THE FOAM STORY – AEROGEL FOAM – The lightest solid in the world – Part 2_The Aerosol :its current and future applications.

At the time in the early 1980s, aerogels were so expensive to make that they could only live in labs where money was no object. CERN was one such lab, but soon NASA followed. The first applications of silica aerogels in space exploration were to insulate equipment from extreme temperatures. Aerogels are particularly suitable for this application because not only are they the best insulators in the world, but they are also extremely light and when you’re launching a spacecraft out of the gravitational pull of the Earth, reducing weight matters rather a lot.

Aerogel was used first in 1997 on the Mars Pathfinder mission and has been used as an insulator on spacecraft ever since. But once the scientists at NASA found that aerogel could cope with space travel they realized that the material had another possible use. If you look up into the sky on a clear night you might see a shooting star, which appears as a bright trail of light crossing the
sky. For a long time it has been known that these are meteors which enter the Earth’s atmosphere at high speeds and burn brightly as they heat up. It is thought that most of these are space dust, which is leftover material from the creation of the Solar System 4,5 billion years ago, along with comets and other asteroids.

Determining exactly what materials these heavenly bodies are made has been of interest for many years, since this information could help us understand how the Solar System was formed and may also account for the chemical composition of the Earth. Analyzing the material composition of meteorites has given us some tantalizing clues, but the problem with these specimens is that they have all been heated to extremely high temperatures by their passage through the atmosphere. Wouldn’t it be nice, the people at NASA thought, if they could capture some of these objects out in Space and bring them back to Earth in a pristine state?

The first problem with this idea is that objects in Space tend to be travelling rather fast. Space dust is often going at speeds of 50 kilometres per second, which equates to 18,000 kilometers per hour, a lot faster than a bullet. Catching an object like that is not easy. As with stopping a bullet with, say, your body either the force of the bullet exceeds the rupture pressure of your skin, meaning it goes through you, or you employ a bulletproof vest made of a high rupture strength material, such as Kevlar, which results in a compressed and deformed bullet. Either way, it’s a risky business.

However, in principle, it is quite possible – just as when catching a cricket ball or baseball with “soft” hands, the trick is to spread and dissipate the ball´s energy rather than bracing yourself for a single, high-pressure impact. What NASA needed, then, was a way to slow the dust down from 18,000 km/hour to zero without damaging the dust or the spacecraft – ideally a material with a very low density, so that the dust particles would be slowed gently without being damaged; ideally one that could do so within the space of a few millimetres; and ideally one that would be transparent, so that scientists could find the tiny specks of dust once they were buried in it. That such a material existed was a minor miracle. That NASA had already used it in space flights was extraordinary. It was, of course, silica aerogel.

The mechanism by which aerogel pulls off this feat is the same as the one used to protect stunt actors in movies when they fall off tall buildings: a mountain of cardboard boxes, each box absorbing some of the energy of the
impact as it collapses beneath the actor’s weight, and the more boxes, the better. In the same way, each foam wall within aerogel absorbs a tiny amount of energy when it is struck by the dust particle, but since there are billions of them per cubic centimetre, there are enough of them to bring it to a halt relatively unharmed. NASA built an entire space mission around the ability of
aerogel to gently collect stardust.

On 7 February 1999, the Starcecraft dust spacecraft was launched, containing all of the equipment necessary to take a trip through the Solar System, while also being programmed to by past a comet called Wild 2. The idea was that it would collect interstellar dust from deep space as well as the dust being ejected from a comet, allowing NASA to study the material composition of both. In order to do this, they developed a tool that resembled a giant tennis racquet, but instead of holes between the strings there was aerogel.

During the summer and autumn of 2002, while in deep space, millions of kilometres from any planet, the Stardust spacecraft opened a hatch and poked out its giant tennis racquet fitted with aerogel. It had no opponent in this game of interstellar tennis and the balls it was looking for were microscopically small: the remains of other stars long gone, the leftover ingredients of our own Solar System still flying around. The Stardust spacecraft
couldn´t hang around in deep space too long because it had an appointment to keep with the comet Wild 2 now hurtling from the outer reaches of the Solar System and approaching the centre, which it does every 6.5 years. Having withdrawn its aerogel tennis racquet, the spacecraft sped off for its meeting. It took over a year to get to the right position, but on 2 January 2004 the spacecraft found itself on a collision course with comet, which was five kilometres in diameter and speeding off around the sun. Once it had manoeuvred itself into the slipstream of the comet, 237 km behind it, the spacecraft opened its hatch and once again poked out its aerogel tennis racquet, this time using the B-side, and started to collect, for the first time in
human history, virgin comet dust. Having collected the comet dust, the Stardust spacecraft returned to Earth, arriving back two years later.

As it approached the Earth it veered away, jettisoning a small capsule, which fell under Earth’s gravity, entering the atmosphere at a speed of 12.9 krn/s, the fastest re-entry speed ever recorded, and so becoming for a while a shooting star itself. After 15 seconds of free-fall, and having reached red-hot temperatures, the capsule deployed a drogue parachute to slow down the rate of descent. A few minutes later, at a height of 3km above the Utah desert, the capsule jettisoned the drogue chute and deployed the main parachute. At this point the recovery crews on the ground had a good idea of where the capsule was going to land and headed out into the desert to welcome it back from its 7-year, 4-billion-kilometre round trip. The capsule hit the sand of the Utah desert at 10.12 GMT on Sunday, 15 January 2006.

“We feel like parents awaiting the return of a child who left us young and innocent, who now returns holding answers to the most profound questions of our solar system,” said the project manager, Tom Duxbury, of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

However, until they opened the capsule and started examining the aerogel samples scientists had no idea whether they held any answers to anything. Perhaps the space dust would have passed straight through the aerogel. Or perhaps the violence and deceleration of re-entry would have disintegrated the the aerogel into meaningless powder. Or perhaps there would be no dust at all. They need not have worried. Once they got the capsule back to the NASA laboratories and opened it up, they found that the aerogel was fully intact and almost completely perfect. There were minuscule puncture marks in the surface and it was these that were subsequently shown to be the entry points for the space dust. Aerogel had done the job that no other material could do: it had brought back pristine samples of dust from a comet formed before the Earth even existed.

Microscopic comet particle tracks in aerogel (NASA)

Since the return of the aerogel capsule, it has taken NASA’s scientists many years to find the tiny pieces of dust embedded within he aerogel, and the work continues to this day. The dust they are looking for is invisible to the naked eye, and so it must be found by microscopic examination of the samples, which has taken years. The project is so massive that NASA has enlisted the public to help with the search. The scheme Stardust@Home
trains volunteers to use their home computers to look through thousands of microscopie images of the aerogel samples and try to spot the signs that a piece of space dust is present. The work so far has thrown up a number of interesting results the most surprising of which is that most of the dust
from the comet Wild 2 shows the presence of aluminium-rich melt droplets. It’s very hard to understand how these compounds could have formed in a comet that had only ever experienced the icy conditions of Space, since they require temperatures of more than 12000 °C to do so. Since comets are thought to be frozen rocks that date back to the birth of the Solar System, this has come as a bit of a surprise to say the least. The results seem to indicate that the standard model of comet formation is wrong, or there is a lot more we don’t understand about how our Solar System formed.

Meanwhile, having completed its mission the Stardust spacecraft has now run out of onboard fuel. On 24 March 2011, when it was 312 million kilometres away from Earth, it responded to a final command from NASA to shut down communications. It acknowledged this command, and said its final goodbye. It is currently travelling off into deep space, a kind of man-made comet. Now that the Stardust mission is over, will this be the fate of aerogel too, to end in obscuriry? It is all too possible.

Although aerogels are the best insulators we have, they are very expensive and it is not clear that even now we care about energy conservation enough to value aerogels economically. There are several companies selling aerogel for such thermal insulation applications, but at the moment the main ones are for extreme environments such as drilling operations. It’s possible that, because of environment al considerations, our energy costs will get higher and higher. In a sufficiently high-cost energy future, it is conceivable that the monolithic double glazing we are all used to may be replaced with a much more sophisticated glass material based on aerogel technology. Research on developing new aerogels has been taking place at an increasingly rapid pace.

There are now a number of aerogel technologies that result in a material that is not rigid and brittle, as silica aerogels are, but flexible and bendy. These so-called x-aerogels are made flexible by a neat piece of chemistry that detaches the rigid foam walls of an aerogel from one another and inserts between them polymer molecules that act like hinges within the material. These x-aerogels can be made into flexible materials such as textiles and could be used to make the warmest but lightest blankets in the world, potentially replacing duvets, sleeping bags and the like.

Because they are so light they would also be perfect for outdoor clothes and boots designed for extreme environments. They could even replace the foam soles in sports shoes that make that type of footwear so springy. Recently, a family of carbon aerogels have been created which conduct electricity, as well as super-absorbent aerogels that can suck up toxic waste and gases. So the future of aerogels may yet be part of our everyday lives, the answer perhaps to living in a more extreme and volatile climate. But although as a materials scientist it’s good to know that we are likely to have the right materials to offer the world the event that global warming is not averted, this is not the kind of future I want for my children.

In a world where we have industrialized so many materials, including those ‘we used to hold sacred, such as gold and diamond, I like to think there may again be a place for a material valued solely for its beauty and significance. Most people will never hold a piece of aerogel in their hand, but those that do never forget it. It is a unique experience. There is no weight to it that you can perceive, and its edges fade away so imperceptibly that it is impossible to see
where the material stops and the air begins. Add to this its ghostly blue colour and it really is like holding a piece of sky in your hand. Aerogels seem to have the ability to compel you to search your brain for some excuse to be involved with them. Like an enigmatic party guest, you just want to be near them, even if you can’t think of anything to say. These materials deserve a different future, not of oblivion or embedded in a particle accelerator, but to be valued for themselves.

Aerogels were created out of pure curiosity, ingenuity and wonder. In a world where we say we value such creativivity and give out medals to reward its success, it’s odd that we still use gold, silver and bronze to do so. For If ever there was a material that represented mankind’s ability to look up to the sky and wonder who we are; if ever there was a material that represented our ability to turn a rocky planet into a bountiful and marvellous place; if ever there was a material that represented our ability to explore the vastness of the Solar System while at the same time speaking of the fragility of human existence, if ever there was a blue-sky material, it is aerogel.

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