Comet science: Rosetta and beyond

在有记载的历史中，彗星使人们着迷，也使人们恐惧。明亮彗星的不可预测的出现，即使在“流浪星”（行星）的运动被很好地理解的时候，也意味着它们被怀疑。今天，我们对彗星的认识已经有了很大的进步，但仍有许多谜团有待解决。当我写这篇文章的时候，散布在太阳系各处的望远镜和宇宙飞船正在跟踪彗星伊森，因为它正冲向太阳，但我们仍然无法预测它是否会像一颗“大彗星”一样照亮天空，或者在它到达之前就消失了。现在我们对彗星感兴趣，不是因为它是厄运的预兆，而是因为我们可以利用它们来了解形成行星的岩石和冰的圆盘。彗星和小行星是太阳系早期遗留下来的“积木”;幸存者没有被纳入行星之一，撞向太阳，或者被完全抛出系统。今后几年将是彗星科学的一个激动人心的时刻，因为欧空局的“罗塞塔”使命将详细探索67 P/丘留莫夫-格拉西缅科彗星。这项大胆的使命将有史以来第一次与彗星会合，并在其绕太阳的轨道上跟踪它。它的一系列科学仪器将观察彗星的活动演变。这一使命不仅将比以往任何航天器更接近彗星（下降到距离表面只有几公里，在气体和尘埃喷流的深处飞行），并成为第一个花很长时间观察彗星变化的人;它还将在彗星表面着陆一个探测器，以查明彗星的确切组成。我将处理罗塞塔号返回的数据，特别是通过OSIRIS相机系统，以及对彗星的平行地面观测。这样的地面背景观测至关重要：虽然航天器将返回关于一个特定彗星的核和内部彗发的非常详细的信息，但我们需要能够看到这些细节如何与望远镜可以观察到的大规模活动相对应，以便将其与一般彗星联系起来。我正在协调一项全球望远镜观测活动。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的重大贡献。这些数据将共同揭示彗星活动是如何运作的。除了67 P，我还将观察其他彗星进行比较。这将包括那些相同类别的短周期“木星家族”彗星，以及其他类型的主带和长周期彗星，包括彗星ISON。太阳和彗星之间的相互作用，如ISON，非常接近太阳表面，不仅告诉我们高度活跃的彗星在极端加热条件下的行为，而且还可以揭示关于太阳本身及其周围环境的新信息。主带彗星位于彗星活动范围的另一端：它们是最近发现的冰彗星和岩石小行星之间的缺失环节，具有小行星般的轨道，但像彗星一样的活动很弱。它们也是地球水的潜在来源：从外部小行星带向早期地球输送水比从“正常”彗星的撞击中解释所有这些水要容易得多。我的工作将着眼于三类彗星（高活动性的近太阳彗星、中等活动性的木星族彗星和低活动性的主带彗星）活动的差异，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。这个关于彗星的“先天与后天”的问题将为行星形成盘的不同区域的性质以及太阳系年龄段内冰冷天体的演变提供线索。

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.