Borderlands of Science

There is another way to distinguish physics from chemistry. Physics is needed in describing the world of the very small (atoms and down) and the very large (stars and up). Chemistry works with everything in between, from molecules to planets. So although physics is vital to us (where would we be without gravity and sunlight?), chemical processes largely control our everyday lives.

An exception to this rough rule has been created in the last century, as a result of human activities. Lasers, nuclear power, and all electronics from computers to television derive from the sub-atomic world of physics.

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5.1 Isaac Asimov and the Timonium engine. Isaac Asimov was a famous science fiction writer, justly proud of the breadth of his knowledge. He wrote books on everything that you can think of, inside and outside science.

In the nineteenth century, the following jingle was made up about Benjamin Jowett, a famously learned Oxford Don:

"I am Master of this college,

What I don't know, isn't knowledge."

About Asimov, we might offer this variation:

"I am science fiction's guru,

What I don't know, don't say you do."

This doesn't actually rhyme, but it makes a point. Asimov knew lots. So when he, on a panel at a science fiction convention in Baltimore, heard one of the other speakers refer to a spaceship whose "engines were powered by timonium," and he noticed that all the audience laughed in a knowing way, he was quite put out.

Only later did he learn that this was a purely local reference. Timonium sounds like the name of an element, similar to titanium or plutonium, but it is actually a suburb of Baltimore. There was little chance that Asimov would know about it, and the other speaker relied on that fact.

Why, though, was Asimov so sure that timonium wasn't an element, perhaps a newly-discovered one between, say, titanium and chromium? Simply because there is already an element, vanadium, between titanium and chromium; and there cannot be any others. Titanium has atomic number 22, vanadium 23, chromium 24. If you say in a story that someone has found another element in there, is it much like saying that you have discovered a new whole number between eight and nine.

In fact, although the normal ordering of the elements corresponds to their atomic weights, their numbering is just the number of electrons that surround the nucleus. Hydrogen has one, helium has two, lithium has three, beryllium has four, and so on. Nothing has, or can have, a fractional number of electrons.

Moreover, the electrons are not buzzing around the nucleus at random, they are structured into electron "shells," each of which holds a specific number of electrons. Chemical reactions involve electrons only in the outermost shells of an atom, and this decides all their chemical properties. The shells have been given names. Proceeding outward from the nucleus, we have K, L, M, N, O, P, and Q. Easy to remember, but of no physical significance.

The innermost shell can hold only two electrons. Hydrogen has one electron, so it has a place for one more, or alternatively we might think that it has one electron to spare. Hydrogen can take an electron from another atom, or it can share its single electron with something else. Helium, on the other hand, has two electrons. That makes a filled shell, and in consequence helium has no tendency to share electrons. Helium does not react appreciably with anything. If you write a story in which a race of aliens are helium-breathers, you'd better have a mighty unusual explanation.

Other elements with closed shells are neon (complete shell of two plus complete shell of eight), argon, (shell of two plus shell of eight plus shell of eight), krypton, xenon, and radon. You may recognize these as the "noble gases" or "inert gases." All resist combination with other elements. Radon, element number 86, decays radioactively, but fairly slowly, over a period of days. Radioactive decay, since it involves the nucleus of the atom, is by our definition a subject for physics rather than chemistry.

Atoms with one space left in a filled shell also combine very readily, particularly when that "hole" can be matched up with an extra electron in some other element which has one too many for a filled shell. Elements with one electron less than a filled shell include hydrogen, fluorine, chlorine, bromine, iodine, and astatine. These elements are all strongly reactive, and collectively they are known as "halogens." Halogen means "salt-producing," for the good reason that these elements, in combination, all produce various forms of salts. The last element in the halogen list, astatine, element number 85, is also unstable. In a few hours it decays radioactively. However, the properties of astatine — while it lasts — are very similar chemically to the properties of iodine.

Atoms with one electron too many for a filled shell include the elements lithium, sodium, potassium, rubidium, and caesium. Known collectively as the alkali metals, they combine readily with the halogens (and many other elements) and form stable compounds. Note that we can choose to regard hydrogen as a halogen, an alkali metal, or both.

The actual number of electrons in each shell is decided by quantum theory. However, long before quantum theory had been dreamed up and before anyone knew of electrons, the elements had been formed into groups in terms of their general chemical properties. This was done by Dmitri Mendeleyev, who in 1871 developed a "periodic table" of the elements. It is still in use today, suitably extended by elements discovered since Mendeleyev's time — discovered, in large part, because he had used the periodic table to predict that they ought to be there.

We repeat, for emphasis: the chemical properties of an element are completely decided by the number of electrons in its outer shell only. There is no scope for adding new elements "in the cracks" of the periodic table, and little scope for new chemical properties beyond those known today.

Can we find a way around that hindrance, and give the writer some room to maneuver?

We can. The so-called "natural" elements begin with hydrogen, atomic number 1, and end with uranium, atomic number 92. Uranium is itself radioactive, so over a long enough period (billions of years) it decays to form lighter elements. Heavier elements than uranium are not impossible to make, but they are unstable. In a short time — for elements with high enough atomic number, small fractions of a second -- they decay and become some other, lighter element.

If we could just make stable elements heavier than uranium, or anything else known to our laboratories today, a whole new field of chemistry would open up. These new "transuranic" elements could have who-knows-what interesting properties.

Now, as heavier and heavier elements are created beyond uranium, their radioactive decay to other elements normally takes less and less time. It seems hopeless to look for new stable elements. However, there is one ray of hope. The neutrons and protons that make up the atomic nucleus form, like the electrons outside it, "shells." When the number of neutrons in a nucleus has certain values (known as "magic" numbers), the corresponding element is unusually stable. Similarly, extra stability is achieved when the number of protons in a nucleus has "magic" values. If a nucleus has the right number of protons (usually written Z) and the right number of neutrons (usually written N), it is known as "doubly magic," and is correspondingly doubly stable.

Magic numbers, computed from the shell theory of the nucleus (and generally agreeing with experiment), are 2, 8, 20, 28, 50, 82, 114, 126, and 184. The theoretical calculations provide the higher values, but these are not seen in naturally occurring elements which end at uranium with Z = 92. In principle, doubly-magic numbers would occur with any pair of these magic numbers, such as Z = 20 and N = 8. In practice, in every heavy nucleus, the number of neutrons is greater than or equal to the number of protons. Also, a nucleus is unstable if N exceeds Z (known as the "neutron excess") by a large factor. Given these two rules, we might expect nuclei of extra stability for Z = 2, N = 2; Z = 8, N = 8; Z = 20, N = 20; Z = 28, N = 28; and so on.

What we find in practice is that Z = 2, N = 2 is helium, and the nucleus is highly stable. Z = 2, N = 8 is not stable at all, because the excess of neutrons over protons is too large. Z = 8, N = 8 is oxygen, and it is very stable. So is Z = 20, N = 20 (calcium, stable), and Z = 82, N = 126 (lead, also stable).

We now see a possibility that doubly-magic, extra-stable elements might exist with Z = 114 or Z = 126, and a suitably high neutron number, N, of 184. Experiments so far have not led to any such elements, but the existence of an "island of stability" somewhere between element 114 and element 126 is a suitable offshore location for science fictional use.

Even more interesting is the possibility that humans might someday discover a way to stabilize naturally radioactive materials against decay. We know of no way to do this at the moment, but we can argue that it might be possible, with an analogy provided by Nature. A free neutron, left to itself, will usually decay in a quarter of an hour to yield a proton and an electron. Bound within a nucleus, however, the same neutron acts as a stable particle. The helium nucleus, two protons and two neutrons, is one of the most stable structures known. Perhaps, by embedding a super-heavy nucleus from the island of stability within some larger structure, we can prevent its decay for an indefinitely long period. The super-heavy transuranic elements might then share a property of the quark, that while we understand their properties, we never actually observe them.

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5.2 The limits of strength. The strength and flexibility of a material, anything from chalk to cheese, depend on the bonds between its atoms and molecules. Since interactions at the atomic level take place only between the electrons in the outer shell of atoms, the strength of those bonds is decided by them.

The density and mass of a material, on the other hand, is decided mainly by the atomic nucleus. For every electron in an atom there must be a proton in the nucleus, plus possible neutrons, and the neutron and proton each outweigh the electron by a factor of almost two thousand.

We thus have an odd contrast:

Strength: determined by outer electrons.

Weight: determined by nucleus.

Using these two facts, without any other information whatsoever, we can reach a conclusion. The strongest materials, for a given weight, are likely to be those which have the most outer electrons, relative to the total number of electrons, and the least number of neutrons (which are purely wasted weight) relative to the number of protons.

The elements with the most electrons in the outer shell relative to the total number of electrons are hydrogen and helium. As we know already, helium is a poor candidate for any form of strong bond. Also, whereas the hydrogen nucleus has no neutrons, the helium nucleus has as many neutrons as protons. We conclude, on theoretical grounds, that the strongest possible material for its weight ought to be some form of hydrogen. Of course, hydrogen is a gas at everyday temperatures, but that should not deter us.

TABLE 5.1 shows the strength/weight ratio of different materials. It confirms our theoretical conclusion. Hydrogen, in solid form, ought to be the strongest "natural" material -- once we have produced it.

We have added to the table "below the line" a couple of extra items with a decidedly science fictional flavor. Muonium is like hydrogen, but we have replaced the single electron in the hydrogen atom with a muon, 207 times as massive. The resulting atom will be 207 times smaller than hydrogen, and should have correspondingly higher bonding strength.

Muonium, considered as a building material, is not without its problems. The muon has a lifetime of only a millionth of a second. In addition, because the muon spends a good part of its time close to the proton of the muonium atom, there is a fair probability of spontaneous proton-proton fusion.

Positronium takes the logical final step of getting rid of the wasted mass of the nucleus completely. It replaces the proton of the hydrogen atom by a positron, an electron with a positive charge. Positronium, like muonium, has been made in the lab, but it too is unstable. It comes in two varieties, depending on spin alignments. Para-positronium decays in a tenth of a nanosecond. Ortho-positronium lasts a thousand times as long, a full ten-millionth of a second.

As in the case of our transuranic elements, we rely upon future technology to find some way to stabilize them.

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5.3 The production of energy. Burning and the strength of materials do not sound to have much in common, but they are alike in this: they both depend on the relationship of the outer electrons of atoms to each other. We use the word "burning" to mean not only the combination of another element with oxygen, but to describe the combination of any elements by chemical means. We also include the very rapid burning that would normally be called an explosion.

We would like to know the maximum possible energy that can be obtained by chemical combination of a fixed total amount of any materials. This information will prove particularly important for space travel.

One way to find out the energy content of typical fuels using standard oxygen burning is to look up their "caloric value." We can also determine, for any particular material, how much weight of oxygen is needed per unit of fuel. Hence, looking ahead to the needs of Chapter 8, we can calculate how fast the total burned material would travel if the heat produced were entirely converted to energy of motion.

The result does not give a wide range of answers for many of the best known fuels. Pure carbon (coal) gives an associated velocity of 4.3 kilometers a second. Ethyl alcohol leads to the same value. Methane and ethane have almost identical values, at 4.7 kms/sec. The highest value is achieved with hydrogen, at 5.6 kms/sec.

These values are more than can be achieved in practice, because all the energy produced does not go into kinetic energy. Otherwise, the combustion products would be at room temperature (assuming they started there — liquid hydrogen and liquid oxygen, which together make an excellent rocket fuel, must be stored at several hundred degrees below zero). Also, from thermodynamic arguments, conversion of heat energy to motion energy can never be one hundred percent efficient.

Why assume that oxygen must be one component in the fuel? Only because oxygen is in plentiful supply in our atmosphere. However, it turns out that oxygen is a good choice for another reason: it combines fiercely with other elements. It is also relatively light (atomic weight, 16). When we examine other combinations of elements, the only one better than a hydrogen-oxygen combination is hydrogen and fluorine. Fluorine, the next element in the atomic table to oxygen, is a halogen, which as we have already noted means it is strongly reactive. When we burn hydrogen and fluorine, and convert all the energy to motion, we find a velocity of 5.64 kms/sec.

This is a very small gain over hydrogen/oxygen, and there are other disadvantages to using fluorine. The result of the combustion of hydrogen and oxygen is water, as user-friendly a compound as one can find. The combination of hydrogen and fluorine, however, yields hydrofluoric acid, a most unpleasant substance. Among other things, it dissolves flesh. Release it into the atmosphere in a rocket launch, and you will have the Environmental Protection Agency jumping all over you. The disadvantages of the hydrogen-fluorine combination exceed its advantages. Hydrogen and oxygen provide the best fuel in practice.

Are there other options to improve performance? One possibility is suggested by something that we noted in discussing the strength of materials. Chemical reactions, like chemical bonds, are decided by the interaction of the electrons that surround the nucleus of an atom. However, the weight of an atom is provided almost completely by the nucleus. We must have protons, to make the atom electrically neutral. But if we want to accelerate a material to high speed, the neutrons in the nucleus are just dead weight.

We would achieve a higher final speed from combustion if we replaced normal oxygen by some lighter form of it. Such an idea is not impossible, because many elements have something known as isotopes. An isotope of an element has the usual number of protons, but a different number of neutrons. For example, hydrogen comes in three isotopic forms: H¹ is "normal" hydrogen, the familiar element with one proton and one electron; H² has one proton, one neutron, and one electron. It is a stable form, with its own name, deuterium. Finally, H³ has one proton, two neutrons, and one electron. It is slightly unstable, decaying radioactively over a period of years.

However, this takes us in the wrong direction. We are interested in isotopes with fewer neutrons than usual, not more. Oxygen has a total of eight isotopes. The most common form of the atom, O¹⁶, has eight protons, eight neutrons, and eight electrons. The four heavier isotopes, O¹⁷, O¹⁸, O¹⁹, and O²⁰, all have more neutrons and are of no interest. There is, however, an isotope O¹³, with only five neutrons. If we use this in place of normal oxygen, the maximum speed associated with hydrogen-oxygen combustion increases from 5.6 kms/sec to 6.15 kms/sec. Unfortunately, O¹³ decays radioactively in a fraction of a second; however, O¹⁴ is longer-lived, and its use gives a maximum speed of 5.95 kms/sec. The similar use of a lighter isotope of fluorine, F¹⁷, gives a speed of 5.8 kms/sec.

It seems fair to say that 6 kms/sec provides an absolute upper limit for an exhaust speed generated using chemical fuels. Putting on our science fiction hats, can we see any possible way to do better than this?

Chemical combustion involves two atoms, originally independent, that combine to share one or more of their electrons. Also, as we have seen, neutrons take no part in this process. They just provide useless weight. We would therefore expect the ideal chemical fuel would be one in which no neutrons are involved, and in which the energy contribution from the electrons is as large as possible.

The best conceivable situation should thus involve only hydrogen (H¹, a single proton with no neutron), and obtain the largest possible energy release involving an electron. This occurs when a free electron approaches a single proton, to form a neutral hydrogen atom. The energy release for this case is well-known. It is termed the ionization potential for hydrogen, and it is measured in a particular form of unit known as an electron volt. One electron volt (shortened to eV) is the energy required to move an electron a distance of one centimeter in an electric field of one volt. That sounds like a very strange choice of unit, but it proves highly convenient in the atomic and nuclear world, where most of the numbers we have to deal with are nicely expressed in electron volts. The masses of nuclear particles, recognizing the equivalence of mass and energy, are normally written in eV or MeV (million electron volts) rather than in kilograms or some other inconveniently large unit (an electron masses only 9.109 x 10^-31 kilograms).

The ionization potential of hydrogen is 13.6 eV. The mass of an electron is 0.511 MeV, and of a proton 938.26 MeV. Knowing these facts and nothing else, we have enough to calculate the maximum speed obtained when neutral hydrogen forms from a proton and a free electron. Write the kinetic energy of the product as ½mv², where m is the mass of electron plus proton, and so equals 938.77 MeV. To convert this from the form of an energy to a mass, we invoke E = mc² from Chapter 2, and divide by c². The energy provided by the electron is 13.6 Ev. Equating these two, we have ½ x 938.77 x 1,000,000 (v/c)² = 13.6. Using c = 300,000 kms/sec and solving for v, v = 51.06 kms/sec.

This is the absolute, ultimate maximum velocity we can ever hope to achieve using chemical means. It is also surely unattainable. To do better, or even as well in practice, we must turn to the realm of physics and the violent processes of the subatomic world.

Orders of magnitude more energy are available there. To give an example: the ionization potential of hydrogen is 13.6 eV, so this is the energy released when a free electron and a free proton combine to form a hydrogen atom. The nuclear equivalent, combining two protons and two neutrons to form a helium nucleus, yields 28 MeV — over two million times as much.

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5.4 Organic and inorganic: building an alien. Those grandparents of modern chemistry, the alchemists of five hundred years ago, had a number of things on their wish list. One, however, dominated all the others. The alchemists sought the philosopher's stone, able to convert base metals to gold.

Claiming to be able to transmute metals, and failing, had stiff penalties in the fifteenth and sixteenth centuries. Marco Bragadino was hanged by the Elector of Bavaria, William de Krohnemann by the Margrave of Bayreuth, David Benther killed himself before he could be executed by Elector Augustus of Saxony, and Marie Ziglerin, one of the few female alchemists, was burned at the stake by Duke Julius of Brunswick. Frederick of Wurtzburg had a special gallows, gold painted, for the execution of those who promised to make gold and failed.

The inscription on a gibbet where an alchemist was hanged read: "Once I knew how to fix mercury, and now I am fixed myself."

Today, we know that the philosopher's stone is a problem not of chemistry, but of physics. The transmutation of elements was first shown to be possible in the early 1900s, by Lord Rutherford, when he demonstrated how one element could change to another by radioactive decay, or through bombardment with subatomic particles. For this achievement, the physicist Rutherford — ironically, and to his disgust -- was awarded the 1908 Nobel Prize. In chemistry.

Next on the alchemists' list was the universal solvent, capable of dissolving any material. That problem has vanished with the advance of chemical understanding and knowledge of the structure of compounds. Today, we have solvents for any given material. Aqua regia, known to the alchemists, is a mixture of one part concentrated nitric acid with three parts concentrated hydrochloric acid. It will dissolve most things, including gold. Hydrofluoric acid is only a moderately strong acid (unlike, say, hydrochloric acid) but it will dissolve glass. However, even today we have no single, universal solvent.

The old question when considering the problem still exists: If you did make the solvent, what would you keep it in?

The third item on the alchemist's list of desirable discoveries was the elixir of life. This would, when drunk or perhaps bathed in, confer perpetual youth. The quest for it not only occupied the alchemists in their smoky laboratories, but sent explorers wandering the globe. Juan Ponce de Leon was told by Indians in Puerto Rico that he would find the Fountain of Youth in America. He sailed west and discovered not the fountain but Florida, a region today noted less for perpetual youth than for perpetual old age. Cosmetic surgery, aerobics, and vitamins notwithstanding, the elixir eludes us still. But as we will see in the next chapter, we may be on the threshold of a breakthrough.

The three alchemical searches are often grouped together, but the third one is fundamentally different from the other two. The first pair belong totally to the chemical world. The elixir of life crosses the borderline, to the place where chemistry interacts with an organism — humans, in this case — to produce a desired effect.

Five hundred years ago, people were certainly doing this in other ways. That is what medical drugs are all about. However, there was a strong conviction that living organisms were not just an assembly of chemicals. Plants and animals were thought to be basically different from inorganic forms. They contained a "vital force" unique to living things.

It was easy to hold this view when almost every substance found in the human body could not be made in the alchemist's retorts. The doubts began to grow when chemists such as the Frenchman, Chevreul, were unable to detect any differences between certain fats occurring in both plants and animals. The key step was taken in 1828, when Friedrich Wöhler was able to synthesize urea, a substance never before found outside a living organism (actually, this is not quite true; urea had been prepared in 1811 by John Davy, but not recognized).

From that beginning, the chemists of the nineteenth and twentieth centuries one by one produced, from raw materials having nothing to do with plants or animals, many of the sugars, proteins, fats, and vitamins found in the bodies of animals and humans. With their success, it slowly became clear why vitalism had seemed reasonable for so long. The molecules of simple compounds are made up of a few atoms; carbon dioxide, for example, is one atom of carbon and two of oxygen. Copper sulfate is one atom of copper, one of sulfur, and four of oxygen. By contrast, many of the molecules of our bodies contain thousands of atoms. The difference between the molecules of living things and those of non-living materials is largely one of scale.

More than that, biological compounds depend for their properties very much on the way they are constructed. Two big molecules can have exactly the same number of atoms of each element, but because of their different connecting structure they have totally different properties (such molecules, like in composition yet un-alike in structure, are called isomers). Wöhler's success with urea was due at least in part to the fact that it is, as biological molecules go, simple and small, containing only eight atoms. In fact, urea is not so much a building block of a living organism, as a convenient way of dealing with the excretion of ammonia, an undesirable by-product of other reactions.

Chemists noticed one other thing. The big molecules of biochemistry all seem to contain carbon. In fact, the presence of carbon is so strong an indicator of organic matter, the terms "organic" and "inorganic" in chemistry have nothing to do with the origin of a material. Organic chemistry is, quite simply, the chemistry of materials that contain carbon. Inorganic chemistry is everything else. The distinction is not quite foolproof. Few people would refer to the study of a very simple molecule, such as carbon dioxide or methane (CH₄), as organic chemistry. They reserve the term for the study of substantial molecules that contain carbon. "Biogenic" is a better term than "organic" to describe the chemistry of living things, but today the latter is used to refer to both biogenic and carbon chemistry.

Why is carbon so important? What is there about carbon that makes it so different, so able to aid in the construction of giant molecules? This question is particularly important if we want to devise alien chemistries. Is it absolutely necessary that alien life-forms, no matter their star or planet of origin, be based on carbon?

Let us return to the shell model of the atom. Each shell around the nucleus can hold a specific number of electrons, and chemical reactions involve only those electrons in the outermost shell.

The innermost shell can hold two electrons. The next will hold another eight, for a total of ten. Atoms with spaces in a filled shell match up such electron "holes" with the extra electrons of other substances outside a filled shell.

Now note the curious situation of carbon. It has six electrons surrounding its nucleus. Thus, it has four extra electrons beyond the two of the first filled shell. On the other hand, it is four electrons short of filling a second shell; thus it has both four extra electrons, and four "holes" to be filled by other electrons. This "ambivalence" (a chemical joke; literally, two valences or strengths) of carbon makes it capable of elaborate and complex combinations with other elements. It is also, as we will see later in this chapter, capable of making elaborate and surprising combinations with itself.

Is carbon unique? Again we consider the shell model. The third shell can hold another eight electrons. Thus, an element with four electrons more than needed to fill the second shell, namely, fourteen, will be four electrons short of filling the third shell. Like carbon, it will have four extra electrons, and at the same time four electron holes to be filled.

Element fourteen is silicon. We have been led to it, by a very natural and simple logic, as a substance with the same capacity as carbon to form complex molecules. It can serve as the basis for a "silico-organic" chemistry, the stuff of aliens.

There will of course be differences between carbon-based and silicon-based life forms. For example, carbon dioxide is a gas at room temperature. Silicon dioxide is a solid with several different forms (quartz, glass, and flint are the most familiar), and remains solid to high temperatures. These differences are an interesting challenge to the writer. Just don't use the carbon/silicon analogy blindly. An alien who breathes in oxygen and excretes silicon dioxide is not impossible, but does deserve some explanation.

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5.5 Building a horse. There is no real difference between the chemistry of life and the chemistry of the inanimate world, other than complexity. "Vitalism" is dead. This simple fact demolishes the idea of a "food pill," found in rather old and rather bad science fiction.

The food pill is an aspirin-sized object that taken twice a day, with water, supplies all the body's needs. Apart from the sheer unpleasantness of the idea (no more pizza, no more veal cordon bleu, no more ice cream), it won't work. The body runs just like any other engine, burning organic fuel to produce energy and waste products. We have seen that there are definite limits to the energy produced by chemical reactions. A couple of small pills a day is not enough, no matter how efficiently they are used. To get by on a food pill, the human body would first have to go nuclear.

Chemistry and biochemistry are subject to identical physical laws. If we like, we can regard biochemistry as no more than a branch of all chemistry. Conversely, we can use the chemistry of living organisms to perform the functions of general chemistry.

To take one example, the marine organism known as a tunicate has a curious ability to concentrate vanadium from sea water. If we want vanadium, it makes sense to use this "biological concentrator" (which also provides its own fuel supply and makes its own copies). In science fiction, it is quite permissible to presume that an organism can be developed to concentrate any material at all — gold, silver, uranium, whatever the story demands. It is also reasonable to assume that the principle employed by the tunicate will eventually be understood, so that we can make a "vanadium concentrator" along the same lines, but without the tunicate.

The analogy between chemical and biological systems is not always a fruitful one. When I was pondering the question of the most efficient chemical rocket fuel, I noted that energy was always wasted in heating the exhaust. A hot exhaust jet does not deliver more thrust than the same mass expelled cold. Greater efficiency would therefore be obtained if the exhaust could somehow be at room temperature.

That sounds impossible, but we and all other animals have in our bodies large numbers of specialized proteins known as enzymes. The purpose of an enzyme is to control chemical reactions, making them proceed at much lower temperatures than usual, or faster or slower.

Suppose we build an "enzymatic engine" in which the chemical fuels are combined to release as much energy as usual, but the temperature remains low? We might then have a better rocket for launches to space.

I soon realized that the idea would not work, because as we point out in Chapter 8, the whole idea in using rockets for a launch is to burn the fuel as fast as possible. There is no way to achieve a fast burn, yet avoid a temperature rise in the fuel's combustion products. Forget that, then. But what about an enzymatic engine for other purposes? Say, to power a vehicle that moves on the ground. For such a use, slow and steady fuel consumption is preferable to a single, giant, near-explosion. There would be other advantages, too. The intense heat generated when we burn fuels such as gasoline is a big factor in a vehicle's wear and tear. The heat also generates nitrogen oxides, which are a serious form of air pollution.

Let us imagine, then, a method of ground transportation, powered by some kind of slow-burning enzymatic engine that can operate at close to room temperature. Such a device would have numerous uses, and be free of environmental problems.

I was expounding on this idea with some fervor when a more hard-headed friend of mine pointed out that I seemed to be designing a horse.

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5.6 Fullerenes: a chemical surprise. Textbooks on inorganic chemistry for the past couple of centuries have stated, without a hint of doubt, that carbon occurs in two and only two elementary forms: diamond, and graphite. In diamond, the carbon atoms form tetrahedra, triangular pyramids with one carbon atom at each vertex and one in the center. This is a strong and stable configuration, so diamond is famously hard. In graphite, the carbon atoms form hexagons with an atom at each vertex, and the hexagons line up as layers of flat sheets. Since the sheets are not strongly coupled with each other, graphite is famously slippery and a well-known lubricant.

The discovery in 1985 of a third elementary form of carbon was a shock in two different ways. First, the existence of the third form could have been predicted, or at least conjectured, since the middle of the eighteenth century. In fact, its existence was suggested in 1966, as a piece of near-whimsical speculation, by a columnist in the New Scientist magazine. No one took any notice. Second, and almost a disgrace to a self-respecting chemist, the third form is not at all hard to make. In fact, it had been around, waiting to be discovered, in every layer of soot produced by a hot carbon fire. Every time you light a candle, at least some of the soot will be this new and previously unknown form of carbon.

I said, this new form of carbon, but actually there is a family of them. The simplest form, C₆₀, is sixty carbon atoms arranged in a round hollow shape involving 12 pentagons and 20 hexagons. Technically, this form is called a truncated icosahedron, but the name is neither suggestive nor catchy. However, the structure looks exactly like a tiny soccer ball.

Leonhard Euler, the great Swiss mathematician, studied the possible geometry of closed spheroidal structures more than two hundred years ago, and proved that while they must have exactly 12 pentagons, the number of hexagons may vary. And vary they do. Continuing the sporting motif, the next simplest form, C₇₀, is an oblong spheroid of 12 pentagons and 25 hexagons that closely resembles a rugby ball. And after that there are carbon molecules with 76, 84, 90, and 94 atoms, and still bigger versions that form hollow closed tubes. All of these are known by the generic name of "fullerenes," or if they are round, "buckyballs." The form with 60 atoms, C₆₀, is the simplest, most stable, and most abundant form, with C₇₀ in second place. Not surprisingly, C₆₀ was the first form to be discovered.

So how was it discovered? Not, as one might think, by direct observation. The C₆₀ molecule is less than a millionth of a millimeter across (about 7x10^-10 meters), but it is big enough to be seen using a scanning tunneling microscope. It wasn't, though. It was found by a very curious and apparently improbable route. A British chemist, Harold Kroto, was studying how carbon-rich stars might lead to the production of long chains of carbon molecules in open space. In the United States, at the Houston campus of Rice University, American chemists Robert Curl and Richard Smalley had suitable lab equipment to simulate the carbon-rich star environment and see what might be happening.

The team did indeed find evidence of a variety of carbon clusters, but as the carbon vapor was allowed to condense, everything else seemed to fade away except for a 60-atom cluster, and, much less abundant, a 70-atom cluster. It seemed that there must be a very stable form of carbon with just 60 atoms, and another, rather less stable, with 70 atoms.

At this point, the team faced a problem. Carbon is highly reactive. If the cluster had the form of a flat sheet, like graphite, it ought to have free edges which would latch on to other carbon atoms, and so grow rapidly in size. The only way around that would be if the structure could somehow close in on itself, and tie up all the loose ends.

The research team was guided at that point not by the 18th century mathematical researches of Euler, but by the geodesic dome idea of Buckminster Fuller. That, too, is a closed structure of pentagons and hexagons. With faith that a closed sixty atom sphere like a geodesic dome was the only plausible structure for the cluster, the researchers went ahead and named it "buckminsterfullerene." They did have the grace to apologize for such a mouthful of a name, and it was quickly shortened to "fullerenes" when it was realized that there was not one but a multitude of molecules.

The first fullerenes were produced in minute quantities. Research on them was therefore difficult. Then in 1990 a German team discovered a shockingly simple production method. By burning a graphite rod electrically, the resulting soot contained a substantial percentage of C₆₀. Combining this with the suitable use of a benzene solvent, an almost-pure mixture of fullerenes was formed. Now anyone who wants fullerenes for research can easily buy them. And they are doing so, in ever-increasing numbers. The buckyball was named "Molecule of the Year" by Science magazine in 1991, and today the most frequently cited chemistry papers all seem to be on the subject of fullerenes. The 1996 Nobel Prize in chemistry went to Robert Curl, Richard Smalley, and Harold (now Sir Harold) Kroto.

One natural question is, all right, so fullerenes exist, and they are of scientific interest. But what are they good for, apart from winning Nobel Prizes? Potentially, many things. Because they are hollow, buckyballs can be used to trap other atoms inside them and to provide miniature "chemical test sites." They are phenomenally robust and stable, and could be the basis for materials stronger than anything we have today. They have been proposed as nanotechnology building blocks. They are already being used to improve the growth of diamond films. And they have interesting properties and potential as superconductors.

The best answer to the question, though, is that it is too soon to say. Like lasers in 1965, five years after the first one was built, fullerenes seem to be a solution waiting for a problem. And like lasers, fullerenes will almost certainly become enormously valuable technological tools in the next thirty years.

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5.7 A burning home: the oxygen planet. Why is combustion normally referred to as combination with oxygen? Why did we discuss aliens breathing oxygen, and exhaling carbon dioxide?

Only because we live on a planet in which free oxygen is a major component of the atmosphere. We do not think of this as unusual, but we ought to. As pointed out earlier, oxygen combines readily — even fiercely — with other elements. A planet with an oxygen atmosphere is unstable. If a world starts out with an atmosphere of pure oxygen, before long the normal processes of combustion will combine the oxygen to other, more stable compounds.

Clearly, that has not happened to the Earth. The presence of life, and in particular of plant life that performs photosynthesis, makes all the difference. Using the energy from sunlight, a plant reverses the process of combustion. It takes carbon dioxide and water, producing from them hydrocarbons, and releasing pure oxygen into the atmosphere. This is a dynamic, self-adjusting process. If there is more carbon dioxide in the air, plant activity will increase, serving to remove carbon dioxide and increase oxygen. Too little carbon dioxide, and plant growth decreases.

Because we grew up with this process, we tend not to realize how extraordinary it is. But the first life on Earth had to deal with an atmosphere containing no oxygen, but plenty of hydrogen. When the first photosynthetic organism (almost certainly, some form of cyanobacteria) developed, a huge but unchronicled battle took place. To hydrogen-tolerant life, free oxygen is a caustic and poisonous gas. To oxygen-tolerant life, free hydrogen is an explosive.

The oxygen-producers and oxygen-breathers won, to become oaks and marigolds and tigers and humans. The hydrogen lovers remain as single-celled organisms, the anaerobic bacteria.

Free oxygen is so much a hallmark of life, Lovelock (Lovelock, 1979) insists that the detection of substantial amounts of oxygen in a planetary atmosphere would prove, beyond doubt, that life must be present there. The converse, as shown by the early history of Earth, is not true: absence of oxygen does not mean absence of life. The science fiction writer is free to suppose that life has developed on other worlds in an atmosphere of hydrogen, or oxygen, or methane, or nitrogen, or carbon dioxide, or many other gases. Combinations are permitted, as our own atmosphere shows. But if you take the route of an exotic atmosphere, the chemical consequences must be worked out in detail. No atmosphere, please, of mixed oxygen and hydrogen.

The master of the design of alien planets and biospheres is Hal Clement. If you want to see how carefully and lovingly it can be done, read his MISSION OF GRAVITY (1953 — with hydrogen-breathing natives, no less); CYCLE OF FIRE (1957); CLOSE TO CRITICAL (1964); and ICEWORLD (1953). If you want to see fascinating and exotic worlds that won't stand up to such close scrutiny, consult Larry Niven's RINGWORLD (1970), or wander the wild variety of planets to be found in his multiple volumes known collectively as TALES OF KNOWN SPACE.

Materials, potential strengths