number and structures of planetary systems around other stars has shown that, while
our Sun is not unique in having planets, the diversity in the distribution of planets
within the other solar systems is far greater than imagined 25 years ago. Hence, there
remains a need to understand how our specific system formed and evolved. Carl
Sagan once remarked that “you have to know the past to understand the present” and
one may invert this by saying that one can get to know something of the past by
studying evolution in the present.
The perturbation of comet orbits by the planets raises further questions about the
significance of comets for planetary evolution. Their motion with respect to the
rather uniform quasi-circular orbits of the planets provides a means of transporting
material over a large range of heliocentric distances. Although the present numbers
of comets may be small, Fig. 1 also indicates that there were many more of these
objects in the early Solar System. Perturbation followed by impact with accreting
planets was a means of incorporating objects formed at many different heliocentric
distances into the growing proto-planets. The significance of this mass transport is
not well established, but its implications are profound. It has been known for
35 years that the main driving volatile in comets when they reach the inner Solar
System is water ice. Furthermore, it was shown during the detailed observations of
comet 1P/Halley in 1985–1986 that the less volatile components contain copious
amounts of organic material. This combination of ice, organics, and large relative
motion has led to the idea that the Earth obtained most, if not all, of its water and
organics from comets and that it was this influx of material that ultimately led to the
development of life. There is no doubt that the surfaces of the terrestrial planets have
been impacted by comets many times over the lifetime of the Solar System. It should
also be noted that meteor showers, which are the products of dust ejected from
comets, enter the Earth’s atmosphere on a regular basis. But the full significance
remains unclear and provides further grounds for detailed investigation of cometary
material.
Given the importance of cometary research, there have been rather few books on
the subject and one of the motivations for this work is the absence of good
introductory texts. Probably the closest in nature to the concept of this book is the
2010 third edition of Physics of Comets by K.S. Krishna Swamy. This has a number
of excellent introductions to various aspects of cometary physics. It is quite focused
on spectroscopy and gas emission, whereas here I have weighted the text more
towards the nucleus and the innermost coma as a direct result of the observations of
comet 67P/Churyumov–Gerasimenko by the European Space Agency’s (ESA)
Rosetta spacecraft. Similarly Brandt and Chapman’s book Introduction to Comets
from 2004 (2nd edition) emphasizes the plasma aspects of comets and is somewhat
out of date with respect to the nucleus.
Although Physics and Chemistry of Comets edited by W.F. Huebner is now
nearly 30 years old, much of the text remains relevant. Huebner collected eight
chapters from experts on all aspects of cometary research and produced an excellent
summary roughly 4 years after the Giotto encounter with 1P/Halley. On the other
hand, it was written prior to the discovery of the first Kuiper–Edgeworth Belt Object
in 1992 and well before several major space missions to comets (including not only
x
Preface (Motivation and Scope)
our Sun is not unique in having planets, the diversity in the distribution of planets
within the other solar systems is far greater than imagined 25 years ago. Hence, there
remains a need to understand how our specific system formed and evolved. Carl
Sagan once remarked that “you have to know the past to understand the present” and
one may invert this by saying that one can get to know something of the past by
studying evolution in the present.
The perturbation of comet orbits by the planets raises further questions about the
significance of comets for planetary evolution. Their motion with respect to the
rather uniform quasi-circular orbits of the planets provides a means of transporting
material over a large range of heliocentric distances. Although the present numbers
of comets may be small, Fig. 1 also indicates that there were many more of these
objects in the early Solar System. Perturbation followed by impact with accreting
planets was a means of incorporating objects formed at many different heliocentric
distances into the growing proto-planets. The significance of this mass transport is
not well established, but its implications are profound. It has been known for
35 years that the main driving volatile in comets when they reach the inner Solar
System is water ice. Furthermore, it was shown during the detailed observations of
comet 1P/Halley in 1985–1986 that the less volatile components contain copious
amounts of organic material. This combination of ice, organics, and large relative
motion has led to the idea that the Earth obtained most, if not all, of its water and
organics from comets and that it was this influx of material that ultimately led to the
development of life. There is no doubt that the surfaces of the terrestrial planets have
been impacted by comets many times over the lifetime of the Solar System. It should
also be noted that meteor showers, which are the products of dust ejected from
comets, enter the Earth’s atmosphere on a regular basis. But the full significance
remains unclear and provides further grounds for detailed investigation of cometary
material.
Given the importance of cometary research, there have been rather few books on
the subject and one of the motivations for this work is the absence of good
introductory texts. Probably the closest in nature to the concept of this book is the
2010 third edition of Physics of Comets by K.S. Krishna Swamy. This has a number
of excellent introductions to various aspects of cometary physics. It is quite focused
on spectroscopy and gas emission, whereas here I have weighted the text more
towards the nucleus and the innermost coma as a direct result of the observations of
comet 67P/Churyumov–Gerasimenko by the European Space Agency’s (ESA)
Rosetta spacecraft. Similarly Brandt and Chapman’s book Introduction to Comets
from 2004 (2nd edition) emphasizes the plasma aspects of comets and is somewhat
out of date with respect to the nucleus.
Although Physics and Chemistry of Comets edited by W.F. Huebner is now
nearly 30 years old, much of the text remains relevant. Huebner collected eight
chapters from experts on all aspects of cometary research and produced an excellent
summary roughly 4 years after the Giotto encounter with 1P/Halley. On the other
hand, it was written prior to the discovery of the first Kuiper–Edgeworth Belt Object
in 1992 and well before several major space missions to comets (including not only
x
Preface (Motivation and Scope)