99942 Apophis Asteroid, possible Impact with Earth in 2036
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99942 Apophis Asteroid
99942 Apophis (previously better known by its provisional
designation 2004 MN4) 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. A future impact on April 13,
2036, is still possible, keeping the asteroid at level 1 on the Torino impact
hazard scale as of September 2005, with an estimated impact-probability
of 1 in 5,560.
Apophis is expected to come close enough that on April 13, 2029 (Friday
the 13th) 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. Throughout recorded history, no other closely-approaching objects
of this size have been visible to the naked eye. As a result of its close
passage, it will move from the Aten (see below) to the Apollo class.
(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
When first discovered, the object
received the provisional designation 2004 MN4 (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. 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 mythical Greek 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. In the first several seasons the show's main antagonist
was an alien named Apophis who took the name for the Egyptian god and sought
to destroy earth. (see "Asteroid Apophis set for a makeover" by Bill Cooke,
Astronomy Magazine, August 18, 2005
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
(The white bar indicates uncertainty in the range
of possible positions.)
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 2004 MN4, and naturally did not yet have its current
name.
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.
At this point the possibility of impact on April 13, 2029 was computed
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 detected the impact possibility
and provided similar predictions.
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 percent (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. The 2029 pass will actually be
much closer than the first predictions (as close as a geosynchronous satellite),
but the uncertainty is so much smaller that the risk of impact then went
away. A pass on April 13, 2036 still carry some risk. As of September 2005,
the odds of impact on that date is 0.018 percent (1 in 5,560) and the Palermo
scale rating for Apophis is −1.35 and the Torino scale rating is 1.
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.
http://en.wikipedia.org/wiki/99942_Apophis
http://en.wikipedia.org/wiki/Wikipedia:Copyrights
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