Asteroid (99942) Apophis

Asteroid (99942) Apophis was discovered on 2004 June 19 by R. A. Tucker, D. J. Tholen and F. Bernardi at Kitt Peak and then lost until it was serendipitously recovered in December of 2004. This Aten class asteroid, approximately 270 meters in diameter, caused a brief period of concern in December 2004 because initial observations indicated a small probability that it would strike the Earth in 2029. Thanks to additional observations, the 2029 threat was eventually ruled out. However, a possibility remained that during the 2029 close encounter with Earth, Apophis would pass through a gravitational  keyhole, a precise region in space no more than about a 600 meters wide, that would set up a future impact on 2036. The principal source of uncertainty is the acceleration due to the Yarkovsky effect, a non-gravitational effect that is a function of the asteroid's rotation period, pole direction, thermal properties, mass, shape, and dimensions. So this asteroid continues to receive considerable attention from amateur and professional astronomers.  

Apophis became (on 2004) the first object rated level 2 on the Torino scale, and for only a short time it was subsequently upgraded to level 4, the record for highest Torino rating. Currently Apophis has been downgraded to 0 in the Torino scale (the currently  highest-scaled objects are 2007 VK184 & 2011 AG5 at level 1).

The following citation is from MPC 54567: "(99942) Apophis = 2004 MN4. Also known as Apep, the Destroyer, Apophis is the Egyptian god of evil and destruction who dwelled  in eternal darkness.  As a  result of its passage within 40 000 km of the earth on 2029 Apr. 13, this minor planet will move from the Aten to the Apollo  class."

Apophis will make a very close Earth approach on April 13, 2029 at 2146UT when it will pass within 0.1 LD (Lunar Distances = ~384,000 kilometers)  or 0.00025 AU (1 AU = ~150 million kilometers). This will be the closest approach by something this large currently known. During the 2029 approach, Apophis' brightness will peak at magnitude 3.3, so it will be visible to the naked eye.

Before the 2029 encounter, Apophis will have a close approach with our planet on January 2013.  Actually, (99942) Apophis will have a close approach with Earth at about 37.6 LD (Lunar Distances = ~384,000 kilometers) or 0.0967 AU (1 AU = ~150 million kilometers) at 1143 UT on Jan. 09, 2013. On that date, the asteroid will reach the magnitude ~15.8 while moving at ~ 3.49"/min. According to Giorgini et al. (2008), it's likely that radar astrometry in 2013 will shrink the uncertainties sufficiently to completely rule out an impact in the 2030s.

We performed some follow-up measurements of this object just few weeks before its 2013 close approach, from the H06 ITelescope network (near Mayhill, NM) on  2012, Dec. 13.4, through a 0.25-m f/3.4 reflector + CCD. Below you can see our image, 19x30-second exposures, taken with the asteroid at magnitude ~17.5 and moving at ~2.57"/min. Click on the image below to see a bigger version. North is up, East is to the left.

Below you can see a short animation showing the movement of (99942) Apophis (each frame is a stack of 19x30-second exposures). North is up, East is to the left. Click on the thumbnail for a bigger version:

UPDATE - January 11, 2013

ESA’s Herschel space observatory made new observations of asteroid Apophis as it approached Earth this weekend. The data shows the asteroid to be bigger than first estimated, and less reflective. "Herschel provided the first thermal infrared observations of Apophis at different wavelengths, which together with optical measurements helped refine estimates of the asteroid’s properties. Previous estimates bracketed the asteroid’s average diameter at 270 ± 60 m; the new Herschel observations returned a more precise diameter of 325 ± 15 m".  The image below shows the asteroid in Herschel’s three PACS wavelengths: 70, 100 and 160 microns, respectively (click on it for a bigger version). For more info about Herschel's find click here.

Credit: ESA/Herschel/PACS/MACH-11/MPE/B.Altieri (ESAC) and C. Kiss (Konkoly Observatory)

After tracking asteroid 99942 Apophis with NASA's Goldstone radar dish, astronomers are now certain that the threatening asteroid has essentially no chance of striking Earth in 2036. See S&T article here for more details.

by Ernesto Guido & Nick Howes