CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 577
and astronomical time scales. For exam-
ple, astronomers estimate an Apollo
object hits Earth once every 250,000
years on average. A typical Apollo with
a diameter of 1 km would strike with
the power of a 100,000-megaton bomb
and dig a crater more than 10 km in
diameter. Th e good news is that we are
certain that no known Apollo object
will hit Earth in the foreseeable future;
the bad news is that there are about
1000 of them 1 km in size or larger.
Asteroid 2004 MN4 was initially
predicted to have a 2.6 percent chance
of striking Earth in 2029. Th at object
is about 400 m ( 1 __
4mile) in diameter,
large enough to do signifi cant damage
over a wide area but not large enough to
alter Earth’s global climate. Fortunately,
further observations and calculations
revealed that the object will not hit
Earth. Th ere will be no impact by 2004
MN4 in 2029, but there are plenty
more asteroids in Earth-crossing orbits
to be discovered. For example, a rock
designated 2009 DD45, estimated to
have been 30 m (100 ft) in diameter, about the size of the
Tunguska impactor, passed only 64,000 km (40,000 mi) from
Earth in March 2009. Th at is only twice the distance of human-
ity’s geosynchronous communications and weather satellites, aabcVisual-wavelength image■ Figure 25-22
(a) Closeup image of the Comet Shoemaker–Levy 9 fragment train on the way
to colliding with Jupiter. (b) A 40-km-long crater chain on Earth’s moon,
and (c) a 140-km-long crater chain on Jupiter’s moon Callisto, probably
formed by the impact of fragmented comet nuclei similar to Shoemaker–Levy
- (NASA)
(a)(b)MexicoUnited StatesYucatánChicxulub crater300 km■ Figure 25-23
The giant impact scar buried in Earth’s crust near the village of Chicxulub in the northern Yucatán
peninsula was formed about 65 million years ago by the impact of a large asteroid or comet.
This gravity map shows the extent of the crater hidden below limestone deposited long after the
impact. (Virgil L. Sharpton, University of Alaska, Fairbanks)