavier elements like silicates, carbon, oxygen, silicon, nickel, and iron. These heavier elements are the products of stellar alchemy—the ashes left behind by the fusion of lighter elements like hydrogen and helium inside the cores of every star throughout the galaxy. When stars reach the end of their lives, they eject most of their mass into space, the more massive ones in violent supernova explosions and the lighter ones in a slow blowing off of their outer layers, enriching the interstellar medium in the process with all the heavier elements they had cooked inside their stellar furnaces. The death throes of these older stars also form the catalyst with which newer generations of stars and planetary systems are formed, now enriched in these heavy elements from their stellar progenitors. Our own Solar System also came into existence through this endless cosmic cycle of creation and destruction approximately 4.5 billion years ago, when a giant molecular gas cloud gravitationally collapsed, possibly from the explosion of a neighboring supernova.
Despite its intrinsic importance to stellar and planetary formation and evolution processes, interstellar dust was originally considered as a nuisance to astronomers, because of its light-obscuring properties. Every dust grain in the interstellar medium absorbs visible and ultraviolet light, which causes it to heat up and radiate in the far-infrared. As a consequence, it renders any background light sources invisible to ground-based telescopes, hiding whole parts of the sky from direct view. It wasn’t until the advent of infrared astronomy during the 20th century that scientists realised the real value of its study. Most of our somewhat limited knowledge of the properties of interstellar dust has come from remote astronomic observations of the interstellar medium and from spectroscopic measurements of the light it either scatters or emits. In addition, space-based, in-situ measurements taken with the dust detector instruments on board NASA’s Ulysses and Galileo spacecraft during the early 1990s revealed the presence of an interstellar dust stream flowing through the Solar System, as the latter orbits the galaxy. Based on these observations, scientists have proposed various theoretical models throughout the years in order to better explain the behaviour and overall dynamics of dust grains in the interstellar medium. Yet, in the absence of any samples of definite interstellar origin that could be thoroughly studied and analyzed, theoretical models until now could only give crude approximations regarding the physical properties and elemental abundances of interstellar dust grains.
Aiming to fill that void, NASA launched the Stardust mission in February 1999 with the goal of collecting the first-ever dust particles from the coma of comet Wild 2, as well as retrieve samples of interstellar dust. Following a 3.2-billion-mile, seven-year journey through the inner Solar System, the spacecraft successfully completed its mission with the soft-landing of its sample return capsule at the Utah Test and Training Range in January 2006 after a dramatic, steep reentry through the Earth’s atmosphere, while carrying with it the first-ever samples of material of extraterrestrial origin from beyond the Moon’s orbit. The latter were captured with the help of a tennis-racket-sized collector array of 132 tiles made of aerogel, an ultralight, porous, and transparent, silica-based material approximately 1,000 less dense than glass, and covered in aluminium foil. Stardust’s collector array was two-sided for avoiding the mixing together of the different types of dust particles in the same collecting area: The A side was pointed toward Wild 2 during the spacecraft’s close flyby of the comet in January 2004, and the B side which pointed toward the interstellar dust stream in the direction of the Ophiuchus constellation over the course of two collection periods between 2000 and 2002 that lasted a total of 195 days.
Every dust particle that hit the collector array at hyper velocity speeds of more than 10 km/sec was effectively decelerated by the aerogel tiles, which kept most of the particles safe from the destructive effects of their high-speed impact. Due to the aerojel’s extremely porous and lightweight nature, each incoming dust particle produced a narrow cone-shaped track on the tiles that was indicative of its trajectory and point of origin, with the cone representing the particle’s point of entry and the cone’s apex representing its final rest point inside the collector. This would allow researchers to determine from which direction in the sky every particle came, and help them separate those of an interstellar origin from those of an interplanetary one.
Parallel to the extensive analysis of the thousands of dust grains from Wild 2’s coma, which yielded some very fascinating and important results, the mission’s science team also began the painstaking process of identifying any interstellar dust grains that might have been captured by Stardust’s interstellar dust collector. To aid them in their efforts, the scientists initiated a citizen science project in 2006 called Stardust@home, inviting the public to participate in the study of the 132 aerogel tiles on Stardust’s collector array and the identification of any possible interstellar dust grains. To that end, the mission’s team used digital micrography to extensively image each individual tile of the collector array more than 40 times at several different focus depths, using a high-precision electronic microscope. Through this process, the mission’s team accumulated more than a million image frames of all 132 aerogel tiles and separated them into small movies, one for every tile, which it later distributed online through the Stardust@home project to more than 30,000 volunteers worldwide who have been visually inspecting them patiently for the last eight years, in the hopes of finally locating the long-sought-for interstellar dust grains.
Now, the Stardust mission team has published the preliminary results of the project’s progress in a new study appearing in the August 15 issue of the journal Science, reporting on the discovery of 71 particle tracks of interest, in 77 out of the total 132 aerogel tiles of the interstellar dust collector that have been scanned to date. Of the 71 particles, 69 were identified by volunteers participating at the Stardust@home project, and the other two were found by the mission’s science team. Following a careful examination with x-ray microscopy and infrared spectroscopy techniques, the researchers discovered that 46 of these tracks were secondary ejecta from impacts on the spacecraft’s aft solar panels. Of the remaining 25 tracks, those that could be studied were mostly found to be either ejecta of aluminium particles that had been extracted from the spacecraft due to micrometeorite impacts, or ejecta from impacts on the lid of the sample return capsule. Nevertheless, the researchers were able to identify three particle traces in the aerogel tiles (named Orion, Hylabrook, and Sorok, respectively, by their Stardust@home discoverers) that were consistent with an interstellar origin. More interestingly, these three traces revealed that their respective particles were quite larger than expected, being approximately two micrometers each and weighting between 3 and 4 picograms (trillionth of a gram). Furthermore, Orion and Hylabrook revealed a fluffy, crystalline structure to researchers similar to that of snowflakes, contrary to the predictions of theoretical models which had suggested that interstellar dust grains should be irregular, amorphous conglomerations of matter. “The fact that the two largest fluffy particles have crystalline material – a magnesium-iron-silicate mineral called olivine – may imply that these are particles that came from the disks around other stars and were modified in the interstellar medium,” says Andrew Westphal, a physicist at UC Berkeley’s Space Sciences Laboratory and lead-author of the study. “We seem to be getting our first glimpse of the surprising diversity of interstellar dust particles, which is impossible to explore through astronomical observations alone.”
In addition to the interstellar dust grain candidates that were found in the aerogel tiles, the researchers also found four additional particle tracks on the aluminium foil between the tiles, which wasn’t originally designed to act as a particle collector. Nevertheless, those four additional tracks were found to have a high probability of interstellar origin as well. Contrary to the large particles that were discovered in the aerogel tiles, these four particles were much smaller, ranging between 0.2 to 0.3 micrometeres in size, and, despite having a chemical composition that was generally consistent with expectations for interstellar dust, three of them were nevertheless found to contain sulfur compounds which astronomers have argued shouldn’t be present in interstellar dust grains. In general, all seven interstellar particle candidates showcase a surprising diversity in their physical properties, which wasn’t predicted by theoretical models. “They were splatted a bit, but the majority of the particles were still there at the bottom of the [impact] crater [at the aluminium foil],” said Stroud. “Their diversity was a surprise, but also these fluffy particles, sort of like a tossed salad, were complex, an agglomeration of other particles, rather than one dense particle suggested by the simplest models of interstellar particles.”
Despite these tantalising results, the researchers point out that additional analysis is needed before a definite confirmation of the particles’ interstellar origin can be made. “The combination of the elemental compositions of the seven interstellar dust candidates with their impact feature characteristics (i.e., track shape and direction, or crater morphology) demonstrates that they are extraterrestrial in origin,” writes the research team in its study. “However, further information is needed to distinguish between a possible interplanetary origin and an interstellar origin. The determination of origin cannot be based on elemental composition alone, because of the similarity of the solar nebula and the local interstellar medium in gas composition, and the overlap in range of temperature and pressure conditions at which dust condenses.” To that end, the researchers plan to continue their analysis of the dust particles, by measuring their relative oxygen isotope abundances using mass spectroscopy. “The most definitive indication of an interstellar origin for a particular particle would be an oxygen isotope composition inconsistent with solar system values,” concludes the research team. “All seven of the captured particles reported here are candidates, for which the oxygen isotope data are either not yet available, or are consistent with solar values. This means that although an interstellar origin cannot be definitively proven for the particles, other origins, including as interplanetary dust, have been determined to be statistically less likely than an interstellar origin.”
Since interstellar dust is generally destroyed within a few hundred million years after its formation, due to the effects of stellar radiation, the samples brought back the Stardust spacecraft—if their interstellar origin is eventually confirmed—have the potential to lead to revolutionary insights regarding the formation and evolution of stars and planetary systems and even life itself. “We can see this material with the naked eye as a black zone running along the center of the Milky Way,” says Dr. Donald Brownlee, a professor of astronomy at the University of Washington in Seattle, and principal investigator for the Stardust mission. “These particles contain the heavy chemical elements that originated in the stars. Since every atom in our bodies came from the inside of stars, by studying these interstellar dust particles we can learn about our cosmic roots.”
In the words of the late astronomer Carl Sagan, studies such are this are “a way for the Cosmos to know itself .”
Below are more electronic microscope pictures of the dust particles collected by Stardust: