Physical Characteristics
Based upon the observed brightness, Apophis' diameter was initially estimated at 450 metres (1,480 ft); a more refined estimate based on spectroscopic observations at NASA's Infrared Telescope Facility in Hawaii by Binzel, Rivkin, Bus, and Tokunaga (2005) is 350 metres (1,150 ft). NASA's impact risk page lists the diameter at 0.270 km, or 270 metres (890 ft) and lists a mass of 27 megatonnes based on an assumed density of 2.6 g/cm3. The mass estimate is somewhat more rough than the diameter estimate, but should be accurate to within a factor of three.
During the 2029 approach, Apophis’ brightness will peak at magnitude 3.3, with a maximum angular speed of 42° per hour. The maximum apparent angular diameter will be ~2 arcseconds, so that it will be barely resolved by telescopes not equipped with adaptive optics.
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