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. Additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029. However there remained a possibility that during the 2029 close encounter with Earth, Apophis would pass through a "gravitational keyhole", a precise region in space no more than about 400 meters across, that would set up a future impact on April 13, 2036. This possibility kept the asteroid at Level 1 on the Torino impact hazard scale until August 2006. It broke the record for the highest level on the Torino Scale, being, for only a short time, a level 4, before it was lowered.^[1]
Additional observations of the trajectory of Apophis revealed the "keyhole" would likely be missed and on August 5 2006, Apophis was lowered to a Level 0 on the Torino Scale. As of October 19 2006, the impact probability for April 13, 2036, is estimated at 1 in 45,000. An additional impact date in 2037 has been identified; however, the impact probability for that encounter is 1 in 12.3 million.
Despite the fact that there is no longer any significant probability of an Earth impact, the Planetary Society is offering $50,000 in prize awards for a few of the best plans to put a tracking device on or near the asteroid.^[2]

Contents

Basic data

Discovery

Naming

Close approaches

History of estimates

Possible impact effects

Popular culture

See also

References

Further reading

External links

Risk assessment

ESA

NASA

Older articles

More recent articles

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 350 m (1150 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 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.^[3].

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 (visible to the naked eye from rural and some darker suburban areas, visible with binoculars from most locations^[4]). 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.
On April 13, 2029, Apophis will pass the earth within the orbits of geosynchronous communication satellites. It will return for another close Earth approach in 2036.
Precovery observations from March 15, 2004 were identified on December 27, and an improved orbit was computed ^[5]. 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. The pass on April 13, 2036 was also determined to carry little risk.
It is important to note that 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 [the next good opportunity], we should be able to predict the location of out to at least 2070." said Jon Giorgini of JPL ^[6].
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. 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 ^[7].

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, 2004 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 of four.

★ Additional observations through 2006 resulted in Apophis being lowered to Torino Scale 0 on August 6 2006.

Possible impact effects

Since the odds of impact are known to be very low, the possible effects of an impact are largely irrelevant. However, the initial reports resulted in widespread discussion, including 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 struck Earth as the equivalent of 1480 megatons of TNT. A more refined later NASA estimate was 880 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 initiation of an impact winter.

Popular culture

★ In March 2007 heavy metal band Type O Negative released the album ''Dead Again'' which contains the track "The Profits of Doom". The song references Apophis and its potential collision in 2029 and uses it as a metaphor for the Biblical book of Revelation.

See also

★ List of noteworthy asteroids

★ Asteroid deflection strategies

★ Asteroid naming conventions

References

1. Near-Earth Asteroid 2004 MN4 Reaches Highest Score To Date On Hazard Scale Don Yeomans, Steve Chesley and Paul Chodas
2. Apophis Mission Design Competition The Planetary Society
3. (naming the asteroid and how Earth's gravity may change its trajectory in 2029)
4. ''The astronomical magnitude scale.'' Harvard-Smithsonian Center for Astrophysics
5. ''MPEC 2004-Y70 : 2004 MN4'' Minor Planet Electronic Circular, issued 2004-12-27
6. Friday the 13th, 2029 (Science@NASA article)
7.



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

Further reading

Cooke, Bill. (2006) Fatal attraction. ''Astronomy'', May 2006, 46-51.

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

★ Near-Earth Asteroid Reaches Highest Score To Date On Hazard Scale (JPL)

★ Possibility of an Earth Impact in 2029 Ruled Out for Asteroid (JPL)

★ Radar Observations Refine the Future Motion of Asteroid (JPL)

★ Animation explaining how impact risk is determined from [1]

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)

More recent articles

★ Astronauts propose 'tractor-pull' of asteroid

★ Closest Flyby of Large Asteroid to be Naked-Eye Visible (2029 approach)

★ Asteroid : A ''Really'' Near Miss! (''Sky and Telescope'')

★ An asteroid, headed our way (Christian Science Monitor, on efforts to deflect the asteroid if needed)

★ Asteroid Impact May Wipe out Life on Earth in 2035 (concern about 2029 orbit trajectory change resulting in later impact)

★ Astronauts want plan to prevent cosmic collision

★ Probabilistic Design of a Planetary Defense System