Over tens of thousands of years, the gases inside the globule continued to fall away from the inside edge of the cocoon, pulled inexorably towards that dense core at the centre. By now, the core of the globule was taking on a definite shape – a gargantuan ball, about the size of the present-day Solar System out to Pluto. Its surface was still too cold to glow optically. But, at last, its central regions had warmed up significantly – to about 10 000 Celsius – and the molecules there had split into atoms of hydrogen.

This marked an important point in the development of the Sun. At this temperature, the cloud core was now hot enough for the radiation it emitted to carry a significant punch. Radiation is composed of tiny packets of energy called photons, each of which can be likened to a subatomic particle. If there are enough of these photons emitted every second they can hit like a hail of bullets, a barrage of electromagnetic force known as radiation pressure. Before this point the core of the globule had been emitting too few photons to exert a noticeable force. Now, though, as the growing waves of radiation streamed away from the warming core they slammed into the outermost regions of the globule where the gases were less dense, and slightly hindered their inbound journey. Thus the contraction of the core slowed, but it did not stop, so overwhelming was the inward pull of gravity. The very centre of the core was also dense enough now that it was beginning to become opaque to the heat radiation generated inside it.

By 3 million years or thereabouts – about 1 million years after the initial collapse of the globule – the protosun had shrunk to a few solar radii. Its temperature at the centre was now around 5 million degrees Celsius, while the surface seethed and bubbled at around 4500 Celsius. At last the object had crossed the line that separates protostars from true stellar objects. It joined the ranks as what astronomers call a T-Tauri star.

Named after a prototypical young stellar object in the constellation Taurus, the T-Tauri phase is one of extreme fury. And as with all T-Tauri stars, this earliest form of solar activity would have been driven – at least in part – by a powerful magnetic field. Because the gases inside the young star were by now fully ionised – a soup of positively and negatively charged elements – their movement as the star rotated effectively amounted to a series of gigantic electric currents. Thus the spinning star developed a global magnetic field in the same way that a wire carrying an electric current does – just as the Sun generates its field even today. During the Sun's T-Tauri phase, though, the star would have been spinning very quickly – once in 8 days compared with once in 30 days – spun up by the swirling gases that had ploughed into it earlier. This means that the T-Tauri Sun's magnetic field was much mightier than at present, and this is what made this phase in the Sun's formation so violent. The Sun was still surrounded by its protoplanetary disc. So, as the Sun whirled around, it dragged its magnetic field through this disc.

Time goes by. And the dead Solar System continues to orbit the galactic centre just as it does today. Its journey takes it past stars young and old; carries it through the ghostly ruins of stars already dead – shrouds of gas ejected by planetary nebulae, or during the cataclysmic explosions known as supernovae. Ultimately the ashes of the star once known as the Sun are spread throughout the entire Milky Way galaxy, mixed indistinguishably with the remains of other stars, replenishing the interstellar medium from which they sprang billions of years earlier.

Then one day…

A massive star reaches the end of its life. It blows itself apart. Shockwaves from the supernova spread outwards from the epicentre through the interstellar medium like concentric rings on the surface of a lake. The waves compress the gas clouds through which they propagate. And eventually, somewhere, part of the cloud begins to contract under its own gravity. Millions of years later, a new star shines in the galaxy, born from the ashes of those long dead – including the Sun. Perhaps planets will also form around this new star – even life. And so it could be that the very atoms that currently comprise our bodies will one day find themselves part of a very different, alien creature. For the Universe is the ultimate recycling machine. We have come full circle.

At last, after a period of perhaps 30 to 50 million years – astronomers still cannot agree on their numbers – the Sun's contraction finally came to an end. Why? Because the Sun's internal temperature had reached an all-time high of 15 000 000 Celsius – and something had begun to happen to its supply of hydrogen.

Hydrogen is the simplest of all elements. Each atom contains just a single subatomic particle called a proton in its nucleus, positively charged. Orbiting this, meanwhile, is a single much smaller particle with exactly the opposite electric charge: an electron. Inside the Sun, these atoms are ionised: the electrons are detached and roam freely in the sea of hydrogen nuclei or protons. Very often, two of these hydrogen nuclei come together. Just as two magnetic poles of like polarity repel each other, so too do two protons. But not if they are brought together with sufficient speed. The speed of particles in a gas can be measured by the gas's temperature. And at 15 000 000 Celsius, the positively charged hydrogen nuclei at the Sun's core were now moving so quickly that when they smashed together they overcame their electrostatic repulsion, and fused as stronger nuclear forces took over. At last, the hydrogen was being consumed, gradually converted into helium in the Sun's core via a chain of nuclear reactions. Energy is a by-product of these reactions. And so the Sun now began to generate a significant amount of power in its core.

The Sun, its nine planets and their satellites, the asteroids and the comets – together, these are the elements that comprise the Solar System. In this book we shall meet them in detail. We shall come to know their properties, their place in the Solar System, what they look like and how they compare with one another. We will learn what they are made of, when and how they were made. We will discover what the Solar System's various contents have endured since their fiery birth. And, lastly, we shall see what will happen to them – to the Solar System as a whole – in the far, distant future, billions of years from now, as the tired star we call the Sun passes into old age, and beyond. These and other issues are all part of a great story – the story of the Solar System.

Overview of the Solar System

What is the shape of the Solar System? Where are the various objects within it to be found, and how do they move in relation to each other? These are important questions. For, unless we can answer them as accurately as possible, we shall be doomed to failure in our treatment of an even more fundamental issue, dealt with in detail in this book: the origin of the Solar System. So perhaps it would be prudent to spend a little time putting together what we currently know about the Solar System of which we are all a part.

The first thing to establish is that the centre of our planetary system is solar territory.

Introduction

In this chapter, we complete our survey of the principal types of remote sensing instrument by discussing those active systems that make direct use of the backscattered power. Optical (lidar) systems are used for sounding clouds, aerosols and other atmospheric constituents, for characterising surface albedo, and for measuring wind speeds. These are discussed briefly in section 9.2. However, the bulk of this chapter is concerned with microwave (radar) systems.

In section 9.3 the ground-work established in chapter 3 is extended to a derivation of the radar equation, which shows how the power detected by a radar system is related to the usual measure of backscattering ability, the differential backscattering cross-section σ0. The remainder of the chapter discusses the main types of system that employ this relationship. The first and simplest is the microwave scatterometer (section 9.4), which measures σ0, usually only for a single region of the surface but often for a range of incidence angles. As described here, this is not an imaging system, although the distinction between microwave scatterometers and imaging radars is not a precise one.

The last two sections discuss the true imaging radars. Section 9.5 describes the side-looking airborne radar (SLAR), or real-aperture radar, which achieves a usefully high spatial resolution in one dimension by time-resolution of a very short pulse. Resolution in the perpendicular direction is achieved by using an antenna with a narrow beamwidth, namely a large antenna.

Definition and origins of remote sensing

‘Remote sensing’ is, broadly but logically speaking, the collection of information about an object without making physical contact with it. This is a simple definition, but too vague to be really useful, so for the purposes of this book we restrict it by confining our attention to the Earth's surface and atmosphere, viewed from above using electromagnetic radiation. This narrower definition excludes such techniques as seismic, geomagnetic and sonar investigations, as well as (for example) medical and planetary imaging, all of which could otherwise reasonably be described as remote sensing, but it does include a broad and reasonably coherent set of techniques, nowadays often described by the alternative name of Earth Observation. These techniques, which now have a huge range of applications in the ‘civilian’ sphere as well as their obvious military uses, make use of information impressed in some way on electromagnetic radiation ranging from ultraviolet to radio frequencies.

The origins of remote sensing can plausibly be traced back to the fourth century bc and Aristotle's camera obscura (or, at least, the instrument described by Aristotle in his Problems, but perhaps known even earlier). Although significant developments in the theory of optics began to be made in the seventeenth century, and glass lenses were known much earlier than this, the first real advance towards our modern conception of remote sensing came in the first half of the nineteenth century with the invention of photography.

Introduction

Chapters 5 to 7 considered passive sensors, detecting naturally occurring radiation. In this chapter and the next we shall discuss active sensors, which emit radiation and analyse the signal that is returned by the Earth's surface or atmosphere. We have already identified three possible classifications of remote sensing systems, distinguishing between passive and active and between imaging and non-imaging, as well as classifying them according to the wavelength of radiation employed. We can also classify active systems according to the use that is made of the returned signal. If we are principally concerned with the time delay between transmission and reception of the signal we shall call the method a ranging technique, whereas if we are also (or mainly) interested in the strength of the returned signal we shall call it a scattering technique. The distinction between the two cannot be made entirely rigorous, but it provides a useful way of thinking about active remote sensing systems. It is clear that ranging systems are simpler both to visualise and, because of their less stringent technical demands, to construct, and we shall therefore consider them first. In chapter 9 we discuss the scattering techniques.

Laser profiling

Laser profiling (or laser altimetry) is the simplest application of the lidar (LIght Detection And Ranging) technique. Conceptually, it is extremely straightforward. A short pulse of ‘light’ (visible or near-infrared radiation) is emitted towards the Earth's surface by the instrument, and its ‘echo’ is detected some time later.

In chapter 3, we discussed principally the interaction of electromagnetic radiation with the surface and bulk of the material being sensed. However, the radiation also has to make at least one journey through at least part of the Earth's atmosphere, and two such journeys in the case of systems that detect reflected radiation, whether artificial or naturally occurring. Each time radiation passes through the atmosphere it is attenuated to some extent. In addition, as we have already seen in section 3.1.2 and figure 3.4, the atmosphere has a refractive index that differs from unity so that radiation travels through it at a speed different from the vacuum speed of 299 792 458 m s−1. These phenomena must be considered if the results of a remotely sensed measurement are to be corrected for the effects of atmospheric propagation, or if they are to be used to infer the properties of the atmosphere itself. We have already considered them in general terms in discussing the radiative transfer equation (section 3.4). In this chapter, we will relate them more directly to the constituents of the atmosphere.

Composition and structure of the gaseous atmosphere

At sea level, the principal constituents of the dry atmosphere are molecules of nitrogen (about 78% by volume), oxygen (21%) and the inert gas argon (1%). There is also a significant but variable (typically 0.1% to 3%) amount of water vapour, often specified by the relative humidity H.

Introduction

In chapter 5 we discussed photographic systems, and although these provide a familiar model for many of the concepts to be addressed in this and subsequent chapters, they nevertheless stand somewhat apart from the types of system to be discussed in chapters 6 to 9. In the case of photographic systems, the radiation is detected through a photochemical process, whereas in the systems we shall now consider it is converted into an electronic signal that can be detected, amplified and subsequently further processed electronically. This clearly has many advantages, not least of which is the comparative simplicity with which the data may be transmitted as a modulated radio signal, recorded digitally and processed in a computer.

In this chapter, we shall consider electro-optical systems, interpreted fairly broadly to include the visible, near-infrared and thermal infrared regions of the electromagnetic spectrum. The reason for this is a pragmatic one, since many instruments combine a response in the visible and near-infrared (VIR) region with a response in the thermal infrared (TIR) region, and much of the technology is common to both. Within this broad definition we shall distinguish imaging systems, designed to form a two-dimensional representation of the two-dimensional distribution of radiance across the target, and systems used for profiling the contents of the atmosphere. It is clear that an imaging system operating in the VIR region has much in common with aerial photography, and systems of this type are in very wide use from both airborne and spaceborne platforms.