At least 14 space agencies have identified ‘in situ resource utilization’ as a necessary capability for long-duration missions, including crewed missions to the Moon, Mars and deep space. Attention is focused on the potential production of rocket fuel from ice and water-bearing minerals. If fuel can be sourced in space, it will not need to be lifted, at great expense, from Earth’s surface. But while the mining of asteroids and other celestial bodies offers benefits, it will also create risks. Mining that is motivated purely by resource extraction could overlook or even destroy important scientific information, while physical interactions with an asteroid could alter its trajectory and, in some circumstances, potentially create a human-caused Earth impact risk. There are presently two competing efforts to develop widely agreed rules on space mining. The first is an industry-friendly effort in which the United States is engaging in bilateral negotiations with dozens of states, encouraging them to sign the non-binding Artemis Accords. The second is a multilateral effort that fully considers the interests of non-spacefaring states and is taking place in the United Nations Committee on the Peaceful Uses of Outer Space.

Some 66 billion years ago, a cataclysmic collision between the Earth and an asteroid ten to 15 kilometres in diameter caused the extinction of the non-avian dinosaurs. In 1908, an asteroid 50 to 70 metres in diameter levelled over 2,000 square kilometres of forest in Siberia, while in 2013 an asteroid 19 metres in diameter produced a shockwave over Chelyabinsk, Russia, sending over a thousand people to the hospital. The field of ‘planetary defence’ involves the detection, characterisation, risk assessment and, if necessary, deflection of asteroids and comets that have the potential to strike Earth. Yet there has been a lack of high-level diplomacy on this issue. In particular, the low probability of a major Earth impact happening in our lifetime makes planetary defence a low priority for political leaders, despite the existential consequences of impacts and their eventual certainty of occurring. There is also a shortage of widely agreed international law, including on the potential use of nuclear explosive devices for deflecting asteroids. Most importantly, there is a lack of agreement on who is responsible for vetting the science, assessing the risks and making decisions if Earth were faced with an actual impact threat. Is it the United Nations Security Council that decides? What if a Security Council decision is blocked by one of its veto-holding permanent members? Would a state that acted unilaterally be excused any illegality because of the necessity of its actions, according to the international law on ‘state responsibility’?

This book depicts a vivid and vibrant image of modern Main Belt asteroid science. In the last decade, thanks to the exploration by the NASA Dawn mission and the advent of high-resolution Earth-bound observations, we have entered a renaissance of Main Belt asteroid science. Formation theories, dynamical models, meteorite geochemical data, remote and in-situ observations synergistically show asteroids are leftover building blocks of planetary formation and tracers of important evolutionary processes (e.g., collisions, orbital migration) that have shaped the evolution of the early Solar System. Planned missions such as NASA’s Lucy and Psyche (scheduled to launch in 2021 and 2022) will surely provide additional colorful strokes to our ever-evolving portrait of the Main Belt.

The asteroid belt was dynamically shaped during and after planet formation. Despite representing a broad ring of stable orbits, the belt contains less than one one-thousandth of an Earth mass. The asteroid orbits are dynamically excited, with a wide range in eccentricity and inclination, and their compositions are diverse (generally dry objects in the inner belt and more water-rich objects in the outer belt). The asteroid belt’s origins and dynamical history are reviewed. The classical view is that the belt was born with several Earth masses in planetesimals, then strongly depleted. However, it is possible that very few planetesimals ever formed in the asteroid region and the belt’s story is one of implantation rather than depletion. Many processes may have implanted asteroids from different regions of the Solar System, dynamically removed them, and excited their orbits. During the gaseous disk phase these include the effects of giant planet growth, migration, and sweeping secular resonances. After this phase these include scattering from resident planetary embryos, chaos in the giant planets’ orbits, giant planet instability, and long-term dynamical evolution. Different global models for Solar System formation imply contrasting dynamical histories of the asteroid belt. Vesta and Ceres may have been implanted from opposite regions of the Solar System – Ceres from the Jupiter–Saturn region and Vesta from the terrestrial planet region – and could therefore represent very different formation conditions.

In 1992, NASA’s planetary efforts were invigorated with the launch of the Discovery Program of principal investigator-led missions. Over the next eight years, a group of planetary scientists and engineers gathered regularly to design and propose to NASA solar-electric propulsion missions targeted to various scientifically important bodies. Ultimately, Dawn, a mission to orbit and explore both Vesta and Ceres, was selected for flight in 2001. It launched in 2007, arrived at Vesta in July 2011, and departed in September 2012 for Ceres. Arrival at Ceres occurred in March 2015, where Dawn operated productively until 31 October 2018, when it exhausted its attitude control propellant. Herein, we summarize the history of Dawn and recount the observations and discoveries made by this pioneering mission.

Classification, physical properties, orbits, evolution of asteroids, comets, and KBOs

An ever-increasing number of laboratory facilities are enabling in situ spectral reflectance measurements of materials under conditions relevant to all the bodies in the Solar System, from Mercury to Pluto and beyond. Results derived from these facilities demonstrate that exposure of different materials to various planetary surface conditions can provide insights into the endogenic and exogenic processes that operate to modify their surface spectra, and their relative importance. Temperature, surface atmospheric pressure, atmospheric composition, radiation environment, and exposure to the space environment have all been shown to measurably affect reflectance and emittance spectra of a wide range of materials. Planetary surfaces are dynamic environments, and as our ability to reproduce a wider range of planetary surface conditions improves, so will our ability to better determine the surface composition of these bodies, and by extension, their geologic history.

New visible and infrared data of minor bodies, including minor planet 1 Ceres, asteroids 4 Vesta, 21 Lutetia, 2867 Steins and comet 67P/Churyumov–Gerasimenko (hereafter 67P/CG) have been collected in the last years by remote sensing instruments aboard NASA-Dawn and ESA-Rosetta missions. These minor bodies are among the most primitive bodies in the Solar System, and the understanding of their composition, surface morphology and evolution history is a fundamental step to shed light on the processes that occurred during planetary formation.By merging spatial and spectral information retrieved from the surfaces of these objects it is possible to infer their composition and physical properties and to correlate them with local morphology and geological processes. A discussion about spectral indicators, modeling, and mapping is given for both asteroids and comet 67P/CG. Given that the remote sensing observation techniques are very similar between Dawn and Rosetta missions, a comparative approach is used for the entire chapter and methods and interpretation for the results of these different objects are given together.

Asteroids represent a time capsule, storing information about the composition of and conditions in the solar nebula as well as processes that have affected the Solar System. The asteroid population includes primitive bodies, partially melted material, and the result of full melting and differentiation of planetesimals. Asteroidal minerals, organic molecules, and ices that are relevant to uncovering the history of the Solar System are accessible spectroscopically. Reflection and thermal emission spectroscopy from ground-based telescopes, space telescopes, and spacecraft provide a rich view of asteroid surfaces. Analysis techniques, including taxonomic classification, direct comparisons to meteorites and pure materials, band parameter analysis, and physical models of light scattering, are customized to the specific science question under study. In recent years, spacecraft missions to asteroids have provided ground-truth to more remote spectral analyses, corroborating many inferences from ground-based observations, while enabling new discoveries and a richer, deeper view of asteroid surfaces. These compositional studies provide an important complement to and constraint on dynamical investigations of Solar System evolution. The future of asteroid science is bright, with advances expected in the areas of sample return, additional reconnaissance missions, improved wavelength coverage spectroscopy, and significant increases in the size of the database of asteroid spectra.