5 - Target Earth


Geology is the study of the Earth including its formation and its relationships within the solar system. So far in this course we have looked at some hazards originating right here at home but actually the biggest potential hazard, although very rare, comes from elsewhere in our solar system. There are a huge number of objects greater than 10m in diameter that cross the orbit of the Earth. These are termed "Near Earth Objects" or NEOs. These mostly burn up or break up in the atmosphere before reaching the surface (Table 5-1) but about one object in the 10m size range hits the Earth each year. The ones we really have to worry about are those greater than about 50m in diameter. As we will see, life on the Earth has been affected by collisions with space objects in the past, and collisions will most likely affect the Earth in the future. Note that many of the numbers listed in this chapter are not absolute because more and more extraterrestrial objects are being found every year and their orbits are being computed more accurately.

Learning Objectives

Upon completion of this unit you should be able to:

1. Explain the classification of extraterrestrial objects.
2. Understand the different types of meteorites and how they are related to the Earth's internal structure.
3. Say where asteroids come from and how they might have been moved into Earth-crossing orbits.
4. Define the different types of comets and say where they come from.
5. Discuss the factors that relate to energy release during an impact event.
6. Discuss the many things that happen during an impact event.
7. Understand the various ways in which impact events are recorded in the rocks
8. Discuss the many things that happen on Earth after a major impact event.
9. Understand clearly the theory of dinosaur extinction as it relates to a giant impact event.
10.  Discuss the ways in which humans are trying to predict the path of extraterrestrial object and how successful the efforts have been.
11. Discuss the merits of various techniques suggested for deflecting large extraterrestrial objects away from the Earth

Required Readings

Abbott, P.L., 2014. Natural Disasters, Ninth Edition: Readings From the Old Book

Abbott, P.L., 2012. Natural Disasters, Eighth Edition:

Glossary Terms

Achondrite K/T Boundary
Amino Acid Kuiper Belt
Asteroid Belt Mars Meteorite
Astrobleme NEA
Barringer Crater Nuclear Winter
Chondrite Oort Cloud
Chondrule Planetesimal
Comet Tempel 1 Richter Magnitude
Cosmic Winter Shatter Cone
Differentiated Stishovite
Hadean Shocked Quartz 
Hypercane Solar System
Hypothesis Tektite
Impact Breccia Torino Scale
Impact Crater Widmanstätten Pattern
Kinetic Energy