Asteroids: Linking Meteorites and Planets

The Japanese spacecraft Hayabusa returned samples from the surface of an asteroid (near-Earth S-type asteroid 25143 Itokawa) for the first time in human history. This article describes the results of the initial analysis of the mineralogy, micropetrology, and elemental and isotopic compositions of regolith particles from Itokawa measuring 30–180 µm in diameter. The results show a direct link between ordinary chondrites and S-type asteroids. The regolith particles provide evidence of space-weathering rims and grain abrasion, and the information obtained has elucidated various processes on the airless surface of Itokawa, such as the impact of small objects, grain motion, and irradiation by solar wind.

Most asteroids are collisional rubble from eons past, and few of them have survived intact. Vesta, the second most massive asteroid, is the only differentiated, rocky body in this category. This asteroid provides a unique view of the kinds of planetesimals that accreted to form the terrestrial planets. We know more about this asteroid than any other, thanks to its recently completed exploration by the orbiting Dawn spacecraft and studies of the ~1000 meteorites derived from it. The synergy provided by in situ analyses and samples has allowed an unparalleled understanding of Vesta’s mineralogy, petrology, geochemistry, and geochronology.

On October 6, 2008, the small (~4 m) asteroid 2008 TC3 was discovered and predicted to hit Earth within ~19 hours. Photometric data and a refl ectance spectrum were obtained. The asteroid fragmented at ~37 km altitude above Sudan. Approximately 700 centimeter-sized fragments were recovered and constitute the meteorite Almahata Sitta. It is a unique meteorite breccia, consisting of ~50–70% ureilitic materials, plus samples of nearly every major chondrite group. The refl ectance spectrum of 2008 TC3 is closest to that of F-class asteroids, not previously associated with any meteorite type. 2008 TC3/Almahata Sitta records a complex history of fragmentation, migration, and reaccretion of materials in the Solar System.

Asteroids are arguably the most accessible remnants of building blocks of the early Solar System and an essential piece of the terrestrial planet–formation puzzle. Determining their compositions and physical properties can provide important and otherwise unobtainable information concerning the origin, structure, and dynamic history of the Solar System, as well as insights into the sources of materials from which the terrestrial planets were constructed. Our understanding of the compositional structure of the asteroid belt and of individual asteroids has advanced significantly since the 1970s. Strong associations between asteroids and meteorites are emerging thanks to multitechnique observations, the synthesis of observations and modeling, in situ measurements, and sample-return missions.

Asteroids are the leftover precursors to the terrestrial planets. Before the first images of them were sent from space, our knowledge of asteroids relied entirely on ground-based observations and meteorite analysis. Spacecraft images revolutionized our knowledge and geological understanding of their physical properties. They also showed us that asteroids are subjected to various kinds of processes and are incredibly diverse in size, shape, structure, composition, and rotational properties. Therefore, space missions remain necessary to enhance our knowledge of the various components of the asteroid population. In addition, numerical modeling is required to interpret spacecraft images and improve our understanding of the physical processes asteroids experience over their lifetime.

At present, we know of ~600,000 asteroids in the asteroid belt, and there are very likely millions more. Orbiting the Sun between Mars and Jupiter, they are thought to be the shattered remnants of small bodies formed within the young Sun’s solar nebula that never accreted enough material to become planets. These “minor bodies” are therefore keys to understanding how the Solar System formed and evolved. As leftover planetary building blocks, they are of great importance in understanding planetary compositions. They may also provide clues to the origin of life, as similar bodies may have delivered organics and water to the early Earth. For these reasons, several international space agencies have funded sample-return missions to asteroids.