the majority of near-earth objects have orbits that don’t bring them very close to earth, and therefore pose no risk of impact, but a small fraction of them – called potentially hazardous asteroids – require more attention. these objects are defined as asteroids that are more than about 460 feet (140 meters) in size with orbits that bring them as close as within 4.6 million miles (7.5 million kilometers) of earth’s orbit around the sun. cneos continuously monitors all known near-earth objects to assess any impact risk they may pose.  the orbital positions of near-earth objects come from the databases of the minor planet center, the internationally recognized clearinghouse for small-body position measurements. this data is collected by observatories around the world, including significant contributions from amateur observers. the vast majority of asteroid-tracking data, however, is collected by large nasa-funded observatories (such as pan-starrs, the catalina sky survey, nasa’s neowise mission and, in the future, neo surveyor). planetary radar projects (including jpl’s goldstone solar system radar group) are another key component of nasa’s neo observations program.  the center for near-earth object studies is home of the sentry impact-monitoring system, which continuously performs long-term analyses of possible future orbits of hazardous asteroids. there is currently no known significant threat of impact for the next hundred years or more. the center also maintains the scout system that continually monitors brand-new potential near-earth object detections, even before they have been confirmed as new discoveries, to see whether any of these generally very small asteroids might pose a threat of short-term (possibly imminent) impact.  cneos also supports nasa’s planetary defense efforts by leading hypothetical impact exercises to help educate national and international space and disaster response agencies on the issues they would face in an actual asteroid impact scenario. the exercises inform scientists and key decision-makers as to the warning systems and impact mitigation strategies that could be employed in the event a threatening object is identified.  more information about near-earth objects: for more information about the center for near-earth object studies and to access close approach and impact-risk data for all known near-earth objects, see: https://cneos.jpl.nasa.gov/  to learn more about the planetary defense coordination office and nasa’s planetary defense strategy, go to: https://www.nasa.gov/planetarydefense/overview  for asteroid and comet news and updates, follow @asteroidwatch on twitter.
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