Remote sensing of comets and asteroids

Abstract

In the past Quadrennium, we have observed a fragmented comet collide with Jupiter, and discovered a population of objects of the order of hundreds of kilometers in diameter beyond the orbit of Neptune, a probable source region of short period comets. We are still in the age of discovery as studies of the solar system continue. The discovery of many small asteroids with the same surface composition as the second largest main belt asteroid, 4 Vesta, has opened our eyes to a pathway by which certain meteorites reach earth and by which asteroids collide and their orbits evolve. This evidence for delivery of meteorites was found by pushing existing observing techniques to their limits. An expanded capability for detecting returned radar echoes from asteroids permits the extraction of basic information about their position, shape, spin state, size and surface texture which is equivalent to that acquired from a flyby of a robotic spacecraft. We will never send spacecraft to as many asteroids as can be studied from the ground, so the contributions of ground‐based astronomy at all wavelengths provides insights into the chemical and physical diversity of the asteroids and comets. Remote sensing is carried out by ground‐based and Earth‐orbiting telescopic studies, and robotic spacecraft traveling through the solar system. In the past Quadrennium, we have viewed data returned from two main belt asteroid flybys missions which represent the transition of asteroids as targets of astronomical study t o subjects of geological study. Scientists using the Galileo spacecraft additionally made the serendipitous discovery of Dactyl, a satellite of 243 Ida, the first direct proof that satellites orbit asteroids. The significant advances discussed below have occurred because of 1) chance discovery (the discovery of Comet Shoemaker‐Levy 9 and Dactyl), 2) the interaction between theoretical predictions and observational programs (discovery of trans‐ Neptunian objects, 3) the use of available technology pushed to its limits of sensitivity (discovery of small asteroids with the same surface composition as 4 Vesta,) and 4) the consequence of technical upgrading of facilities. The latter provides fundamental facts about the diverse physical characteristics of small solar system bodies. A prime example is advancing knowledge derived from radar astronomy. Our new knowledge has generated a modified view of the number and nature of the small bodies in the solar system and how they formed and evolved through time.