Based upon the observed brightness, Apophis's 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 330 metres (1,080 ft) and lists a mass of 4×10¹⁰ kg based on an assumed density of 2.6 g/cm³. The mass estimate is more approximate than the diameter estimate, but should be accurate to within a factor of three.
Apophis's surface composition probably matches that of LL chondrites.

Based on Goldstone and Arecibo radar images taken in 2012–2013, Brozović et al.
have estimated that Apophis is an elongated object 450 × 170 metres in size, and that it is bilobed (possibly a contact binary) with a relatively bright surface albedo of 0.35±0.10.
Its rotation axis has an obliquity of −59° against the ecliptic, which means that Apophis is a retrograde rotator.

During the 2029 approach, Apophis's brightness will peak at magnitude 3.1, easily visible to the naked eye if one knows where to look, 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 ground-based telescopes not equipped with adaptive optics.
Due to the closeness of the approach, it is likely that tidal forces will alter Apophis's rotation axis.
A partial resurfacing of the asteroid is possible, which might change its spectral class from a weathered Sq- to an unweathered Q-type.
Based upon the observed brightness, Apophis's 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 330 metres (1,080 ft) and lists a mass of 4×10¹⁰ kg based on an assumed density of 2.6 g/cm³. The mass estimate is more approximate than the diameter estimate, but should be accurate to within a factor of three. Apophis's surface composition probably matches that of LL chondrites.
Based on Goldstone and Arecibo radar images taken in 2012–2013, Brozović et al. have estimated that Apophis is an elongated object 450 × 170 metres in size, and that it is bilobed (possibly a contact binary) with a relatively bright surface albedo of 0.35±0.10. Its rotation axis has an obliquity of −59° against the ecliptic, which means that Apophis is a retrograde rotator.
During the 2029 approach, Apophis's brightness will peak at magnitude 3.1, easily visible to the naked eye if one knows where to look, 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 ground-based telescopes not equipped with adaptive optics. Due to the closeness of the approach, it is likely that tidal forces will alter Apophis's rotation axis. A partial resurfacing of the asteroid is possible, which might change its spectral class from a weathered Sq- to an unweathered Q-type.
99942 Apophis