Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 228

of this scale are not likely to have a serious effect upon the biosphere and cause
mass extinctions (Wetherill and Shoemaker, 1982). Events such as Tunguska, an
airburst about 8 km above Siberia, occur on a timescale of hundreds of years.
Impacts with Earth are thought to occur randomly in both space and time.
The next large impact with the Earth could be an 'impact-winter' producing
event or even a K--T-sized event (Earth Impact Database, 2003). To emphasise
this point, in March 1989 an asteroidal body named 1989 FC passed within
640 000 km of the Earth. This Earth-crossing body was not discovered until it
had passed the Earth (NASA, 2004). It was estimated to be in the 0.5 km size-
range, weighing 50 million tons and travelling at 74 000 km h − 1 .Anobject of this
size is capable of producing a Zhamanshin-sized crater or a devastating tsunami;
however, it missed the Earth by only 6 hours (Earth Impact Database, 2003). At
present, no systems or procedures are in place for mitigating the effects of an
asteroid impact. In February 2004, the 'Protecting the Earth from Asteroids' con-
ference intheUnited States concluded that the world's governments were not
prepared for a short-term threat from a potential asteroid impact. The confer-
ence recommended that future resources be channelled to developing response
strategies for an imminent impact and that all avenues to dislodge an asteroid
from its collision course (including nuclear weapons) should be pursued (NASA,
2004).
The asteroid, known as 1997 XF11, will pass within the Moon's distance of
the Earth in October 2028 (NASA, 2004). This distance may seem large in human
terms, but it is less than has previously been predicted in advance for any other
known asteroid during the foreseeable future.
Historical events
The most famous asteroid impact was 65 million years ago, when about
70% of all species then living on Earth disappeared within a very short period
(Figs. 9.1 and 9.2). The K--T impact occurred in the Yucatan region of Mexico. It is
thought that the asteroid was about 10 km wide. Chicxulub, the name given to
the asteroid, impacted at a velocity of 11 km s − 1 creating a 180 km diameter ring
structure (Zebrowski, 1997). The event threw huge amounts of matter into the
atmosphere and generated 600 m high waves that may have emptied the Gulf
of Mexico (Zebrowski, 1997). It is thought that the impact created months of
darkness and much cooler temperatures globally (Cockell, 1999). The relic impact
crater formed by Chicxulub is centred on the present coastline of Yucatan and
now lies beneath several hundred metres of sedimentary deposits. It was only
discovered in 1990 from geophysical data taken 10 years earlier while searching
for oil (Zebrowski, 1997; Earth Impact Database, 2003).