Fig. 55. albedo versus scattering angle measured in 12 comets 
Both albedos are related by: AB = Apq. Here, q is the phase integral and is equal to 4 in case of isotropic scattering where AB = 1 and Ap = 0.25. In general q is given as:
Figure 55 shows the albedo versus scattering angle measured in 12 comets . The albedo is derived from measurements made quasi simultaneously at thermal IR and optical wavelengths. The scattering function can be derived when comets are observed over a wide range of phase angles. This is however, only rarely possible. Therefore, often a "standard" phase function is assumed based on . We note, however, that the phase function for individual comets can be quite different to the standard phase curve (e.g., ).
At the beginning of this chapter, we recalled the main motivation for cometary studies: How did comets form and what do we learn from them about the formation of our solar system? Answering this fundamental questions requires knowledge of many different fields of physics, such as gas dynamics, plasma
physics, chemistry, scattering theory, molecular physics, etc., including their applications to the conditions found in comets. In the previous sections, we looked at some of the basic concepts scientists use to address the problems in the different areas of comet research. Of course, students specialize in their field of interest to obtain a much deeper understanding in that area. However, I encourage students to have a broader view on the context of their specialist research fields to be able to see interrelations and avoid misconceptions.
An enormous progress has been made in the past 20 years in our understanding of comets, although important key processes are still far from being understood (e.g., activity). Space missions have been sent to observe comets directly and provide in situ data. Ground-based technology progressed with new detector technologies and telescopes allowing us to observe comets not only in the optical but also at radio and near-IR wavelengths with high sensitivity. In addition, space telescopes provide access to wavelengths regions we can not investigate from the ground. The technical progress resulted in an enlarged volume of high quality data, which stimulated the development of theory to explain what had been observed. Nevertheless, there is still no generally accepted theory on comet formation. We need to proceed and obtain further data and improve our theory in the future. Future investigations need to include
- Direct investigations of nucleus material by landers.
- Sample return of cometary material (nucleus and coma).
- Improved statistical database of cometary gas and dust composition.
- Measurements of gas and dust activity over wide ranges of heliocentric distances.
- Laboratory comet simulation experiments (e.g., outgassing processes, ice formation).
- Numerical models of cometary activity.
- Understanding of the relation to Kuiper belt objects.
Some of these point will be already addressed in the near future by space missions. The Stardust mission has already transported in situ samples of coma dust particles to Earth . ESA's Rosetta  mission will follow a comet over a substantial part of its orbit and will drop a lander on its nucleus to investigate the upper surface layers directly. In parallel, ground-based facilities, such as the radio telescope array ALMA [29, 202] will provide a new wealth of data on parent gas molecules. The field of cometary science therefore needs young motivated students educated in cometary science to explore the information to come.
Solving the problem of comet formation is like solving a big puzzle. We have to ask the right questions to find the right puzzle pieces and then find the right places for them to be able to solve "the big picture" of comet and planet formation. Each student works on his/her own small or larger "puzzle piece." It is the combination of all of them that will help us to obtain the whole picture.
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