using wafers made of different elements. The molecular structure of these wafers allows the collection of various sizes of particles. Collector arrays, such as those flown on Genesis, are ultra-pure in order to ensure maximal collection efficiency, durability, and analytical distinguishability. Collector arrays are useful for collecting tiny, fast-moving atoms such as those expelled by the Sun through the solar wind, but can also be used for collection of larger particles such as those found in the coma of a comet. The NASA spacecraft known as Stardust implemented this technique. However, due to the high speeds and size of the particles that make up the coma and the area nearby, a dense solid-state collector array was not viable. As a result, another means for collecting samples had to be designed to preserve the safety of the spacecraft and the samples themselves. Aerogel Aerogel is a silica-based porous solid with a sponge-like structure, 99.8% of whose volume is empty space. Aerogel has about 1/1000 of the density of glass. An aerogel was used in the Stardust spacecraft because the dust particles the spacecraft was to collect would have an impact speed of about 6 km/s. A collision with a dense solid at that speed could alter their chemical composition or perhaps vaporize them completely. Since the aerogel is mostly transparent, and the particles leave a carrot-shaped path once they penetrate the surface, scientists can easily find and retrieve them. Since its pores are on the nanometer scale, particles, even ones smaller than a grain of sand, do not merely pass through the aerogel completely. Instead, they slow to a stop and then are embedded within it. The Stardust spacecraft has a tennis-racket-shaped collector with aerogel fitted to it. The collector is retracted into its capsule for safe storage and delivery back to Earth. Aerogel is quite strong and easily survives both launching and outer-space environments. Robotic excavation and return Some of the riskiest and most difficult types of sample-return missions are those that require landing on an extraterrestrial body such as an asteroid, moon, or planet. It takes a great deal of time, money, and technical ability to even initiate such plans. It is a difficult feat that requires that everything from launch to landing to retrieval and launch back to Earth is planned out with high precision and accuracy. This type of sample return, although having the most risks, is