Complex sentence: Based upon the observed brightness, Apophis's diameter was initially estimated at; a more refined estimate based on spectroscopic observations at NASA's Infrared Telescope Facility in Hawaii by Binzel, Rivkin, Bus, and Tokunaga (2005) is.
Split into simpler sentences: Based upon the observed brightness, Apophis's diameter was initially estimated at. A more refined estimate based on spectroscopic observations at NASA's Infrared Telescope Facility in Hawaii by Binzel, Rivkin, Bus, and Tokunaga (2005) is.