Philosophy

The discipline of radiation hydrodynamics is the branch of hydrodynamics in which the moving fluid absorbs and emits electromagnetic radiation, and in so doing modifies its dynamical behavior. That is, the net gain or loss of energy by parcels of the fluid material through absorption or emission of radiation is sufficient to change the pressure of the material, and therefore change its motion; alternatively, the net momentum exchange between radiation and matter may alter the motion of the matter directly. Ignoring the radiation contributions to energy and momentum will give a wrong prediction of the hydrodynamic motion when the correct description is radiation hydrodynamics.

Of course, there are circumstances when a large quantity of radiation is present, yet can be ignored without causing the model to be in error. This happens when radiation from an exterior source streams through the problem, but the latter is so transparent that the energy and momentum coupling is negligible. Everything we say about radiation hydrodynamics applies equally well to neutrinos and photons (apart from the Einstein relations, specific to bosons), but in almost every area of astrophysics neutrino hydrodynamics is ignored, simply because the systems are exceedingly transparent to neutrinos, even though the energy flux in neutrinos may be substantial.

Another place where we can do “radiation hydrodynamics” without using any sophisticated theory is deep within stars or other bodies, where the material is so opaque to the radiation that the mean free path of photons is entirely negligible compared with the size of the system, the distance over which any fluid quantity varies, and so on. In this case we can suppose that the radiation is in equilibrium with the matter locally, and its energy, pressure, and momentum can be lumped in with those of the rest of the fluid.

Non-LTE

The title non-LTE refers to a method for analyzing the interaction between a gas and the radiation field that accounts for the modification of the excitation and ionization state of the atoms in the gas by the influence of the radiation field. If the radiation field is weak or the density is large, then the occupation probabilities of the atomic states are governed by the dominant collisional processes. In a wide variety of cases the rates of electron–electron and ion–ion collisions, and usually electron–ion collisions as well, are so great that the relaxation of all these species to a single kinetic temperature is essentially complete. So when collisional processes dominate the atomic excitation rates, the result is occupation probabilities that agree with thermodynamic equilibrium, no matter what the radiation intensities may be. This is LTE – local thermodynamic equilibrium. So non-LTE is the other situation. In non-LTE the relaxation of the velocity distributions of the electrons and atoms to a single kinetic temperature is still supposed to be true. But what is not true in non-LTE is that collisional processes are generally dominant over the competing radiative processes for populating and depopulating the atomic states.

Non-LTE is a large subject. Good references for non-LTE line transport are Mihalas's Stellar Atmospheres (1978), Athay (1972b) and Ivanov (1973). Approximations to the electron-impact excitation rate are discussed by Sobel'man (1979) and by Sampson and Zhang (1992, 1996).

Kinetic equations

As a system for doing calculations, non-LTE consists of solving for the radiation field Iv and the set of atomic occupations Nij, also often called the “populations.” Here i is an index for elements, and j is an index for an energy level belonging to that element.

We turn now to the subject of radiation transport. As much as possible, the present goal is to demystify this subject. Photons are just particles like the others that make up our systems; they just happen to go faster and farther, and are therefore often of special importance in carrying energy and momentum from one place to another. In kinetic theory we introduce the phase-space distribution function for the atoms, develop the theory of the Boltzmann transport equation, and come up with some satisfactory approximate methods for solving it. Radiation transport is exactly the same; the transport equation is about the same, and the approximate methods are about the same as well. The difference is that the subject of radiation transport was elaborated by different people than was kinetic theory, using an entirely different notation, and we have that difference with us today. In the last two or three decades yet another community has joined the discussion of radiation transport, and these are the nuclear engineers, who have evolved a collection of methods for describing neutron transport, methods that are useful for photons as well as neutrons. The present discussion will not attempt to show, Rashomon-like, the same physical concepts from the varied points of view of several disciplines. We will stick with one, mainly the astrophysical notation found, for example, in Mihalas and Mihalas (1984). The elementary definitions of the radiation field quantities are found in many astrophysics books. One good treatment is Mihalas's Stellar Atmospheres (1978), and this is also found in Mihalas and Mihalas (1984).

Introduction

The spectacular collision of the Shoemaker-Levy 9 comet with Jupiter in July 1994 was a dramatic reminder of the fact that the Earth has and will continue to experience such catastrophic events. While the frequency of such massive collisions is very low, smaller objects collide with the Earth regularly and do damage that would be intolerable in any populated region. As an example, the Tunguska (Siberia) event of 1908 is estimated to have involved a 60-m object exploding at a height of 8 km and produced devastation over an area almost the same as that devastated by the eruption of Mt. St. Helens (Morrison et al. 1994). The famous 1-km Meteor Crater in Arizona was made by the impact of an even smaller body only 30 m in diameter (Adushkin and Nemchinov 1994). Human casualties due to direct meteorite strikes are rare but known (Yau 1994). The greater danger is due to the fact that the time between large impacts, such as the Tunguska impact which released tens of megatons of TNT equivalent energy, is significant compared to a human lifetime and there is a small chance that any impact will be in a populated area. The relative scarcity of such areas on the Earth may not offer the protection one might think as recent calculations suggest that larger bodies might do more damage if they didn't hit land; predicting that an impact anywhere in the Atlantic Ocean by a 400-m asteroid would devastate the (well-populated) coasts on both sides of the ocean with tsunamis over 60 m high (Hills et al. 1994).

The chapters in this book are based on a series of invited lectures given at the “Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids,” which was held in Arlington, Virginia, on September 3–6, 2002. The focus of the workshop was to determine what needs to be done to ensure that an adequate base of scientific knowledge can be created that will allow efficient development of a reliable, but as yet undefined, collision mitigation system when it is needed in the future.

To achieve this goal essentially all aspects of near-Earth objects were discussed at the workshop, including the completeness of our knowledge about the population of potential impactors, their physical and compositional characteristics, the properties of surveys that need to be done to find hazardous objects smaller than 1 km in size, our theoretical understanding of impact phenomena, new laboratory results on the impact process, the need for space missions of specific types, education of the public, public responsibility for dealing with the threat, and the possible roles in the United States of the National Aeronautics and Space Administration (NASA), the military, and other government agencies in mitigating the threat.

Most of these topics are, we believe, well covered by the material contained within this volume and so it should serve both as a snapshot of the state of the collision hazard issue in the United States in late 2002 and also a useful sourcebook for reference into the associated technical literature.

Introduction

In the twenty-first century, we must consider the asteroid and comet impact hazard in a context in which citizens of many nations are apprehensive about hazards associated with foods, disease, accidents, natural disasters, terrorism, and war. The ways we respond psychologically to such threats to our lives and well-being, and the degrees to which we expect our societal institutions (both governmental and private) to respond, are not directly proportional to actuarial estimates of the causes of human mortality, nor to forecasts of likely economic consequences. Our concerns about particular hazards are often heavily influenced by other factors, and they vary from year to year. Citizens of different nations demonstrate different degrees of concern about risks in the modern world (for example, reactions to eating genetically modified food or living near a nuclear power plant). Yet one would hope that public officials would base decisions at least in part on the best information available about the risks and costs, and scientists have a responsibility to assist them to reach defensible conclusions.

Objective estimates of the potential damage due to asteroid impacts (consequences multiplied by risk) are within the range of other risks that governments often take very seriously (Morrison et al. 1994). Moreover, public interest is high, fueled by increasing discovery rates and the continuing interests of the international news media. In this chapter we consider the past, present, and future of interactions by scientists with the public and media on the subject of the impact hazard.

Introduction

Over the last several decades, evidence has steadily mounted that asteroids and comets have impacted the Earth over solar system history. This population is commonly referred to as “near-Earth objects” (NEOs). By convention, NEOs have perihelion distances q ≤ 1.3 AU and aphelion distances Q ≥ 0.983 AU (e.g., Rabinowitz et al. 1994). Sub-categories of the NEO population include the Apollos (a ≥ 1.0 AU; q ≤ 1.0167 AU) and Atens (a < 1.0 AU; Q ≥ 0.983 AU), which are on Earth-crossing orbits, and the Amors (1.0167 AU < q ≤ 1.3 AU) which are on nearly-Earth-crossing orbits and can become Earth-crossers over relatively short timescales. Another group of related objects that are not yet been considered part of the “formal” NEO population are the IEOs, or those objects located inside Earth's orbit (Q < 0.983 AU). To avoid confusion with standard conventions, we treat the IEOs here as a population distinct from the NEOs. The combined NEO and IEO populations are comprised of bodies ranging in size from dust-sized fragments to objects tens of kilometers in diameter (Shoemaker 1983).

It is now generally accepted that impacts of large NEOs represent a hazard to human civilization. This issue was brought into focus by the pioneering work of Alvarez et al. (1980), who showed that the extinction of numerous species at the Cretaceous–Tertiary geologic boundary was almost certainly caused by the impact of a massive asteroid (at a site later identified with the Chicxulub crater in the Yucatan peninsula) (Hildebrand et al. 1991).

Introduction

One of the most fundamental aspects of mitigating an impact threat by moving an asteroid or comet involves physical interaction with the body. Whether one is bathing the body's surface with neutrons, zapping it with a laser or solar-reflected beam, bolting an ion thruster or mass driver onto the surface, or trying to penetrate the surface in order to implant a device below the surface, we need to understand the physical attributes of the surface and sub-surface. Of course, we would critically wish to understand the surface of the particular body that is, most unluckily, found to be headed for Earth impact – should that eventuality come to pass. But, in the event that we have relatively little warning time, it might behoove us to examine well in advance the potential range of small-body surface environments that we might have to deal with. It will improve our ability to design experiments and understand data concerning the particular body if we have evaluated, beforehand, the range of surface properties we might encounter and have specified the kinds of measurement techniques that will robustly determine the important parameters that we would want to know.

We already know, from meteorite falls, that asteroidal materials can range from strong nickel–iron alloy (of which most smaller crater-forming meteorites, like Canyon Diablo, are made) to mud-like materials (like the remnants of the Tagish Lake fireball event). But the diversity could be even greater, especially on the softer/weaker end of the spectrum, because the Earth's atmosphere filters out such materials.

Why radio tomography?

So far, the planetary explorations have focused on gathering information about the atmosphere, the ionosphere, and the surface of the planets. Most of the remote-sensing techniques have focused on observation of planets and small objects in visible and near-visible range using cameras, or GHz-range radars for surface mapping of planets (e.g., Magellan radar for Venus, Shuttle Imaging Radar (SIR A, B, and C), Shuttle Radar Topography Mapping SRTM and TOPEX/Poseidon for Earth). The radio science techniques used to study the gravity field are important for exploring planetary interior. Although these techniques have provided a wealth of information, there are still a large number of questions that cannot be answered unless we probe the sub-surface. Example of questions that radio tomography can answer are: (1) sub-surface stratigraphy on planets, (2) the existence of paleo-channels, (3) the depth of CO2 and H2O ice layers on Mars, (4) the existence of liquid water under the surface of Mars, (5) the existence of an ocean on Europa, and (6) looking inside comets, and testing the rubble-pile hypothesis for asteroids. It is well known that low-frequency electromagnetic waves (specifically, high-frequency (HF) and very-high-frequency (VHF) regimes) can penetrate ice and rocks to a depth of hundreds of meters to a few kilometers. Such radars have been used on Earth for sub-surface investigations. Also, airborne radar sounders have been deployed over the past few decades to investigate glaciers and to measure ice-layer thickness (Gudmandsen et al. 1975).

Overview

Understanding the interior structure and composition of asteroids and comets is important for understanding their origin and evolution. In addition to basic science objectives, understanding the interior structure of near-Earth objects (NEOs) will be essential to addressing mitigation techniques should it become apparent that such an object has the potential to impact Earth. NEOs are comprised of asteroids and comets in near-Earth space. Work is progressing to find, catalog, and determine the orbits of NEOs larger than 1 km, but little is known about NEOs' bulk properties, such as strength and structure (Huebner et al. 2001; Greenberg and Huebner 2002). Should a Potentially Hazardous Object (PHO) threaten Earth, attention would focus on countermeasures. All conceived countermeasures rely on knowledge of the bulk material properties of NEOs, in particular material strength, structure, mass distribution, and density. It is believed that NEO compositions range from nickel–iron through stony and carbonaceous to ice-and-dust mixtures. Their structure can be monolithic, fractured, assemblages of fragmented rock held together only by self-gravity (“rubble piles”), porous, or fluffy. Types of collision mitigation and countermeasures will vary widely depending on composition and structure.

Much of the lack of knowledge of the interior properties of NEOs is due to the fact that most study has been by remote sensing. Information such as rotation rate and shape can be determined remotely through radar and light curve analysis, but determining the geophysical and geological properties requires something more. Two techniques of imaging the interior are apparent: radio tomography (electromagnetic waves) and seismology (mechanical waves).

Introduction

The method to be used for mitigating the impact of an asteroid on Earth depends on the nature of the asteroid. A compact rock would react very differently to almost any violent mechanical event than would an object that consisted of unconsolidated dust and fragments. A water-rich, comet-like object would react very differently to laser heating than a completely hydrated object. Thus, impact mitigation begins with scientific investigation.

We have been investigating physical processes likely to be occurring on asteroids in connection with our efforts to understand the origin and history of meteorites and their relationship to asteroids. In this connection, we have been developing proposals for a near-Earth asteroid sample return mission called Hera (Sears et al. 2002c) (Fig. 15.1). Hera will visit three near-Earth asteroids, spend 3 months to 1 year in reconnaissance, and then nudge itself gently down to the surface to collect three samples from each asteroid at geologically significant sites (Britt et al. 2001). By returning weakly consolidated surface samples, the Hera mission will clarify many issues relating to the asteroid–meteorite connection and the origin and evolution of the solar system (Sears et al. 2001). In addition, interstellar grains in the samples will shed light on the relationship between our Sun and other stars.

The major challenge of the Hera mission is the design of the collector and this depends on a knowledge of the nature of the surface.

Introduction

Impacts of near-Earth objects (NEOs) onto our planet are natural events where the effects of each single impact mainly depend on NEO size, structure, relative velocity, and impact location. To determine if a newly discovered object might impact on Earth one day, the object's orbit has to be numerically computed into the future. The accuracy of this orbit prediction basically depends on the optical and eventually radar measurements available for that object, and of course on the completeness and precision of the numerical perturbation models. Care has to be taken when handling few observations, long prediction periods, and unendorsed NEO properties, which might lead to large uncertainties in collision probability prediction (e.g., Giorgini et al. 2002).

NEOs larger than 150 m in diameter and approaching Earth's orbit closer than 7.5 million km (0.05 AU) are called Potentially Hazardous Objects (PHOs). Due to their susceptibility to small orbit disturbances on short timescales they are candidates for future collisions with Earth. Typical near-Earth asteroid (NEA) impact velocities onto the Earth range from 11.18 to about 25 km s–1, whereas comets typically impact at higher velocities up to 73.65 km s–1 if on a retrograde orbit (e.g., Gritzner 1996).