What’s new at Stardust@home, the groundbreaking program that asked volunteers to help find interstellar dust particles collected by the spacecraft Stardust? “Things are good” said Andrew Westphal of the Space Sciences Laboratory at UC Berkeley, the project’s director. “We have positively identified more than 100 tracks” he reported, “each one representing a particle embedded in the collector.” The vast majority of them were found by Stardust@home volunteers. But there is more: 7 of these tracks just might be what Westphal and his team had been searching for all along: pristine particles of interstellar dust, picked up in the depth of space.
It has been a while since I last reported on the work of Westphal and his dedicated dusters, so here’s a quick refresher. On February 7, 1999, the spacecraft Stardust launched on a 7-year Odyssey to collect particles from deep space. Five years later, with its trademark tennis racquet-shaped collector extended, Stardust passed within 150 miles of core of comet Wild-2, flying through a hailstorm of pebbles and rocky debris ejected from the comet’s core only minutes before. The spacecraft then turned home, and on January 15, 2006, as it swooped by the Earth, it released a sealed capsule, which parachuted down to the Utah desert. Inside was the collector, containing the rare cometary particles, dating from the earliest days of the solar system. They have been keeping scientists busy ever since.
But cometary dust and rocks weren’t the only particles picked up by Stardust’s collector. On the way to its dramatic encounter with Wild-2 the spacecraft twice crossed a region of space between Mars and Jupiter, where a thin stream of dust particles from interstellar space flows through the solar system. These particles, formed in the hearts of distant stars, are sometimes referred to as “the building blocks of the universe,” and are of enormous interest to scientists. As it sped through, Stardust raised its collector in the hope of capturing a few of these pristine grains, as they travel through space. And so, when the sample return capsule landed in the desert it contained samples from Wild-2 on one side of the collector, and samples from the interstellar dust stream on the other. All that remained was for scientists to retrieve the dust grains from the collector, and study them in their laboratories. New insights about the origins of the solar system, and even the universe, were sure to follow.
But extracting the particles, as it turned out, was much easier said than done. Each side of the Stardust collector was made of 130 tiles of aerogel – an ultra-light semi-transparent substance sometimes called “frozen smoke.” To deep-space particles, traveling at speeds of up to 90 times the speed of sound, the aerogel was like a feather-bed, capturing without destroying them. Consequently, the particles collected by Stardust were all embedded deep inside the aerogel tiles, and had to be extracted before they could be studied. This was not so much a problem for the particles from comet Wild-2, which are both numerous and relatively large, and the craters they created in the aerogel could mostly be seen with the naked eye. Using computer-controlled blades, scientists cut out aerogel “keystones” containing the most interesting particles, and proceeded to study them systematically.
The interstellar dust grains were a different matter. They were far fewer than the cometary particles, and even the most optimistic estimates put their number at only a few dozen. To make things even harder for scientists, these particles are miniscule, just a few microns in size on average, some as small as one hundredth of a micron. Such particles are invisible to the naked eye, and even the tracks they made as they bore into the aerogel are miniscule and nearly indistinguishable from normal cracks. And how can scientists extract dust grains, not to mention study them, if they can’t even find them?
The problem seemed all but insoluble, until Westphal came up with a radical new idea. Inspired by his SSL neighbor, Dan Werthimer, who was running SETI@home from down the hall, he decided to turn to the public for help. What a few of hard-working scientists could not accomplish, and even computerized microscopes could not do, maybe the thousands of eyes of dedicated volunteers could. To make this possible, Westphal arranged to have the entire collector microscopically scanned, bit by bit, recording the image of each tiny portion of the aerogel. Each location was scanned at multiple depths, which together formed a “movie” of what it would look like to travel through the aerogel at that particular point. With help from The Planetary Society, Westphal archived the movies and made them available to anyone who was interested. Using a simple piece of software called a “virtual microscope,” dusters (as they came to be called) from all over the world could now scan the Stardust collector looking for the miniscule tunnels bored in the aerogel by miniscule dust grains traveling at a blistering speed. Stardust@home was born.
It has now been eight years since Stardust@home first launched in August of 2006. During this time an ever-increasing portion of the collector has been covered by the automated microscope at the Johnson Space Center, until fully one half of the aerogel surface has now been scanned and turned into movies. Training procedures for dusters have improved, control movies have been produced to test the skills of individual volunteers, and the dusters themselves have become ever more adept at distinguishing harmless cracks in the aerogel from possible trails of interstellar particles. And now, after thousands of movies and untold hours of patient work by thousands of dusters, the first results are finally trickling in.
To date, dusters have found 130 confirmed tracks in the interstellar dust collector, where particles had been captured and were now embedded in the aerogel. Using a computer-controlled needle and a technique perfected over time, the Stardust@home team have extracted a selected set of these particles from the collector, leaving them encased in tiny slivers of aerogel called “pico-keystones.” One additional track was discovered by Westphal’s science team at the edge of the collector, and it too was cut out, as were several others, detected in the collector’s aluminum foil by automated electron microscopy. Westphal and his team then took the aerogel particles to be analyzed for their composition at shopping-mall sized instruments called synchrotrons, one of which is in Berkeley. The particles in foils were analyzed by transmission electron microscopes.
The results of the synchrotron tests were revealing: all but two of the particles discovered by the dusters showed high levels of either cerium, or aluminum metal. This is significant because these elements are extremely rare in nature, and the chances that they would be found in interstellar dust particles are exceedingly slim. They are, however, plentiful on the Stardust spacecraft itself: the glass protecting the solar panels that are Stardust’s chief power-source are rich in cerium, and the lid of the sample return capsule (SRC), which would ultimately seal the capsule before delivering it to Earth, is made of machined aluminum. If particles containing these elements ended up in the aerogel collector, they almost certainly came not from space, but from the solar panels and the SRC lid.
But how did particles from the spacecraft end up embedded in the aerogel collector? This, explains Westphal, is the effect of micrometeoroids– tiny rocks traveling through space at high speed. As the spacecraft sped through space with its racquet-shaped collector extended, it was struck by a constant drizzle of these objects. Some of them struck the solar panels and the SRC lid at precisely the right angles to send shards flying into the collector. Analysis of the trajectories of the particles as they struck the aerogel confirmed that they indeed came from the direction of the solar panels and the aluminum lid. But to dusters, scouring tirelessly through movies of the aerogel, the tracks they left were indistinguishable from true interstellar dust grains.
This left only seven particles that could potentially be Interstellar Dust particles. Two of them were found by dusters Bruce Hudson and Naomi Wordsworth, who named them Orion and Hylabrook respectively. One was discovered at the edge of the collector by the science team, which named it Sorok. And four others were detected by automated electron microscopy in the aluminum foil. These last are miniscule, perhaps one thousandth the size of the particles found by dusters and likely too small to be detected in the aerogel. Appropriately for particles found by a machine, they have no names but are designated I1044N.3, I1061N.3, I1061N.4, and I1061N.5.
And with these seven grains, Westphal says, the signs are very promising. The trajectories of the particles captured in aerogel are consistent with particles from the interstellar stream, and so is the fact that they do not contain traces of elements from the spacecraft. All of this strongly suggests that these seven particles just might be the real thing: pristine interstellar dust grains, the building blocks of the universe.
Finding only seven particles in the portion of the collector scanned so far poses something of a riddle to scientists. Even if the total doubles when the entire collector is covered, it will still be well below the dozens of particles that were originally predicted. But those estimates, said Westphal, are based on the data from two spacecraft, Ulysses and Galileo, which took their measurements at a different location from where Stardust collected its samples. Furthermore, he insists, collecting only a handful of particles is perfectly consistent with measurements from Earth of the density of the dust stream – more so in fact than the measurements of Ulysses and Galileo. Scientists will now have to work to explain the discrepancy, but that is how science moves forward.
For most of us, the recent results from Stardust@home would be a cause for celebration: it seems almost certain that the volunteer dusters had succeeded in their mission of finding those elusive interstellar dust particles. Scientists, however, are a picky lot, and to them “almost certain” is still a far cry from “scientifically certain.” To reach scientific certainty, Westphal explained, the particles would have to undergo an analysis to determine the relative distribution of different isotopes within each grain. All matter that originates in our solar system has pretty much the same distribution of isotopes, since all of it emerged from the same primordial cloud of gas and dust, four and a half billion years ago. In particular, almost all the oxygen found in the solar system (over 99.7%) has a mass number of 16, although isotopes with mass numbers 17 and 18 occur as well. If a particle captured by Stardust showed a significantly different distribution of oxygen isotopes, then it is unquestionably not from the solar system. It is, rather, part of the interstellar dust stream, precisely the particle that Westphal and his team are looking for.
In order to determine the relative abundance of isotopes in a sample, scientists rely on an instrument called a mass spectrometer. Unfortunately, unlike the synchrotron, which analyzed the particles inside their pico-keystone, analysis by a mass-spectrometer would require that they would be completely removed from the aerogel. This, acknowledges Westphal, is a very tricky business, requiring use of a computerized diamond knife and infinite precision. “Any false move,” he explained, “could easily destroy the particle.” “If we had hundreds of dust grains,” he mused, “we might risk it; but with only 7 grains in existence, there is no room for error.”
Consequently, the members of the Stardust@home science team are taking it slowly. They are systematically developing the methods that would allow them to extract the dust particles from the aerogel and aluminum, gradually increasing their precision and perfecting their methods. Only when they have full confidence that their method is 100% accurate and reliable will they apply it to the Stardust particles themselves. And that, Westphal acknowledges, could take time.
Meanwhile Stardust@home is moving forward. More tiles are being scanned at the Johnson Space Center, more movies produced and made available. All over the world dusters are still inspecting them closely and noting anything that might look like an actual particle track. But for Westphal’s crew and dusters alike, something has nevertheless changed: Now, for the first time, they know that their work and patience over a span of years had born fruit. Stardust@home has almost certainly found the first interstellar dust particles ever harvested in space, and brought to Earth.