Comet science: Rosetta and beyond

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Comets have fascinated, and terrified, people throughout recorded history. The unpredictable appearances of bright comets, even in times when the motions of the "wandering stars" (planets) were well understood, meant that they were treated with suspicion. Today, our knowledge of comets has come a long way, but there are still many puzzles left to solve. As I write this, telescopes and spacecraft scattered all over the solar system are tracking comet ISON as it plunges toward the Sun, but we still cannot predict whether it will light up the sky as a "Great comet" or fizzle out before it arrives.Nowadays we are interested in comets not as harbingers of doom, but because we can use them to learn about the disc of rock and ice that formed the planets. Comets and asteroids are the leftover "building blocks" from the early days of the solar system; the survivors that didn't get incorporated into one of the planets, crash into the Sun, or get thrown out of the system altogether. The next few years will be an exciting time in cometary science, as the ESA mission "Rosetta" will be exploring comet 67P/Churyumov-Gerasimenko in great detail. This audacious mission will, for the first time ever, rendezvous with a comet and follow it on its orbit around the Sun. Its array of scientific instruments will watch as the activity of the comet evolves. This mission will not only get closer to a comet than any previous spacecraft (down to only a few km from the surface, flying deep within jets of gas and dust), and be the first to spend an extended period of time watching changes on the comet; it will also land a probe on the surface, to find out exactly what the comet is made of.I will work on the data that will be returned by Rosetta, especially through the camera system OSIRIS, and also by parallel ground-based observations of the comet. Such ground-based context observations are of critical importance: While the spacecraft will return very detailed information about the nucleus and inner coma of one particular comet, we need to be able to see how these details correspond to the large-scale activity that can be observed with telescopes in order to relate this to comets in general. I am coordinating a campaign of observations from telescopes worldwide. This will include a significant contribution from amateur astronomers, and from UK schools via the Faulkes Telescope project, when the comet is bright enough. Together, these data will reveal how cometary activity works.In addition to 67P, I will also observe other comets for comparison. These will include those of the same class, the short-period "Jupiter family" comets, and also other types such as main belt and long-period comets, including comet ISON. The interaction between the Sun and comets like ISON, which get very close to the solar surface, not only tells us about how highly active comets behave under extreme heating conditions, but can also reveal new information about the Sun itself and the environment around it. Main belt comets are at the other end of the cometary activity scale: They are a recently discovered missing link between icy comets and rocky asteroids, with asteroid-like orbits but weak comet-like activity. They are also a potential source of Earth's water: It is much easier to have delivered water from the outer asteroid belt to the early Earth than it is to account for all of it from the impact of "normal" comets. My work will look at the differences in the activity of the three classes of comet (high-activity near-Sun comets, medium-activity Jupiter family comets, and low-activity main belt comets), and the question of whether the variation is due to the different source regions they come from, or due to the environment in which they are active now. This "nature vs. nurture" question for comets will provide clues about the properties of different areas of the planet-forming disc, and the evolution of icy bodies over the age of the solar system.