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CHAPTER 30

Physics and Chemistry

of Comets

John C. Brandt

Department of Physics and Astronomy

University of New Mexico

Albuquerque, New Mexico

1. Space Missions to Comets 6. Comet Chemistry


2. A Brief History of Comet Studies 7. Formation and Ultimate Fate of Comets


3. Physics of the Nucleus 8. Summary


4. Coma and Hydrogen Cloud Bibliography


5. Tails

T

he spectacular sight of a bright comet with a tail stretch-
ing across the sky (Fig. 1) prompts questions about the
nature of the object and the physical processes at work.
The current era is one of major comet research, with several
space missions to comets producing pioneering results. The
images and data that are becoming available often prompt
new questions and challenge old ideas. Two decades have
passed since the first space missions to comets, and comet
science has reached a level of maturity that was unimagin-
able not long ago.

1. Space Missions to Comets

Many lines of evidence indicate that the source of all
cometary phenomena is a rather small central body called
thenucleus. Typical dimensions are in the range 1–10 km.
Viewing an object, say, 3 km across from a distance of 0.2
AU (or 3× 107 km) means that the object subtends an angle
of 1/50th of an arc second. Typical resolution from ground-
based observatories is about 1.0 arc second and, for large
telescopes, is due to the effects of the Earth’s atmosphere.
Mountaintop observatories in good locations can do bet-
ter, and theHubble Space Telescope (HST)has a resolution
of about 0.1 arc seconds. From Earth, except in extraordi-
nary circumstances, the nucleus cannot be resolved, and no

detail on the surface can be seen. The solution is to send

spacecraft with imaging systems close to the cometary nu-

clei. In situ measurements of gas, dust, plasma, magnetic

fields, and energetic particles can be obtained while the

spacecraft is near the comet. The imaging and in situ data

provide a major source of information on comets.

Table 1 summarizes completed missions to comets. Of

course, analysis often continues for years. In this section,

only the missions with imaging are discussed. The missions

to comet Halley in 1986 were collectively called the Hal-

ley Armada, and three of them had imaging. TwoVEGA

spacecraft were sent by the Soviet Union and passed within

8890 km (VEGA 1) and 8030 km (VEGA 2) of the comet.

The images from theVEGAs are valuable, but they were

somewhat noisy and were taken from larger distances than

those taken byGiotto.

The European Space Agency (ESA) sent theGiotto

spacecraft to pass the nucleus of Halley’s comet within

596 km. The spacecraft carried the Halley Multicolor

Camera (HMC), which obtained images of the nucleus un-

til approximately the time of closest approach when it was

damaged by the impacts of dust particles. Figure 2A is an

overall view of the nucleus composed of 68 individual im-

ages. The nucleus was not spherical but was a potato-shaped

object with a long axis of approximately 15 km and short

axes of approximately 7 km. The nucleus showed features

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