Environmental Engineering Reference
In-Depth Information
EARTH-CROSSING OBJECTS AND DEEP IMPACTS
new developments
Earth's orbit takes it on an annual journey of 1·052 × 10
9
km (1·052 billion km) around
the sun at a velocity of 120,000 km hr
−1
or 2,000 km min
−1
. Crossing its path are 10
6-9
clumps of interstellar dust and larger fragments or planetesimals similar to the matter
from which Earth itself accreted 4·6 Ba ago. Most are probably stony or metallic
fragments of
asteroids
. Orbiting between Mars and Jupiter, they may represent a 'failed'
terrestrial planet whose accretion was frustrated by Jupiter's powerful gravity field.
Debris from the outer solar system orbits as ice-rich comets which vaporize nearer the
sun.
The vast majority of fragments cross our orbital path some distance from Earth, or are
small enough (well below 5 m in diameter) to burn up in our protective, oxygen-rich
atmosphere. Relatively few reach Earth's surface as
meteorites
. The moon's heavily
cratered surface is a reminder of how Earth would look without its atmosphere under this
space bombardment. Fragments of iron, stone and ice with diameters exceeding 20 m, 50
m and 150 m respectively can penetrate to Earth's surface. Space exploration of Earth
and our solar system provides evidence of how relatively common, rather than rare,
impacts have been in the geological past - and the extent of future risks. It is estimated
that 10
6-7
of Earth-crossing fragments exceed 10 m diameter, 10
5-6
exceed the critical
threshold diameter of 100 m and 10
2-3
are larger than 1 km. Recent passes of the Hale-
Bopp and Halley comets, and the observed collision of the Shoemaker-Levy 9 comet with
Jupiter in 1994, enhance public awareness.
Science fiction has become reality as collisions with near-Earth objects are now
recognized as a major geological hazard confronting human life. Many more crater
structures are revealed by satellite images than traditional surface geological surveys,
many of them tens of kilometres in diameter. It is widely believed that the 200 km-wide
Chicxulub crater, in the Yucatan peninsula of central America, was the impact site of a
10 km object which triggered catastrophic ocean-atmosphere changes at the K/T
(Cretaceous/Tertiary) boundary 65 Ma ago. This may have rendered 50 per cent of all
species extinct, including, most notably, the dinosaurs. The environmental effects of such
impacts and the risk of future
extinction-level
events have become issues of serious
international concern. Estimates of the size and number of Earth-crossing objects suggest
that the
recurrence interval
of 1 km, 5 km and 10 km objects is 10
3-4
, 10
4-5
and 10
5-7
years respectively. However, this gives us little breathing space, as recent near-passes
show.
The air blast of a fragment cut a 50 km wide swathe through Siberian forest at
Tunguska in 1908. Newly discovered asteroid fragments will make close Earth passes
during our lifetimes and planetary gravity may increase collision risk as they approach
Earth. Recent misses by just a few hours expose our current poor understanding of impact
risk. We may be aware of the existence and orbits of only 1 per cent of 1 km and larger
fragments. A 1 km impact would wreak immense damage through the vaporization and
ejection of Earth materials at impact sites, triggering earthquakes, fires and hurricane-
force winds devastating life and property over 10
4-5
km
−2
. Most impacts would occur at
sea, stimulating huge
tsunami
or tidal waves. Long-term effects would alter weather and
climate
biological
mutations
and
extinctions
We
must
greatly
enhance
our
risk
