Structure, composition, and origin of meteoroids

自从太阳系形成以来，彗星和小行星就没有什么变化。与行星不同的是，彗星和原始小行星含有行星形成之前就存在的基本颗粒。彗星尤其难以直接研究，因为它们在接近地球时移动得非常快，这使得航天器很难观测到它们。“星尘”号只从一颗彗星“狂野2号”上带回了样本。宇宙飞船取样的微小颗粒包括在高温下形成的固体岩石颗粒：这些固体颗粒是如何形成的，这些固体颗粒一定是在星云中最热、密度最大的地方靠近太阳的地方形成的，它们是如何形成冰冷的彗星的，而冰彗星显然是在远离太阳的地方形成的，这是一个谜。流星体是从彗星和小行星母体中分离出来的小天体，每天都有大量的流星体撞击地球。我们将使用一种新的高分辨率观测系统来计算三种不同类型流星体的精细结构和组成：小行星、形成于巨行星之间的彗星和形成于海王星轨道之外的彗星。这个新系统使我们能够观察流星体在与地球大气层碰撞时是如何分裂的，这将使我们能够判断它们是坚固的固体颗粒还是微弱的尘埃团。光谱系统将为我们提供有关单个颗粒矿物组成的线索。特别是，我们将寻找小行星上常见的那种固体颗粒。了解这些固体颗粒是否存在于每个种群中，以及它们有多普遍，将有助于我们在早期太阳系的许多模型中做出选择，并帮助我们了解行星的形成。

Comets and asteroids are relatively unchanged since the formation of the solar system. Unlike the planets, which have melted and differentiated, comets and primitive asteroids contain the basic grains which were present before the planets were formed. Comets in particular are very difficult to study directly, because they are moving very quickly as they approach the Earth, making it difficult for spacecraft to visit them. Samples have been returned from just one comet, Wild 2, by the Stardust mission. The tiny grains sampled by that spacecraft include solid, rocky grains that formed at high temperature: how such solid grains, which must have formed close to the sun in the hottest and densest part of the nebula, were incorporated into icy comets, which obviously formed far from the sun, is a mystery. Meteoroids are small objects which have been separated from comet and asteroid parent bodies, and they strike the Earth in large numbers every day. We will use a new, high-resolution observing system to work out the fine structure and composition of meteoroids from three different populations: asteroids, comets which formed among the giant planets, and comets which formed beyond Neptune's orbit. The new system allows us to watch how meteoroids break up as they collide with the Earth's atmosphere, which will allow us to tell if they are strong, solid particles or weak clusters of dust. A spectral system will give us clues about the mineral composition of the individual grains. In particular, we will search for the sort of solid particles commonly found in asteroids. Knowing whether these solid grains are present in each population, and how common they are, will help to choose among many models of the early solar system, and help us to understand the formation of the planets.