Structure, composition, and origin of meteoroids

彗星和小行星自太阳系形成以来相对没有变化。与行星不同，彗星和原始小行星含有在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，使得航天器很难访问它们。星尘使命只从一颗彗星--怀尔德2号--上带回了样本。该航天器采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，必须在星云最热和最密集的部分靠近太阳形成，如何被合并到冰冷的彗星中，这些彗星显然是远离太阳形成的，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物体，它们每天都大量撞击地球。我们将使用一个新的高分辨率观测系统来计算三种不同群体流星体的精细结构和组成：小行星、在巨行星之间形成的彗星和在海王星轨道之外形成的彗星。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破碎的，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的矿物成分的线索。特别是，我们将寻找在小行星中常见的固体颗粒。了解这些固体颗粒是否存在于每个种群中，以及它们有多常见，将有助于在早期太阳系的许多模型中进行选择，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.