Aerogel Helps Scientists Unravel Mysteries of Comets
Strange stuff called 'aerogel' that looks like a semi-transparent, blue cloud, but that is solid, is carrying captured comet dust to Earth for a Jan. 15, 2006, landing in a Utah desert.
In January 2004, the Stardust spacecraft flew within 147 miles (236 kilometers) of the comet Wild 2 (VILT-TWO) and survived the high-speed impact of millions of dust particles and small rocks up to nearly two-tenths of an inch (one-half centimeter) across. With its tennis-racket-shaped collector extended, Stardust captured thousands of comet particles in the see-through aerogel, which includes silica and oxygen.
Aerogel samples: effects of light scattering off the microstructure + View Larger Image
"It's a little bit like collecting BBs by shooting them into Styrofoam," said Scott Sandford, an astrophysicist and Stardust mission co-investigator based at NASA Ames Research Center in California's Silicon Valley. "Some of the grains are likely to have exotic isotopic ratios that will give us an indication that we're looking at materials that aren't as old as the solar system, but that are, in fact, older than the solar system," Sandford asserted.
Another mission objective was to expose the spacecraft to the interstellar dust stream for 150 days to grab particles. After collecting them, the aerogel collector retracted into the spacecraft's capsule. Stardust will be the first mission to capture and return a substantial sample from outside Earth's moon system.
Making sure that precious comet and interstellar particles imbedded in the aerogel are not affected by earthly contaminants was an important task to complete before the Stardust spacecraft was launched on Feb. 7, 1999, from Cape Canaveral Air Station, Florida. aboard a Delta II rocket.
"Under Dr. Sandford's guidance, I performed the lab analysis of the aerogel using infrared (IR) light to determine the level of organic contamination," said Max Bernstein, a scientist at NASA
Ames. "These and other preliminary lab tests ultimately led the Stardust aerogel development team to devise a bake-vacuum-bake cycle to reduce the carbon content in aerogel," Bernstein said.
"Aerogel is made mostly of sand (silica), and what we're interested in is the organic material in the cometary samples," Bernstein said. "We measured organic contamination in aerogel early on. We raised a concern, and Peter Tsou and the aerogel team at the Jet Propulsion Laboratory in Pasadena, Calif., devised a method to reduce carbon content in aerogel by a factor of 10."
Infrared light that astronomers use to detect organic molecules in space also can be used to measure organic molecules in the laboratory. In their laboratory, Ames scientists shined IR light though a piece of an early batch of test aerogel, and they saw organic contamination. Because infrared is light that is not visible to the human eye, scientists use special detectors to 'see' IR. If scientists detect a specific IR color scheme, they can tell that a specific molecular fragment is moving and is present in the sample of material they are examining.
"If you understand that color scheme, then when you make the measurement, you can say, 'ah hah, I spotted colors corresponding to a carbon-hydrogen motion, so there must be carbons and hydrogen in the aerogel, not just silicon and oxygen,'" Bernstein explained. "Thanks in part to our measurements, we now have cleaner aerogel, which is flying on the Stardust spacecraft."
In cooperation with Bernstein, graduate student Maegan K. Spencer of Stanford University, Stanford, Calif., is conducting more sophisticated aerogel organic contamination tests in the laboratories of Stanford Professor Richard Zare.
The returning Stardust capsule will strike Earth's atmosphere at eight miles (12.8 kilometers) per second - more than 10 times faster than a speeding bullet. That is fast enough to go from San Francisco to Los Angeles in only one minute. The 101-pound (45.7 kilogram) conical object will hurtle through the atmosphere and slow before the spacecraft finally parachutes down to Earth in a Utah dry lake. The landing will occur on Sunday, Jan. 15, 2006, at about 3 a.m. MST, in a restricted area - the Utah Test and Training Range, located southwest of Salt Lake City.
"There will be a team of scientists at Johnson Space Center who will assess what we actually got back from the comet so we can verify we did get a useful sample," Sandford said. "A small portion of the samples will then be used to make a preliminary study of the returned material. After the preliminary examination is complete, all the samples will be made available to the general scientific community for more detailed study. My guess is people will be asking for and working on these samples for decades to come."
NASA Ames Research Center, Moffett Field, Calif.