(Close approach of Apophis on April 13th, 2029)

Apophis remains at level one on the Torino scale because of a very low but non-zero probability of impact in 2036. However, the approach in 2029 will substantially alter the object's orbit, making predictions uncertain without more data. "If we get radar ranging in 2013 [the next good opportunity], we should be able to predict the location of 2004 MN4 out to at least 2070." said Jon Giorgini of JPL

In July 2005, former Apollo astronaut Rusty Schweickart, as chairman of the B612 Foundation, formally asked NASA to investigate the possibility that the asteroid's post-2029 orbit could be in orbital resonance with the Earth, which would increase the probability of future impacts. For this to happen, Apophis would have to pass precisely through a certain very narrow region of space during the 2029 close approach, a "gravitational keyhole" no more than about 600 m across. Schweickart asked for an investigation of the necessity of placing a transponder on the asteroid for more accurate tracking of how its orbit is affected by the Yarkovsky effect.

Naming

Basic data

Apophis belongs to a group called the "Aten asteroids", asteroids with an orbital semi-major axis less than one astronomical unit. This particular one has an orbital period about the Sun of 323 days, and its path brings it across Earth's orbit twice on each passage around the Sun.

Based upon the observed brightness, Apophis's length was estimated at 415 m (1350 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 320 m (1050 ft). Its mass is estimated to be 4.6×1010 kg.

As of February 2005 it is predicted that the asteroid will pass about 36,350 km (22,600 mi) from the Earth's surface on April 13, 2029, slightly higher than the altitude of geosynchronous satellites, which is 35,786 km (22,300 mi). Its brightness will be about magnitude 3.3, with a peak angular speed of 42° per hour. Such a close approach by an asteroid of this size is expected to occur only every 1,300 years or so. The maximum apparent angular diameter will be only 2 arcseconds, which means it will be a starlike point of light in all but the very largest telescopes

Discovery

History of estimates

• The original NASA report on December 24 mentioned impact chances of "around 1 in 300", which was widely reported in the media. The actual NASA estimates at the time were 1 in 233; they resulted in the Torino scale rating of 2, the first time any asteroid had received a rating above 1.
• Later that day, based on a total of 64 observations, the estimates were changed to 1 in 62 (1.6 percent), resulting in an update to the initial report and an upgrade to a Torino scale rating of 4.
• On December 25, the chances were first reported as 1 in 42 (2.4 percent) and later that day (based on 101 observations) as 1 in 45 (2.2 percent). At the same time, the asteroid's estimated diameter was lowered from 440 m to 390 m and its mass from 1.2×1011 kg to 8.3×1010 kg.
• On December 26 (based on a total of 169 observations), the impact probability was still estimated as 1 in 45 (2.2 percent), the estimates for diameter and mass were lowered to 380 m and 7.5×1010 kg, respectively.
• On December 27 (based on a total of 176 observations), the impact probability was raised to 1 in 37 (2.7 percent); diameter was increased to 390 m, and mass to 7.9×1010 kg.
• On December 27 in the afternoon, a precovery increased the span of observations to 287 days and allowed more accurate calculations to re-rate the asteroid's 2029 approach as level zero on the Torino scale (no threat). The cumulative impact probability was estimated to be around 0.004 percent, a lower risk than asteroid 2004 VD17, which once again became the greatest risk object (a position it had held since late November 2004). A 2053 approach to the earth still poses a minor risk of impact, and Apophis was still rated at level one on the Torino scale for this orbit.
• On December 28 at 12:23 GMT and (based on a total of 139 observations), produced a value of one on the Torino scale for 2044-04-13.29 and 2053-04-13.51.
• By 01:10 GMT on December 29 the only pass rated 1 on the Torino scale was for 2053-04-13.51 based on 139 observations spanning 287.71 days (2004-Mar-15.1104 to 2004-Dec-27.8243).
• By 19:18 GMT on December 29 this was still the case based upon 147 observations spanning 288.92 days (2004-Mar-15.1104 to 2004-Dec-29.02821), though the close encounters have changed and been reduced to 4 in total.
• By 13:46 GMT on December 30 no passes were rated above 0, based upon 157 observations spanning 289.33 days (2004-Mar-15.1104 to 2004-Dec-29.44434). The most dangerous pass was rated at 1 in 7,143,000.
• By 22:34 GMT on December 30, 157 observations spanning 289.33 days (2004-Mar-15.1104 to 2004-Dec-29.44434). One pass at 1 (Torino Scale) 3 other passes.
• By 03:57 GMT on January 2, 182 observations spanning 290.97 days (2004-Mar-15.1104 to 2004-Dec-31.07992) One pass at 1 (Torino Scale) 19 other passes.
• By 14:49 GMT on January 3, 204 observations spanning 292.72 days (2004-Mar-15.1104 to 2005-Jan-01.82787) One pass at 1 (Torino Scale) 15 other passes.
• Extremely precise radar observations at Arecibo Observatory on January 27, 28, and 30 refine the orbit further and show that the April, 2029 close approach will occur at only 5.6 Earth radii, approximately one-half the distance previously estimated.
• A radar observation on August 7 refines the orbit further and eliminates the possibility of an impact in 2035. Only the pass in 2036 remains at Torino Scale

Possible impact effects

It must be stressed that the odds of impact are now known to be very low. Hence, the possible effects of an impact are largely irrelevant.

However, the initial reports resulted in widespread discussion on many Internet forums, including armchair speculation about exactly where Apophis (then known only as 2004 MN4) would hit and what would happen when it did.

NASA initially estimated the energy that Apophis would have released if it impacted Earth as the equivalent of 1480 megatons of TNT (114,000 times the energy from the nuclear bomb Little Boy, dropped by the United States on Hiroshima, Japan). A more refined later estimate was 850 megatons. The impacts which created the Barringer Crater or caused the Tunguska event are estimated to be in the 10-20 megaton range. The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons.

The exact effects of any impact would have varied based on the asteroid's composition, and the location and angle of impact. Any impact would have been extremely detrimental to an area of thousands of square kilometres, but would have been unlikely to have long-lasting global effects, such as the precipitation of an impact winter.

Based on the predicted time of impact (0.89 of a day, or about 21:20 UTC) and the fact that the asteroid would be approaching the Earth from outside of its orbit, the impact was likely to occur in the Eastern Hemisphere (time zones UTC +3 to UTC +10).

Any potential impact would have occurred at a velocity of 12.59 km/s.