"Budget cuts have hit NASA's science missions hard. NEOCam is not certain to fly, and the foundation worries that, although NASA has already catalogued most of the biggest, civilisation-ending asteroids, thousands of smaller rocks, of similar dimensions to the one that exploded over Siberia, remain undetected. If one were to hit the wrong part of the planet it would cause a catastrophe. Hence the shift in focus from deflection to discovery. Sentinel's mission will be broadly similar to NEOCam's. Both telescopes will have 50cm mirrors. Both will scan the sky in the infra-red spectrum, where dark but comparatively warm asteroids should show up brightly against the cold of deep space. Both will inhabit orbits between Earth and the sun, in order to get the best possible vantage point. The foundation's ambition is to produce an asteroid map that records 90% of near-Earth objects that are more than 140 metres across, and half of those bigger than 50 metres. Armed with data on their orbits and velocities, astronomers should be able to calculate which pose a threat over the coming century or so."
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