Introduction

In the 1994 book edited by Gehrels, Hazards due to Comets and Asteroids, chapters by Ahrens and Harris (1994), Shafer et al. (1994), Simonenko et al. (1994), Solem and Snell (1994), and Melosh et al. (1994) present and study a number of ways of preventing an oncoming asteroid from colliding with the Earth. Most methods considered nudging it sufficiently at 10 or so years before the impending collision to change its course so it would miss the Earth.

The methods studied include the use of conventional or nuclear explosives on or below the surface, the impact by large masses at high velocities, the blowing off of material by standoff nuclear weapons or by the concentration of solar energy using giant mirrors or by zapping it with lasers, and more gentle methods such as simply attaching a propulsion rocket, a solar sail or launching surface material off at sufficient velocity to escape the asteroid.

The analyses of the different methods rely primarily on data and estimates accumulated for cratering and disruption using the material properties of terrestrial materials. In most cases those were silicate materials with mass densities of ∼3 g cm–3 or iron asteroids of density ∼8 g cm–3. However, it is becoming generally accepted that many of the asteroids are re-accumulated rubble pile bodies of very low density and strength, and comets have been thought for some time to have that structure.

Introduction

Understanding the inventory and size distribution of those bodies which during their orbital evolution can intersect the orbit of the Earth with a non-zero probability of collision is a high priority task for modern planetary science. Apart from obvious considerations about mitigation of the impact hazard for the terrestrial biosphere, this is also a challenging theoretical problem, with important implications for our understanding of the orbital and physical evolution of the minor bodies of our solar system.

Several mechanisms have been discovered and analyzed in recent years to explain the steady influx of bodies from different regions of the solar system to the zone of the terrestrial planets. Several unstable regions in the orbital element space have been identified in the asteroid main belt, which can lead bodies to be decoupled from the belt and evolve into near-Earth object (NEO) orbits (Morbidelli et al. 2002). Both conventional collisional mechanisms and dynamical non-gravitational mechanisms (Yarkovsky effect) can be responsible for a steady injection of main-belt asteroids into these unstable orbits. Even in the absence of unstable regions, the Yarkovsky effect can cause a continuous drift in semimajor axis and eccentricity, eventually leading to a close encounter with a terrestrial planet, and removal from the main asteroid belt (Spitale and Greenberg 2001, 2002). The NEO production rate and the resulting NEO inventory and size distributions can be theoretically estimated and compared with observations (Bottke et al. 2002). It should be noted that the effectiveness of the different supply mechanisms is eminently size dependent.

This edition of The Clementine Atlas of the Moon represents a significant enhancement over the first edition. The primary improvement is that we use new mission data for the annotated pages of the atlas. Specifically we use a new global topography product generated using data acquired by LROC, the imaging camera on board NASA's Lunar Reconnaissance Orbiter. For allowing us to use these wonderful data we wish to thank Dr Mark Robinson, the LROC team, and Frank Scholten and colleagues at the DLR Institute of Planetary Research in Berlin, Germany. We'd like to acknowledge the LOLA team whose high-resolution topography of the lunar polar regions was used for LACs 1 and 144.

Also since the initial release of the atlas, the Astrogeology branch of the United States Geological Survey has released a comprehensive set of annotated LACs that use Lunar Orbiter data as their basemap. We thank Jenny Blue and her colleagues for this invaluable resource for lunar scientists and enthusiasts. Finally we thank Danny Caes of Ghent, Belgium, whose meticulous and careful reading of the atlas uncovered several typographical errors in the first edition.

The Clementine data

The Clementine spacecraft entered a 400–3000 km elliptical lunar orbit on 19 February 1994. It remained in this 5 h period orbit for 71 days, systematically mapping the surface of the Moon with its instrument suite (Table 4). Because of the elliptical nature of the orbit, perilune (the point of closest approach the lunar surface) was moved from 28° S to 30° N approximately half-way through the mapping phase of the mission (Figure 20). This ensured that the entire surface of the Moon was mapped at approximately the same resolution.

The imaging data set shown in this atlas comes from Clementine's ultraviolet-visual (UV–VIS) instrument. The UV-VIS camera was essentially a digital camera with a 384 × 288 pixel array. This camera contained five filters and imaged over 99% of the lunar surface at an average resolution of 200 m/pixel. The images used here were taken with the 750 nm filter as they are close to the visible part of the spectrum and are of better quality than the 415 nm images.

Because of the redundant overlap in surface coverage at high latitudes between consecutive orbits, the Clementine team devised a mapping strategy that conserved data volume. In a type A orbit the spacecraft (travelling from south to north) began mapping at 70° S and finished imaging when it was above the North Pole. In a type B orbit, mapping began at 90° S and finished at 70° N. Type A and B orbits were alternated on each revolution, resulting in a seamless, global digital image of the Moon (Figure 20).

An atlas serves many purposes: the need to have a ready compilation of maps to locate features, a desire casually to explore an unknown territory, or a summary of existing knowledge about a barely familiar place. In our case, the impetus and inspiration to make an atlas based on data from the highly successful Clementine mission in 1994 began many years ago, even before the Clementine mission. An out-of-print book, The Times Atlas of the Moon (edited by H. A. G. Lewis, Times Newspapers Limited Printing House, London, 1969) has been a boon to serious lunar students for many years. This book, although containing much ‘slick’ front matter hyping the impending landing of Apollo 11 on the Moon, conveniently bound together all of the published US Air Force LAC (Lunar Astronautical Charts) series of lunar maps, originally published separately at a scale of 1:1000000. The LAC series consisted of airbrushed, shaded-relief maps, overlain by topographic contour lines (determined from Earth-based telescopic images) and the nomenclature of lunar features. Having this wonderful map series in a single, bound, easy-to-reference volume was very handy and this book is both used and treasured by working lunar scientists to this day (hence, its rarity and cost on the used-book market).

As wonderful as the Times Atlas was and is, it has several drawbacks, aside from its unavailability. It uses old versions of the LAC charts – these maps were drawn in the early 1960s, from telescopic images, and the subsequent 40 years of spacecraft exploration have produced an abundance of exciting, new images and data with which to compile an atlas.

Earth's Moon is mankind's first offshore island in space, an exotic world with its own unique properties. The Moon's motions and environment create challenges and opportunities. The following is a brief description of the general properties of the Moon, its motions and environment, surface, geology and history.

Basic properties and motions

The Moon is quite large in relation to the planet it orbits, measuring about one-quarter the radius of the Earth (Table 1). In surface area, the Moon at 38 million square kilometres is slightly larger than the continent of Africa. The tenuous lunar atmosphere is a near-perfect vacuum; no weather affects its terrain and the sky is perpetually black. Stars are visible from the surface during daytime, but difficult to see because the glare reflected from the surface dilates the pupils. At high noon, the surface temperature can be over 100 °C and, at midnight, as low as -150 °C. The lunar day (the time it takes to rotate once on its spin axis) is about 29 Earth days or 708 h, and daylight on the Moon (sunrise to sunset) lasts almost two weeks. Because the Moon has only 1% of the mass of the Earth, its surface gravity is only one-sixth as strong. Thus, an astronaut who weighs 200 lb on the Earth will weigh 34 lb on the Moon.

The Moon moves in an elliptical path around the Earth, completing its circuit once every 29 days. This equals the amount of time it takes for the Moon to rotate once on its axis (the lunar day).

The early history of the space program was dominated by the ‘space race’ between the United States of America and the (former) Soviet Union (USSR). The launch of the Soviet satellite Sputnik in October 1957 sent shock waves through the American psyche. With the successful launch of Explorer 1 into Earth orbit by Wernher von Braun and his colleagues in January 1959, America entered the race and the battle was joined. Over the next several years, the United States seemed to be catching up to the Soviets as it orbited many satellites and prepared to send men into the unknown, but once again, the Soviets struck first, as Yuri Gagarin was launched into Earth orbit in April 1961. The new American president, John F. Kennedy, searched desperately for a field to challenge the Soviets successfully. After due consideration, Kennedy set a decade-long goal of landing a man on the Moon and returning him safely to Earth.

Important information was needed about the Moon in advance of human missions to assure a safe voyage and landing. We needed to learn how to control spacecraft at lunar distances, how to maintain an orbit around the Moon, and how to land and operate safely where we didn't know the surface conditions. Having these knowledge requirements ensured the need for precursor missions, missions that would not only blaze the trail for the people to follow, but would invariably advance our understanding of the Moon and its environment in major ways (Table 3).