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Projects: Stardust@homeAerogel: The "Frozen Smoke" that Made Stardust Possibleby Amir Alexander As Stardust@home participants scan their movies through the virtual microscope, they are searching carefully for the telltale signs of interstellar dust particles. Generally, they don't give much thought to what is actually before their eyes the entire time: aerogel, that strange and wonderful material that made up Stardust's particle collectors. Each Stardust@home movie is an image of a tiny segment of an aerogel tile that was part of the Stardust collector. It had traveled seven years and three billion kilometers in space, through the ethereal stream of interstellar dust and the fierce pounding of particles from comet Wild 2, before landing safely in the Utah desert on January 15, 2006. The aerogel survived it all, and returned to Earth in near pristine condition, practically unchanged from the day it was launched – except for the precious particles it brought back with it. When those grains are finally extracted from the collector and begin to yield their cosmic secrets to expecting scientists, it will be aerogel that will make it all possible.
The Stardust particle collector was made of 260 tiles of aerogel, 130 on each side of a tennis racquet-shaped grid. One side was exposed during Stardust's flight through the interstellar dust stream between February and May of 2000 and August and December of 2002, the other received a barrage of cometary particles during the spacecraft's dramatic flight through Wild 2's comma on January 2, 2004. As the grains, racing through space, struck the collector at speeds of up to 90 times the speed of sound, the aerogel absorbed them like a cushion of feathers and brought them to a halt within the space of a few dozen microns (in the case of interstellar dust), or just 1 or 2 centimeters (in the case of the cometary particles). There is no other material known today that is capable of stopping and capturing particles traveling at such dazzling speeds in such a short distance, without destroying them in the process. If you have ever seen or held a piece of aerogel in your hand, you will always remember the ghostly feel of this remarkable substance. It is nearly transparent (unless deliberately dyed) but not quite, its outlines visible through their slight blue-grey tinge. When held, it feels just as unreal, nearly weightless but unmistakably solid, soft and spongy, yet ultimately well defined and unyielding. Aerogel of the type used on Stardust is 99.8% empty space, which explains why it made the Guinness Book of Records in 2002 as the lightest known solid. There really is nothing like it. Curious to learn more about the substance that made the Stardust mission possible I went straight to the source – Steven M. Jones of the Jet Propulsion Laboratory in Pasadena. Jones, working mostly on his own, has produced practically all of the aerogel used in NASA missions in the past 10 years, and that includes all of the Stardust collector tiles. What strikes one most when visiting Jones' laboratory is how unassuming the place is, considering how crucial its products are to the success of the space program. Composed of two adjacent rooms and the passageways between them, the lab is full of glass bottles, flasks and dishes, glass molds and liquid ingredients. It has little of the high tech electronics and flashing computer screens that one usually associates with the products of the space age. Jones supplies the explanation: "Aerogel has actually been around since the 1930's" he says to my surprise. It was developed by an academic, a physical chemist named Samuel Kistler, who was studying polymer networks. The methods Kistler used to produce aerogel have actually been around for even longer, having been developed as far back as the 19th century.
Aerogel, like glass, is made of silica explained Jones, in this case a substance called tetraethylorthosilicate, known more simply as TEOS. But whereas in order to make glass the silica found in sand is simply melted down, a more complex process is necessary to produce the porosity and low density of aerogel. The TEOS is mixed with a solvent to make a liquid, and then water and a base are added to the mix. With the base acting as a catalyst, the TEOS molecules form into long chains of silicon dioxide called polymers. In practice, this means that within a few hours, the clear liquid that was the TEOS mix has condensed into a gel. The gel is then placed in a metal cylinder that Jones calls the "supercritical solvent extraction vessel," where it is heated to 300 degrees Celsius and pressurized to 800 pounds per square inch. Under these conditions, the solvent within the pores of the gel, turns supercritical, which means it is held in a state that is neither liquid nor gas. If the temperature were lowered, the substance would return to its liquid state; if the pressure were lowered, it would turn to gas; but under these precise conditions, what was formerly liquid is held in the balance in a supercritical state. Once this state has been accomplished, and held in place for a while, Jones's computerized system begins gradually to lower the pressure in the cylinder, while holding the temperature steady. The supercritical fluid is sucked into a metal sphere that Jones calls the "solvent recovery tank.” When all the liquid had been sucked out of the cylinder, what is left is a light porous substance composed of silica– aerogel. Just to be on the safe side, Jones leaves the aerogel inside the tank for several hours, to let it the glass molds cool down slowly before they are exposed to the ambient temperature. Jones has something of a production routine for aerogel, which repeats itself every week: on Mondays he prepares the molds and mixes TEOS with the other ingredients. On Tuesdays the solutions are left to gel and placed in the supercritical extraction vessel. On Wednesdays the vessel is heated and pressurized, and on Thursday the vessel is gradually depressurized and then cooled down . The aerogel is then left in place overnight, before being taken out on Friday. And so the process repeats itself nearly every week, each time producing aerogel of different densities qualities and shapes, depending on what Jones was asked to produce.
Stardust was not the first space mission to use aerogel to capture particles in space. The Russian space station Mir used aerogel to collect microscopic grains of space dust, as did NASA's Spacelab II mission, which flew aboard the Space Shuttle Challenger in the summer of 1985. These however were simple types of aerogel, with a uniform density throughout. For Stardust, things were more complicated: for one thing, Jones had to produce different types of aerogel for each side of the collector: heavier and denser aerogel on the cometary side, which would be subject to a bombardment of relatively large grains, some even visible to the naked eye, and lighter more porous aerogel on the interstellar dust side, where the particles would be no more than a few microns in diameter. But even more challenging was the fact that the aerogel tiles would have to be of varying density: very light and porous on the surface of the collector, but increasingly dense towards the back. The particles, traveling at between 6 kilometers per second (for the cometary particles) and a dizzying 26 kilometers per second (for the interstellar dust particles) would enter the aerogel at the surface, and encounter increasing resistance to their passage, until they were stopped in their tracks several microns to several centimeters into the collector. Only aerogel of carefully controlled and varying density could accomplish this task, and Jones was the man to produce it. He was up to the task, and the Stardust aerogel tiles performed flawlessly on their long journey. The greatest surprise, as both Jones and Stardust team members have noted, is that the tiles seemed as clean and pristine on their return from their epic space journey as they did the day they were first placed in the collector.
Apart from being an ideal particle collector, aerogel has another, no less important but perhaps less glamorous role on space missions: heat insulation.Indeed, aerogel is an outstanding heat insulator, many times more effective than commercially used materials. This, says Jones, is the result of its internal structure. "When you insulate a wall, or a window, you do so by leaving a layer of air in the middle; aerogel does the same, only instead of one layer of air, it has billions of separate layers of air trapped inside its porous interior." In the 1940s. shortly after aerogel was first introduced, there were some attempts to make use of its remarkable properties for commercial purposes, such as insulating refrigerators and picnic coolers. It soon became apparent, however, that mass produced aerogel would cost twice as much as the standard commercial insulators. Not only that, but aerogel is too fragile to endure the kind of treatment usually accorded commercial materials. "You would have to be able to toss whole boxes of it off a truck" Jones said. "You just can't do that with aerogel." For space missions, however, aerogel is ideal. Its outstanding insulation properties are put to full use in the extreme conditions of space, while the vacuum and radiation it encounters affect it not at all. The amounts of aerogel required for the missions are also relatively small, so considerations of mass production and commercial handling do not apply. Aerogel was used successfully on the Mars Pathfinder lander mission in 1997, and is also the primary insulator of the Mars Exploration Rovers, Spirit and Opportunity, who landed on the Red Planet in January of 2004. The aerogel used on the rovers, however, was not the pretty bluish color of the Stardust collector, but rather a dark dirty-looking gray. This, explained Jones, is to prevent radiative heat transfer through the aerogel. To achieve the required effect, Jones added graphite to the usual aerogel mix, producing, in effect, black aerogel.
In addition to space missions, aerogel has also come to play a significant role in a very different field of human exploration – modern sculpture. The unreal texture of the material has attracted the attention of sculptors from around the world for use in their creations. "I get a steady stream of requests from artists" said Jones, who explained he does the work when he has the time. When I visited his lab, he was working on a pair of transparent aerogel cones, about 30 centimeters in height, for a sculpture by a well known London artist. A bluish human profile made of aerogel, seemingly complete, was waiting nearby. This one, said Jones, is heading overseas to an artist in Greece. And so, as Stardust@home users continue their search for microscopic particles through the virtual microscope, they might pause for a second and consider the remarkable material that is right before their eyes. This uncanny substance was developed in the early 20th century based on technology from the 19th, but seemed to have found its true vocation only in the past few decades, as a remarkably versatile resource in the space program. Aerogel has now orbited Earth, traveled through the solar system, and landed on Mars. By all indications, its journey has only just begun. |
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