The Explored Asteroids: Science and Exploration in the Space Age

• Published in: Space science reviews 2015-11, Vol.194 (1-4), p.139-235
• Main Author: Sears, D. W. G.
• Format: Article
• Language: English
• Subjects: Aerospace Technology and Astronautics; Asteroids; Astronomy; Astrophysics and Astroparticles; Comets; Density; Exploration; Fluidization; Geochemistry; Meteorites; Physics; Physics and Astronomy; Planetology; Science; Snowline; Solar system; Space exploration; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Spacecraft; Subsurface water; Surface layer; Texture; Vesta asteroid; Water distribution
• ISSN: 0038-6308; 1572-9672
• DOI: 10.1007/s11214-015-0202-7

Interest in asteroids is currently high in view of their scientific importance, the impact hazard, and the in situ resource opportunities they offer. They are also a case study of the intimate relationship between science and exploration. A detailed review of the twelve asteroids that have been visited by eight robotic spacecraft is presented here. While the twelve explored asteroids have many features in common, like their heavily cratered and regolith covered surfaces, they are a remarkably diverse group. Some have low-eccentricity orbits in the main belt, while some are potentially hazardous objects. They range from dwarf planets to primary planetesimals to fragments of larger precursor objects to tiny shards. One has a moon. Their surface compositions range from basaltic to various chondrite-like compositions. Here their properties are reviewed and what was confirmed and what was newly learned is discussed, and additionally the explored asteroids are compared with comets and meteorites. Several topics are developed. These topics are the internal structure of asteroids, water distribution in the inner solar system and its role in shaping surfaces, and the meteoritic links. It is suggested, that asteroid-scale grooves, ridges, and catenas on several explored asteroids argue against these asteroids having rubble pile interiors, i.e. interiors made when impact fragments reaccumulate. The only body for which this is not true is the tiny Itokawa and it is argued that this asteroid is a regolith breccia. The discovery of water on Vesta, fluidization textures on comets and possibly Eros, and the relatively large number of active asteroids inside the purported snowline, suggests that significant subsurface water may be present on asteroids in the inner solar system and may partly account for their low densities. The explored asteroids have also confirmed the linkage of the HED meteorites with Vesta and Itokawa with the ordinary chondrite meteorites, Eros is somewhat problematical. So while diversity, and the range of sizes, histories, and surface compositions, is the hall mark of the explored asteroids, the number of explored asteroids is small compared with the diversity of material expected on the basis of asteroid astronomy and meteorite geochemistry. The exploration of the solar system’s asteroids has only just begun.