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To Catch a Falling StarThis Saturday, NASA will launch StarDust, a spacecraft designed
to sample and return material from the comet Wild 2 (below).
Above: Comet P/Wild-2 photographed
by K. Meech on Dec. 17, 1990 using an 88 inch reflector telescope
on Mauna Kea, Hawaii.
Right: Headed this way, comet Wild 2 in early February 1999 was between the orbits of Jupiter and Mars. StarDust will fly a high-speed trajectory that crosses just ahead of Wild 2 shortly after it crosses the orbit of Mars on its way back to the outer solar system. See comet Wild's orbit in 3D, from Liftoff! (requires java)
"I like to call this NASA's version of collecting bugs on the windshield," says Dr. David Noever, an aerogel researcher at NASA's Marshall Space Flight Center. "In actuality, aerogel is low enough density to collect the "bugs" without destroying them, even though the satellite will be traveling through the comet debris at 20 times the speed of a bullet. That's a soft touch." When the particles hit the aerogel, they will drill through the material, gradually slowing down, creating furrows that scientists will use to track the paths of the particles. Left: Looking like comets themselves, microscopic tracks in an aerogel sample lead to dust particles captured in ground tests to validate the StarDust "catcher's mitt." The particles coming off comet Wild 2 are projected to travel 6 km/sec, or 20 times the speed of a bullet. |
The
comet travels a path from just outside Jupiter's orbit to just
inside the orbit of Mars. StarDust will sweep through the comet's
coma, the ball of gas surrounding the nucleus of the comet, at
136,000 miles per hour. NASA will use an aerogel "catcher's
mitt" designed by Dr. Peter Tsou and his colleagues at the
Jet Propulsion Laboratory in Pasadena, Calif. Rather than catch
fly balls, this "mitt" will catch particles coming
off the comet. After a 7-year journey, StarDust will return to
Earth, dropping its newly obtained cargo onto the Utah desert.
Because the comet originally came from the Oort cloud, which
extends beyond the orbit of Pluto, the StarDust mission will
bring back matter from the deepest recesses of our solar system.
Passing through cometary
debris, however, can be more dangerous than standing in front
of a machine-gun. The particles coming off the comet will probably
be smaller than grains of sand, but they will be hitting the
aerogel at an extremely high velocity.
This
impact is so powerful that, with any substance other than aerogel,
the particles would either vaporize upon impact or become so
distorted that scientists couldn't study them.
Aerogel is the result of a friendly wager between Dr. Steven Kistler and Dr. Charles Learned, two Stanford University scientists. They competed to see if one of them could replace the liquid inside a jelly jar with gas without causing any shrinkage. Kistler won the bet, and published his findings in a 1931 edition of the journal Nature. But the potential of aerogel didn't come to light until the 1960s and 70s, when many in the aerospace industry were trying to develop an extremely lightweight, heat-resistant material to put on airplanes and spacecraft. To illustrate the impossibility of ever developing such a material, some in the aerospace industry dubbed it "unobtainium."
Right: NASA/Marshall's Dr. David Noever inspects an aerogel specimen. "The whole process is not that different from how coffee is decaffeinated," comments Noever. The end result is "frozen smoke," one of the lightest solid materials known. Researchers are currently developing techniques to dry aerogel under normal atmospheric pressure. Although aerogel looks like it could float away, it has very high compression strength for its mass. "But aerogel is not really so much of interest as a structural material," says Noever. "The real clincher is its incredible insulating effects on any kind of energy transfer: thermal, electrical or acoustic. Aerogel can damp out almost any kind of energy." |
Modern silica aerogel
starts as a liquid (mainly water, alcohol and silica), and then
gels into something that looks like Jello. Just as Kistler replaced
the liquid in jelly with gas without causing the jelly to shrink,
the trick to making areogel is to dry the "aero-Jello"
without collapsing it into a dense slab. This can be accomplished
by exchanging the alcohol with liquid carbon dioxide, and then
by removing the carbon dioxide at high pressure.|
That's the primary reason aerogel was used as insulation on the Sojourner Mars rover in 1997. As night fell on Mars, the temperature dropped down to -67 C (-88 F). Although the temperature outside was colder than Antarctica in winter, it remained a balmy 21 C (70 F) inside the Rover, where sensitive electronics were protected from the hard freeze. The Rover performed spectacularly, gathering information on the surface of Mars for almost three months following the Pathfinder's landing on July 4. A Toast to Jelly
So why aren't we currently using aerogel windows? First, because it is expensive to produce. Also, aeorgel is not perfectly transparent. Aerogel's slightly bluish cast presently makes it only practical for use in skylights or bathroom windows. According to Noever, "The holy grail of aerogel applications is transparent, superinsulating household windows." Left: Crayons - notorious for melting in the back seat when kids leave them in the car - don't even soften when a thin layer of aerogel stands between them and a propane torch. Aerogel, however, is a very fragile substance. In this example, it will begin to disintegrate as a result of its interaction with the blowtorch flame after a minute or so. |
Aerogel's superior
insulation can be used down here on Earth, as well. Currently,
a large portion of a home's heating bill literally goes out the
window. A single one-inch thick windowpane of silica aerogel
is equivalent to the insulation provided by 20 windowpanes of
glass (R-20 insulation factor). Aerogel is such a good insulator
that a house with aerogel windows could be kept warm and toasty
by using only a fraction of the energy currently needed. Aerogel
windows would help people save on their home heating bills,and
subsequently would reduce the world-wide production of carbon
dioxide and other greenhouse gases.|
The problem lies in the size of the material's pores. Most of the pores in aerogel are too small to scatter visible light, but once in a while a few of the pores are larger. The larger pores scatter light as it passes through aerogel, and this creates the hazy appearance. It is thought that the elimination of these larger pores would result in an aerogel of better optical quality. NASA is developing techniques to produce a clearer aerogel. There is evidence that the pore irregularities are diminished when the substance is manufactured in the microgravity environment of space. Experiments on suborbital rockets have shown that producing aerogel in space can reduce the number of large pores that form. "But the rocket only offered 7 minutes of reduced gravity and the results are not yet conclusive," says Dr. Laurent Sibille of the Universities Space Research Association, working at NASA/Marshall. "That is why we are repeating these experiments on the Space Shuttle." Aerogel experiments were conducted on the recent STS-95 mission with Senator John Glenn, and are planned for the upcoming STS-93 mission, which will also launch the Chandra X-Ray Observatory. "The goal of our research is to establish the role of gravity in the manufacturing of materials and suggest Earth-based techniques to modulate or counter these effects," states Sibille. By finding out how gravity affects the production of aerogel, researchers can develop better ways to produce it down here on Earth. As progress continues, we can look forward to a future where the use of clear aerogel insulation will better our chances that neither hazy windows nor atmospheric pollution will cloud our view of the stars. |
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| Web Links | |
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Stardust Mission home page -- from JPL The Science of Stardust -- from JPL Stardust Education web page -- from JPL Why comet Wild-2? -- from the JPL Stardust team The Solar System -- including comet Wild, from Liftoff! to Space Exploration Where is comet Wild-2 right now? -- from the JPL Stardust team, updated every 5 minutes Orbital elements of comet Wild-2 -- from the JPL Stardust team NASA office of Space Science -- news and research More space science headlines -- from Science@NASA |
read about the substance that will be used to capture comet dust particles -- from JPL Right Stuff for the Super Stuff - John Glenn will conduct tests with a space age super-substance called aerogel on STS-95. The house of the future?- Aerogel research could lead to super energy efficient homes. A
24 GHz computer for your desktop.- Aerogel could lead to ultra-fast
personal computers |
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