99942 Apophis

99942 Apophis (previously known by its provisional designation ) is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a relatively large probability that it would strike the Earth in 2029. However, additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029. An impact is still possible on April 13, 2036, keeping the asteroid at level 1 on the Torino impact hazard scale. As of June 2006, this corresponds to an impact probability of 1 in 38,000.

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 kg.

As of February 2005 it is predicted that the asteroid will pass just below the altitude of geosynchronous satellites, which are at 35,786 km (22,300 mi). Apophis' brightness will peak at magnitude 3.3, with a maximum 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 ~2 arcseconds, which means it will be a starlike point of light in all but the very largest telescopes.

Discovery

Apophis was discovered on June 19, 2004, by Roy A. Tucker, David J. Tholen, and Fabrizio Bernardi of the NASA-funded University of Hawaii Asteroid Survey from Kitt Peak National Observatory in Arizona. This group observed for two nights. The new object received the provisional designation .

On December 18, the object was rediscovered from Australia by Gordon Garradd of the Siding Spring Survey, another NASA-funded NEA survey. Further observations from around the globe over the next several days allowed the Minor Planet Center to confirm the connection to the June discovery.

Naming

When first discovered, the object received the provisional designation (sometimes written 2004 MN4), and news and scientific articles about it referred to it by that name. When its orbit was sufficiently well calculated it received the permanent number 99942 (on June 24, 2005), the first numbered asteroid with Earth-impact solutions (to its orbit determination from observations). Receiving a permanent number made it eligible for naming, and it promptly received the name "Apophis" as of July 19 2005. Apophis is the Greek name of the Ancient Egyptian god Apep, "the Destroyer", who dwells in the eternal darkness of the Duat (underworld) and tries to destroy the Sun during its nightly passage.

Although the Greek name for the mythical Egyptian god may be appropriate, Tholen and Tucker (two of the co-discovers of the asteroid) are reportedly fans of the TV series Stargate SG-1. The show's main antagonist in the first several seasons was an alien named Apophis who took the name for the Egyptian god and sought to destroy Earth (naming the asteroid and how Earth's gravity may change its trajectory in 2029).

Close approaches

After the Minor Planet Center confirmed the June discovery of Apophis (then known only as ), the next close approach was computed to be April 13, 2029 by the automatic Sentry system of NASA's Near-Earth Object Program Office. NEODyS, a similar automatic system at the University of Pisa, Italy and the University of Valladolid, Spain also calculated this same approach date. On that date, it will become as bright as magnitude 3.3 (easily visible to the naked eye). This close approach will be visible from Europe, Africa, and western Asia. As a result of its close passage, it will move from the Aten (see below) to the Apollo class.

After the June discovery was confirmed, observatories throughout the world computed the probability of impact. Over the next several days, additional observations allowed for astronomers to narrow the cone of error. As they did, the probability of an impact event climbed, peaking at 2.7% (1 in 37). Combined with its size, this caused Apophis to be assessed at level four out of ten on the Torino Impact Hazard Scale and 1.10 on the Palermo scale, scales scientists use to represent the danger of an asteroid hitting Earth. These are the highest values for which any object has been rated on either scale.

Precovery observations from March 15, 2004 were identified on December 27, and an improved orbit was computed MPEC 2004-Y70 : 2004 MN4 Minor Planet Electronic Circular, issued 2004-12-27. Radar astrometry further refined the orbit. The 2029 pass will actually be much closer than the first predictions , but the uncertainty is such that an impact is ruled out. A pass on April 13, 2036 still carries some risk.

Apophis remains at level one on the Torino scale because of a very low but non-zero probability of impact in 2036. However, the close approach in 2029 will substantially alter the object's orbit, making predictions uncertain without more data. "If we get radar ranging in 2013 next good opportunity, we should be able to predict the location of out to at least 2070." said Jon Giorgini of JPL Friday the 13th, 2029 (Science@NASA article).

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 400 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 .

History of estimates

The original NASA report on December 24, 2004, 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%), 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%) and later that day (based on 101 observations) as 1 in 45 (2.2%). 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%), 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%); 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%, a lower risk than asteroid , 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, 2005, 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, 2005, refines the orbit further and eliminates the possibility of an impact in 2035. Only the pass in 2036 remains at Torino Scale 1.
A new radar observation at Arecibo Observatory on May 6 2006 slightly lowered the Palermo scale rating, but the pass in 2036 remained at Torino Scale 1 despite the impact probability dropping by a factor or four.

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 ) 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 NASA estimate was 880 megatons, which is still around 65,500 times the energy of the bomb mentioned above. 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.

References

Fountain, Henry (27 November 2005). "Saving Earth from the next heavenly blow". New Straits Times, p. F24.

External links

Risk assessment

These sources are updated as new orbital data becomes available:

Impact Risk (NASA JPL)
page and impactor table from NEODyS.

ESA

May we deflect asteroids? (Advanced Concepts Team)

NASA

Older articles

Whew! Asteroid Won't Hit Earth in 2029, Scientists Now Say (SPACE.com)
Asteroid Watch: Odds of 2029 Collision Stuck at 1-in-40 (SPACE.com)
Worrisome Asteroid Underscores Planetary Defense Mission (SPACE.com)

Basic data

Discovery

Naming

Close approaches

History of estimates

Possible impact effects

See also

References

Further reading

External links

Risk assessment

ESA

NASA

Older articles

More recent articles

Gourt Home::Gourt :: 99942 Apophis

Basic data

Discovery

Naming

Close approaches

History of estimates

Possible impact effects

See also

References

Further reading

External links

Risk assessment

ESA

NASA

Older articles

More recent articles