page_title: 99942 Apophis
section_title: Physical characteristics
hierarchical_section_title: 99942 Apophis / Physical characteristics
caption_attribution_description: English: Comparison between the best-fit convex and nonconvex shape models, and the available radar images of (99942) Apophis. The best-fit nonconvex model is bilobed, providing a better fit to the data.
context_page_description: 99942 Apophis is a near-Earth asteroid with a diameter of 370 metres that caused a brief period of concern in December 2004 because initial observations indicated a probability of up to 2.7% that it would hit Earth on April 13, 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029. However, until 2006, a possibility remained that during the 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole, a small region no more than about 800 metres in diameter, that would set up a future impact exactly seven years later on April 13, 2036. This possibility kept it at Level 1 on the Torino impact hazard scale until August 2006, when the probability that Apophis would pass through the keyhole was determined to be very small and Apophis's rating on the Torino scale was lowered to zero. By 2008, the keyhole had been determined to be less than 1 km wide. During the short time when it had been of greatest concern, Apophis set the record for highest rating on the Torino scale, reaching level 4 on December 27, 2004.
context_section_description: 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.
caption: Comparison between the best-fit convex and nonconvex shape models, and some of the available radar images of (99942) Apophis
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