Dale P. Cruikshank - Discovering Pluto: Exploration at the Edge of the Solar System

WITH THE NEW HORIZONS flyby of Pluto in July 2015, the Grand Tour of the Solar System, which began in the 1960s with the Mariner missions and continued with the Voyager flybys of the giant planets, was finally complete. Plutooriginally, when the Grand Tour was first conceived, the ninth major planet of the Solar Systemtogether with its anomalously large moon Charon and the four small satellites of their system, has been surveyed at close range. This book presents the long and fascinating history of searches of trans-Neptunian space by Percival Lowell and his colleagues that led to Clyde Tombaughs discovery of what seemed at the time to have been Lowells Planet X, and the subsequent gradual accumulation of facts about this small icy body on the inner edge of the Kuiper Belt. The book is brought up to date with a summary of the scientific results so far about this surprising world and a look forward to the next steps planned in the exploration of the Kuiper Belt.

Pluto, now classified as a dwarf planet but regarded as the ninth major planet of the Solar System from the time of its discovery by Tombaugh at Lowell Observatory in 1930 until it was reclassified by the International Astronomical Union (IAU) in 2006, remainshowever classified, and despite its smallish size and distanceone of the most charismatic bodies in the SolarSystem. Except perhaps for Mars, no other planet seems of greater interest to the public.

Lowell first identified it as the incarnation of Planet X, a seven-Earth-mass major planet posited because of supposed residuals (unexplained differences between the theory and observed positions) of the planet Uranus. Astronomers hailed its discovery as a triumph of celestial mechanics similar to John Couch Adams and Urbain Le Verriers calculations leading to the discovery of Neptune in 1846and it arguably saved the famously broke Lowell Observatory from extinction. From the first, however, there were doubts about its mass and planetary status. By 1976, Dale Cruikshank, Carl Pilcher, and David Morrison discovered methane ice on Pluto. From then on it was clear that Pluto was much too smallonly about 2,000 km in diameter, or smaller than Earths Moonto have exerted any significant perturbations on the giant planets Uranus or Neptune. The discovery of the largest of its five satellites, Charon, by James Christy at the U.S. Naval Observatory two years later furnished a precise value of the mass.

Since those 1970s discoveries, it was learned that Pluto is a particularly outsized member of the expanse of icy bodies on the edge of the outer Solar System called the Kuiper Belt. Rather than being, as once thought, a major planet in a relatively simple collection of planets, Pluto is representative of a part of the Solar System that contains some two thousand known objects. It is a mini-world or, to use the IAUs nomenclature, a dwarf planet. It and its Kuiper Belt congeners (as well as bodies belonging to the still more distant Oort cloud) point to a Solar System in a state that was once vastly different from what it is now.

This book falls naturally into two parts. Though both authors contributed to all the chapters, the first eight were primarily the work of coauthor Sheehan, while the remaining twelve were chiefly written by coauthor Cruikshank. Cruikshank was a graduate student of Gerard Kuiper and later a planetary scientist specializing in the investigation of the icy outer worlds of the Solar System, and he has had an insider view of the recent science as an investigator on the New Horizons mission.