Structure, composition, and origin of meteoroids

自太阳系形成以来，彗星和小行星相对没有变化。与融化和分化的行星不同，彗星和原始小行星含有行星形成前存在的基本颗粒。特别是彗星很难直接研究，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。星尘任务只从一颗彗星Wild 2上带回了样本。该航天器采集的微小颗粒包括在高温下形成的固体岩石颗粒：这种固体颗粒肯定是在星云最热、最密集的地方靠近太阳形成的，是如何融入冰彗星的，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，它们每天都会大量撞击地球。我们将使用一种新的高分辨率观测系统来研究来自三个不同群体的流星体的精细结构和组成：小行星、巨行星之间形成的彗星和海王星轨道外形成的彗星。新系统允许我们观察流星体在与地球大气层相撞时是如何分解的，这将使我们能够区分它们是强的固体粒子还是弱的尘埃团。光谱系统将为我们提供有关单个颗粒的矿物组成的线索。特别是，我们将寻找在小行星中常见的那种固体粒子。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，将有助于在许多早期太阳系模型中进行选择，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.