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magine yourself lying in an open field on a warm summer evening, watching the sky as it darkens. Suddenly, a streak of light traces across the sky. As you continue to watch you see another, and then another. Over the course of the evening, you lose count of the number of streaks. The meteor shower you are watching offers evidence that the solar system is replete with small pieces of cosmic debris. Most of this material is too small to detect too small, that is, until it collides with Earth's atmos- phere and provides the celestial light show you are witnessing. Meteor showers are only one type of evidence of these bits of wandering rock. While you are counting the meteor trails, the Moon rises into the sky. Studying it with binoculars, you pick out some of the craters that cover the lunar surface craters caused by meteoroids hitting a surface without a protective atmosphere like Earth's. It rarely happens, but sometimes these objects do descend through Earth's atmosphere, impact the surface, and form craters like those seen on the Moon. Though not as numerous as lunar craters, the scars of such impacts are still evident on Earth's surface (see page 14~. Scientifically, the history of impacts on Earth is vital for understanding how the plan- et evolved and how life arose. For example, it has been suggested that most of the water on this planet was delivered by comet impacts (see the Kuiper Belt Objects section, pages 12- 13~. A better-known example of the role of impacts is the Cretaceous- Tertiary event 65 million years ago that led to global mass extinctions, including that of the dinosaurs. Impacts are not now as numerous as they were in the first billion years of the solar system's history, but the potential for a major impact is still there. A close examination of Earth's history shows that there is a 1 percent chance in the next century that Earth will be struck by an object large enough (greater than 300 meters in diameter) to cause significant damage. Current telescopic surveys have identified an estimated 50 percent of near-Earth objects (NEOs; asteroids and comets whose orbits cross that of Earth) that have a diameter of 1 kilo- meter or greater, and approximately 10 to 15 percent of objects between 0.5 and 1 km. NASA's current goal is to finish cataloging the objects larger than 1 km by 2008, but the agency has no formal plans to extend the search to smaller objects. Searching for NEOs demands an exacting observational strategy. To locate NEOs as small as 300 meters requires a survey down to 24th magni- tude (sensitive enough to detect objects 16 million times fainter than the feeblest stars that are visible to the The 19-km-long asteroid 951 Gaspra (above] and the 33-km-long 433 Eros (right] as seen, respectively, by Galileo from a distance of 5,300 km while en route to Jupiter and by the NEAR Shoemaker orbiter from a distance of about 200 km. Faze {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~

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large Synoptic Survey Telescope A simulation of a design for the Large Synoptic Survey Telescope. This ground- based telescope will survey the visible sky once each week. Profile Large Synoptic Survey Telescope Mission Type: Ground-based Facility Cost Class: Small Priority Measurements: SurveytheKuiperBelt. Survey the population of near-Earth objects down to 300 km in diameter. naked eye). Images have to be taken every 10 seconds to allow complete coverage of the sky in a reasonable amount of time, a necessary capability that is almost 100 times greater than that of existing survey telescopes. Furthermore, NEOs spend only a frac- tion of each orbit in Earth's neighbor- hood where they are most easily seen. Repeated observations over a decade would be required to explore the full volume of space populated by these objects. Such a survey would identify Guiding Themes Addressed Important Planetary Science Questions Addressed several hundred NEOs per night and obtain astrometric (positional) meas- urements on the much larger (and growing) number of NEOs already cat- alogued. Precise astrometry is needed to determine the orbits of the NEOs and to assign a hazard assessment to each object. Astrometry at monthly intervals would ensure against losing track of these fast-moving objects in the months and years after discovery. In its most recent decadal survey (gastronomy arid Astrophysics ir' the New Miller~r~ium, National Academy Press, Washington, D.C., 2001), the astrono- my and astrophysics community sin- gled out the proposed Large Synoptic Survey Telescope (LSST) as one of its highest-priority ground-based instru- ments. The SSE Survey echoed this finding and named the LSST as the solar system exploration community's top-ranked ground-based facility. Instruments like the Hubble Space Telescope and the Keck telescopes in Hawaii are designed to study selected, localized regions of the sky with very high sensitivity. Another type of tele- scope is needed to survey the entire sky relatively quickly, so that periodic maps can be constructed that display how objects change in position and/or appearance from week to week. The LSST is a 6.5-m-effective-diam- eter, very wide field (~3 degrees) tele- scope that will produce a digital map of the visible sky every week. For this type of survey observation, the LSST will be a hundred times more capable than the Keck telescopes, the world's largest at present. Not only will LSST carry out an optical survey of the sky far deeper than any previous survey, but also and just as importantly it will add the dimension of time and thereby open up a new realm of dis- covery. By surveying the sky each month for over a decade, LSST would revolutionize our understanding of various topics in astronomy concern- ing objects whose brightnesses vary on time scales of days to years. NEOs, which drift across a largely unchanging sky, are easily identified. The LSST could locate 90 percent of all near-Earth objects down to 300 m in size, enable computations of their orbits, and permit assessment of their threat to Earth. In addition, this facili- ty could be used to discover and track objects in the Kuiper Belt, the largely unexplored, primordial component of our solar system. Beyond the solar sys- tem, it would discover and monitor a wide variety of variable objects, such as the optical afterglows of cosmic gamma-ray bursts. In addition, it would find approximately 100,000 supernovae per year and be useful for many other cosmological observations. At this time, NASA has no system- atic survey capability to discover the population distribution of solar system bodies. The LSST would enable the compilation of a systematic inventory of near-Earth objects that is crucial to an improved understanding of Earth's cosmic environment, especially to the prediction of future hazards posed to our species. Many of the targets are as yet undiscovered, and construction of the LSST provides a necessary first step toward a rational spacecraft explo- ration program for these bodies. ~3

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