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February 26, 2013
As the recent Russian Chelyabinsk meteor impact and (unrelated) close passing of Asteroid 2012 DA14 remind us, the Earth shares its Solar System neighborhood with many other moving objects, some of which are potentially harmful to our planet.
NASA's NEOWISE survey indicates that there are about 20,000 near‐Earth asteroids (NEAs) with sizes of 100‐1000 meters, only 22% of which have already been catalogued. Asteroid 2012 DA14 (~46m) is even smaller, and there may be hundreds of thousands of uncharted NEAs of similar size. Fewer than 10,000 NEAs of all sizes have been identified to date through the efforts of ground‐based observatories and space missions.
LSST will play a significant role in planetary defense against these objects. Its large‐diameter optics, 3200Mpix camera, and sophisticated data processing system will provide both early detection and orbit determination for fainter and smaller objects at greater distances than currently possible. During its 10‐year survey, LSST will provide orbital parameters for more than 100,000 of these NEAs allowing us to be better prepared for the potentially devastating events that are sure to come in the future.
The warning time before impact depends on the asteroid's size, its orbit, and the cadence and sensitivity of the observing system. For 45m objects, the LSST warning time would be about 1‐3 months, depending on their orbits. Note that LSST could also detect such an object during three prior close approaches. Simulations show that after 10 years of surveying, LSST would detect about 50% of the Potentially Hazardous Asteroids (PHAs) down to this size. As an example of a very different hazardous object ‐ the 3 km large comet C/1996 B2 Hyakutake, which passed within 0.10 AU from Earth in 1996, LSST could provide a warning time of 8 years, with over 500 observations over that period.
The chances of an object that caused the 1908 Tunguska event impacting Earth are about once every 100 years. But probability can be misleading: such an improbable event could happen next year. The threat of asteroid impact is serious enough that Congress directed NASA in 2005 to implement a survey that would catalog 90% of PHAs larger than 140 meters in 15 years, the George E. Brown Jr. NEO Survey Act. With an optimized design and operations for the purpose of NEA detection and characterization, this congressional mandate could be reached within 12 years of LSST science operations; details here.
LSST will help assess the hazard to Earth from asteroid impacts by constraining the orbital and size distribution of the near‐Earth population, allowing concrete estimates of the impact frequency as a function of size. LSST's ground‐based optical observations complement potential space observations in the infrared, helping us determine more accurate sizes and composition of these objects. LSST will also identify potential targets for spacecraft missions, searching for objects with small relative velocities to Earth.
A recent National Research Council report1 found that LSST is the most cost effective way of detecting the most likely and potentially most damaging Earth threatening objects.
1 National Research Council. Defending Planet Earth: Near‐Earth Object Surveys and Hazard Mitigation Strategies. Washington, DC: The National Academies Press, 2010.
