Comet science: Rosetta and beyond

在有记录的历史中，彗星让人们既着迷又害怕。明亮的彗星的不可预测的出现，即使在人们很好地理解“游荡的恒星”(行星)的运动的时候，也意味着它们受到了怀疑。今天，我们对彗星的知识已经走了很长的路，但仍然有许多谜团有待解开。彗星和小行星是太阳系早期遗留下来的“积木”；没有融入其中一颗行星的幸存者，撞上了太阳，或者被彻底赶出了太阳系。未来几年将是彗星科学领域激动人心的几年，因为欧空局的“罗塞塔”号任务将非常详细地探测67P/丘留莫夫-格拉西缅科彗星。这项大胆的任务将有史以来第一次与一颗彗星会合，并跟随它绕太阳运行。它的一系列科学仪器将随着彗星活动的演变而观察。我正在协调一项来自世界各地的望远镜观测活动。这将包括业余天文学家的重大贡献，以及当彗星足够明亮时，英国学校通过福克斯望远镜项目做出的贡献。总而言之，这些数据将揭示彗星活动的原理。除了67P，我还将观测其他彗星进行比较。这些彗星将包括同一类的短周期“木星家族”彗星，也包括其他类型的主带彗星和长周期彗星，包括ISON彗星。太阳和像ISON这样非常接近太阳表面的彗星之间的相互作用，不仅可以告诉我们高度活跃的彗星在极端加热条件下的行为，还可以揭示关于太阳本身和它周围环境的新信息。主带彗星位于彗星活动天平的另一端：它们是最近发现的冰彗星和岩石小行星之间缺失的一环，它们的轨道类似小行星，但类似彗星的活动很弱。它们也是地球水的一个潜在来源：从外部小行星带向早期地球输送水要比解释所有来自“正常”彗星的影响要容易得多。我的工作将考察三类彗星(高活动近太阳彗星、中等活动木星家族彗星和低活动主带彗星)的活动差异，以及这种差异是由于它们来自不同的源区，还是由于它们现在活动的环境。关于彗星的这个“先天与后天”的问题将提供有关行星形成盘不同区域的性质的线索，以及随着太阳系年龄的推移冰体的演化。

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.