21

The Final Frontier

In some ways the future has been a great disappointment. When I was first reading science fiction, space travel was almost a defining characteristic of the genre, interplanetary at the least, with luck interstellar. Other technologies are well ahead of schedule; computers are a great deal smaller than most authors expected and used for a much wider variety of everyday purposes, genetic engineering of crops is already a reality. But serious use of space has been limited to near-Earth orbit – our backyard. Even scientific activity has not gotten humans past a very brief visit to the moon. We have sent a few small machines a little farther, and so far that is about it.

One possible explanation is that the slow rate of progress is due to the dominant role of governments, itself in part a result of the obvious military applications. Another is that getting into space was harder than writers thought. The problem with the latter explanation is that we have already done the hard part. The next steps, now that we have learned to get free of the terrible drag of Earth’s gravity, should be much easier. Perhaps, after a brief pause for rest and refreshment, they will be.

VIEW FROM THE BOTTOM OF A WELL

In one of Poul Anderson’s more improbable science fiction stories,¹ a man and a crow successfully transport themselves from one asteroid to another in a spaceship powered by several kegs of beer. From what I know of the author, he probably did the arithmetic to make sure the thing would work.

It would not have gotten far on Earth, but moving in space is in some ways a much easier problem. Our present home is inconveniently located at the bottom of a very deep well. Getting out of that well, lifting something from the surface of Earth into space, takes a lot of work. The price, the charge for satellite launches and similar services, is measured in thousands of dollars a pound. Science fiction writers of the fifties and sixties took it for granted that the point of getting off Earth was to get to Mars, or Venus, or perhaps to a planet circling some distant star. Sometime between then and now it occurred to someone that it made little sense to climb, with enormous effort, out of one well only to jump down another. Planets are traps.

One alternative is an orbital habitat, a giant spaceship permanently located in orbit around a planet or star. The ecology of such a miniature world, like the ecology of the orbiting habitat we now live on, would consist of closed cycles powered by the sun. Recycling on an almost total scale.

The first problem is where to put it. A solar orbit, unless very close to the Earth’s, in which case it will be made unstable by the Earth’s gravity, puts you a long way from home. Orbiting the Earth, far enough out to avoid the clutter of communication satellites and orbital trash, looks more attractive. Unfortunately, such an orbit will eventually decay, even if not quite as fast in the real world as on Star Trek.

The solutions are Lagrangian points 4 and 5, L4 and L5 for short. They are locations in orbit around the Earth sixty degrees ahead and behind the moon. As Joseph Louis Lagrange proved in 1772, they are stable equilibria. A satellite or space habitat placed at L4 or L5 stays there. Like a ball bearing at the bottom of a bowl, if something pushes it a little away from the center, it moves back.

A second problem is what to build your space habitat out of; at $5,000 a pound, materials from Earth are a bit pricey. That suggests an alternative location – the asteroid belt, which consists of a large number of chunks of rock located between the orbits of Mars and Jupiter. If we do not want to live that far from home, we might use asteroids outside of the belt, some of which have orbits that come quite close to that of Earth.

Asteroids are small enough so that their gravity is negligible. Many are large enough to provide adequate quantities of building material. One way of using that material would be to colonize an asteroid, perhaps drilling tunnels in its interior. An alternative, for those who prefer a shorter commute to the neighborhood of the home planet, is to mine an asteroid and ship what you get back to somewhere near Earth – L5, say. It is a long way from the asteroid belt to the Earth, but transportation is easier if you are not starting at the bottom of a well. Delivering material from the asteroid belt might take months, even years, but the forces required are much smaller than those needed to lift the same amount from Earth. If you are not in too much of a hurry you could even try beer.

A future in which some significant number of people are permanent residents of space, living in habitats, asteroids, perhaps fleets of mining ships, raises some interesting issues. The obvious political question is who rules them. Are they the legal equivalent of ships on the high seas, independent states, or something else? A less obvious legal and economic issue is how to define and enforce the relevant property rights – to an orbit (already a problem for communication satellites), chunks of matter floating through space, sunlight for power, or whatever else is scarce and useful.

So far the only reason I have offered for living in space is that it is much easier to get to space from there. Readers may be reminded of the man who explained to his friends that he played golf to stay fit; when asked what he was staying fit for he answered “golf.” There are better answers. An environment with zero gravity and an unlimited supply of almost perfect vacuum could be useful for some forms of production. Asteroids could provide a very large and inexpensive source of raw materials. While their first use will be to build things in space, we do not have to stop there. Getting things down a well is a lot less work than getting them up.

Another answer is that, if Earth gets crowded, we may want to look at other places to live. By mining the asteroid belt we could build structures that would provide living space for enormously more people than presently exist. There need be no shortage of power; the sunlight that falls on Earth is less than one billionth of the total output of the sun. A sufficiently developed space-faring civilization could make use of the rest of it. In the limiting case, one could imagine the sun entirely surrounded by the works of man, visible from other stars only by the vast infrared output of our waste heat. Freeman Dyson has proposed locating technologically sophisticated species a little ahead of ours by searching the heavens for stars like that.²

The final answer is that there are risks to putting all our eggs in one basket. It is possible, indeed not unlikely, that life on Earth will get better and better over the next few decades. But it is far from certain. One can imagine a range of possible catastrophes, from gray goo to global government, that would make somewhere else to be a very attractive option. There is a lot of space in space.

The biggest barrier to the future I have been sketching is the cost of getting off Earth. While a space civilization, once started, might be self-sustaining, it requires a big start. And at $5,000 a pound, not many of us are likely to go.

Which raises the obvious question of whether there might be better ways than on top of a giant firework.

TAKE A VERY LONG ROPE …

Artsutanov proposed to use the initial cable to multiply itself, in a sort of boot-strap operation, until it was strengthened a thousand fold. Then, he calculated, it would be able to handle 500 tons an hour or 12,000 tons a day. When you consider that this is roughly equivalent to one Shuttle flight every minute, you will appreciate that Comrade Artsutanov is not thinking on quite the same scale as NASA. Yet if one extrapolates from Lindbergh to the state of transatlantic air traffic 50 years later, dare we say that he is over-optimistic? It is doubtless a pure coincidence, but the system Artsutanov envisages could just about cope with the current daily increase in the world population, allowing the usual 22 kg of baggage per emigrant….

Arthur C. Clarke³

For a really efficient form of transport, consider the humble elevator. Lifting the elevator itself takes almost no energy, since as the box goes up the counterweight goes down. Energy consumption is reduced to something close to its absolute minimum – the energy required to lift the passengers from one point to a higher point. And if your design is good enough, you can recover most of that energy when they come back down.

The idea of applying this approach to space transport, like the less efficient method we currently use, is due to a Russian.⁴ A multistage rocket was first proposed by Tsiolkovsky in 1895. The space elevator was first proposed by Yuri Artsutanov, a Leningrad engineer, in 1960, and has been independently invented at least half a dozen times since.

You start with a satellite in geosynchronous orbit – over the equator, moving in the direction of the Earth’s rotation, going around the Earth once a day. From the viewpoint of someone on the ground the satellite is standing still, since it orbits the Earth at exactly the same rate that the Earth rotates.

Let out two cables from this satellite, one going up, one down. For the one going up, centrifugal force more than balances gravity, so it tries to pull the satellite up. For the one going down, gravity more than balances centrifugal force, so it pulls the other way. Let out your cables at the right speed and the two effects exactly balance. Continue letting out the cables until the lower one touches the ground. Attach it to a convenient island. Run an elevator up it. You now have a way of getting into space at dollars a pound instead of thousands of dollars a pound.

A space elevator has a number of odd and interesting characteristics, some of which we will get to shortly. Unfortunately, building it faces one very serious technical problem: finding something strong enough and light enough to make a very long rope.

Consider a steel cable hanging vertically. If it is longer than about fifty kilometers, its weight exceeds its strength and it breaks. Making the cable thicker does not help, since each time you double its strength you also double its weight. Kevlar, used for purposes that include bulletproof garments, is considerably stronger for its weight than steel. A Kevlar cable can get to about 200 kilometers before it breaks under its own weight. Geosynchronous orbit is 35,000 kilometers up. Kevlar is not going to do it.

At first glance, it looks as though we need a material almost 200 times stronger for its weight than Kevlar, but the situation is not quite that bad. As you go up the cable, you are getting farther from the Earth – gravity is getting weaker and, since the cable is going around with the satellite (and the Earth), centrifugal force is getting stronger. By the time you get to the satellite, the two balance. So it is only the lower end of the cable that will be really heavy. Furthermore, the lower you go on the cable the less weight is below it to be held up, so you can make a cable longer before it breaks by tapering it. Building a space elevator requires something quite a lot stronger for its weight than Kevlar, but not 200 times stronger.

Such materials exist. Microscopic carbon fibers appear to have the necessary properties. So, according to theoretical calculations, would buckytubes – long fibers of carbon atoms bonded to each other. Neither is in industrial production in the necessary sizes just now, but that may change in the fairly near future.⁵

One nice feature of carbon, aside from its ability to make very strong materials, is that some asteroids are largely made out of it. Move one of them into orbit and equip it with a factory capable of turning carbon into superstrong cable. When you are done, use what is left of the asteroid for a counterweight, attached to the cable that goes from the satellite away from the Earth, letting you hold the lower cable up with a much shorter upper cable. Nobody is taking bids on the project just at the moment, but in principle it is doable.⁶

And Off We Go

Consider a cargo container moving up the cable. At the bottom, its motors have to lift its full weight. As it gets higher, gravity gets weaker, centrifugal force gets stronger, so it becomes easier and easier to move it up. When it reaches the satellite at geosynchronous orbit, the two exactly balance – inside the container, you float. Let the container keep going, following the upper cable into space. Now centrifugal force more than balances gravity. With no motor and no brakes you go faster and faster.

One possibility is to use that process, with careful timing, to launch you into space. In principle, it would be possible to build space elevators on a number of different planets and use them instead of rockets for interplanetary transport. Think of it as a giant game of catch. You get launched from Earth by letting go of its space elevator at just the right time and place. As you approach Mars you adjust your trajectory a little – we probably still need rockets for fine-tuning the system – so that you match velocities with the space elevator whipping around Mars. Let go of that at the right time, after moving a suitable distance in or out, and you are on your way to the asteroid belt, or perhaps Jupiter. Building a space elevator on Jupiter might raise problems even for the best cable nanotechnology could spin; perhaps we should use one of its satellites instead. It’s a dizzying picture.

An alternative is to equip your cargo capsule with regenerative brakes, an idea already implemented in electric and hybrid cars. A regenerative brake is an electric generator that converts the kinetic energy of a car into electricity, slowing the car down and recharging its batteries. On the space elevator, the electricity generated by the brakes keeping one cargo capsule from taking off for Mars could be used to lift the next one from Earth to the satellite.

Skeptical readers may wonder where all this energy, used to fling spaceships around the solar system or lift capsules from Earth, is coming from. The answer is that it is coming from the rotation of the Earth. Every time you lift a load up the elevator it is being accelerated in the direction of the Earth’s rotation, since the higher it is the faster it has to move in order to circle the Earth once a day. For every action there is a reaction; conservation of angular momentum implies that accelerating the load slows down the Earth. Fortunately, the Earth is very much larger than either us or the things we are likely to send up the elevator, so it would be a very long time before the effect became significant.⁷

I have not attempted to calculate how much mass one could shoot off into space before the astronomers complained that their atomic clocks were running fast.

Arthur C. Clarke

The space elevator I have described cannot be built with presently available materials. But there are at least two modified versions of the design that perhaps can. One is called a skyhook. It was proposed in the United States by Hans Moravec in 1977, but Artsutanov had published the idea back in 1969. Here is how it works.

Start, this time, with a satellite much closer to the Earth. Again release two cables, one up, one down. Since this satellite is not in geosynchronous orbit, it is moving relative to the surface of the Earth. That makes it difficult to attach the bottom end of the cable to anything, so we don’t. Instead we rotate the cable, one end below the satellite, one above, like two spokes of an enormous wheel rolling around the Earth.

The satellite is moving around the globe, but the bottom end of the cable, when it is at its lowest point, is standing still; the cable’s motion relative to the satellite just cancels the satellite’s motion relative to the Earth. If that sounds odd, consider a car going down the freeway at sixty miles an hour. The car is moving but the bottom of the tire is standing still, since the rotation of the wheel moves it backwards relative to the car just as fast as the car moves forwards relative to the pavement. The skyhook applies the same principle scaled up a bit.

Seen from Earth, the end of the cable comes down from space, stops at the bottom of its trajectory, goes back up. To use it for space transport, you put your cargo capsule on an airplane, fly up to where the cable is going to be, hook on just as the bottom of the cable reaches its lowest point. The advantage over the space elevator is that the much lower orbit means a much shorter cable, so you can come a lot closer to building it with presently available materials. The physics works, but don’t expect the Civil Aeronautics Board to approve it for passengers any time soon.

A different version that might be workable even sooner has been proposed by researchers at Lockheed Martin’s Skunk Works, source of quite a lot of past aeronautical innovation. It starts with a simple observation: getting something to orbit is much more than twice as hard as getting it halfway to orbit. If you have two entirely different technologies for putting something in orbit, why not let each of them do half the job?

The Skunk Works proposal uses a short space hook, reaching from a satellite in low orbit down to a point above the atmosphere. It combines it with a spaceplane, a cross between an airplane and the space shuttle, capable of taking off from an ordinary airport and lifting its cargo a good deal of the way, but not all the way, to orbit. SpaceShipOne, Burt Rutan’s innovative vehicle that recently won the $10 million Ansari X PRIZE, provides a proof of concept, although its cargo capacity is a bit low. The spaceplane takes the cargo capsule to the skyhook, the skyhook takes it the rest of the way. The engineers that came up with the design believe that it could be built today and that it would bring the cost of lifting material into space down to about $550 a pound. That is quite a lot more than the estimated cost with a space elevator, but about a tenth the cost of using a rocket.

CONCENTRATING THE MIND: THE PROBLEM OF NEAR-EARTH OBJECTS

Nothing so concentrates a man’s mind as the prospect of being hanged in the morning.

Samuel Johnson

A little less than a century ago – in 1908 – Russia was hit with a fifteen-megaton airburst. Fortunately the target was not Moscow but a Siberian swamp. The explosion leveled trees over an area about half the size of the state of Rhode Island. While there is still some uncertainty as to precisely what the Tunguska event was, most researchers agree that it was something from space, perhaps a small asteroid or part of a comet, which hit the Earth. A rough estimate of its diameter is sixty meters. While it was the largest such event in recorded history, there is geological evidence of much larger strikes. One, occurring about 65 million years ago, left a crater 180 kilometers across and a possible explanation for the period of mass extinctions that eliminated the dinosaurs.

2002 CU11 is a near-Earth object, an asteroid in an orbit that will bring it close to the Earth. Its estimated diameter is 730 meters. Since volume goes as the cube of diameter, that means that it probably has more than 1,000 times the mass of the Tunguska meteor and could do a comparably greater amount of damage – quite a lot more than the largest H-bomb ever tested. A little while after it was first observed, 2002 CU11was estimated to have about a 1-in-9,000 chance of striking the Earth in 2049. You will be relieved to know that later observations, allowing a more precise calculation of its orbit, have reduced that probability to essentially zero.

2000 SG344 is a much smaller rock, about forty meters. NASA estimates that it has about a 1-in-500 chance of hitting the Earth sometime between 2068 and 2101. Even a rock that small would produce an explosion very much more powerful than the bomb dropped on Hiroshima.

By current estimates, there are about 1,000 near-Earth objects of 1-kilometer diameter or above and a much larger number of smaller ones. We think we have spotted more than half of the big ones; none appear to be on a collision course. Since an object that will at some point in its orbit pass near Earth may at the moment be a very long distance away, locating all of them is hard.

Our best guess at the moment, from the geological evidence, is that really big asteroids – two kilometers and over – hit the Earth at a rate of about one or two every million years. That makes the odds that such a strike will occur during one person’s life span about 1 in 10,000. Smaller strikes are much more common – one in the megaton range in the last century.

The odds of a big strike are low, but given how much damage it could do it is still worth worrying about. The odds of a small strike, which could do significant damage if it happened to hit a populated area or the sea near a populated coast, are larger.⁸ What can we do?

The first step is to watch for things heading our way. NASA, along with researchers in other countries, has been working at it; that is why I can quote sizes and probabilities for known near-Earth objects. U.S. Congressman Dana Rohrabacher has proposed to supplement that by a more decentralized approach: cash prizes to reward amateur astronomers for spotting previously unknown near-Earth asteroids.⁹ Since the objects are moving in orbits determined by the laws of physics, once we have spotted one of them several times we can make a fairly accurate projection of where it will be for many years into the future. One particularly well-observed asteroid,¹⁰ a little more than one kilometer in diameter, is expected to make a close approach to Earth on March 16 of the year 2880.

Suppose we spot an asteroid on course for Earth. If it is going to hit tomorrow, there is not much you can do other than getting as far as possible from the point of impact and well above sea level. But if we spot it early enough, we may be able to prevent the collision. Moving a large asteroid is hard, but with a decade or more of pushing a fairly small force can change its orbit at least a little. Even a small change in the orbit, acting over a long time, can turn a hit into a near miss.

One approach would be to land on the asteroid, equipped with a small nuclear reactor. Use the reactor to vaporize rock and blow it at space, gently pushing the asteroid in the other direction. A less elegant solution, but one that uses off-the-shelf hardware currently available in excess supply, is to nuke it. Explode a nuclear or thermonuclear bomb on, slightly under, or slightly above the surface of the asteroid. Exploded under the surface it blows chunks of the asteroid – hopefully small enough chunks not to be themselves too dangerous – in one direction and moves the rest of the asteroid in the other. On or near the surface it vaporizes some of the surface and drives that in one direction, giving the asteroid a brief but very hard shove the other way. For an asteroid with a diameter of one kilometer or so spotted a decade or more before it hits us, such an approach might do the job.

The odds of a catastrophic asteroid hit are low, but the downside risk could be substantial. If only the dinosaurs had had an adequate space program they might still be around.

AD ASTRA

We are currently active in Earth’s backyard, putting up communication satellites, spies in the sky, and the like. I have suggested some possibilities for the next step – lowering the cost of getting off Earth by enough to make it possible to establish substantial human populations in space habitats or suitably modified asteroids. The step after that is much harder, because the stars are much farther away. Current physics holds that nothing can move faster than the speed of light. If that remains true, trips to other stars will take years, probably decades, possibly centuries. We might as well start thinking about them now.¹¹

Earlier chapters provide three solutions to the problem of keeping the crew of an interstellar expedition alive long enough to get somewhere. One is life extension. Another is cryonic suspension. A third is to have the ship crewed by programmed computers. If a program gets bored it can save itself to hard disk, or whatever the equivalent then is, and shut down. After, of course, reminding another AI to reboot it when they arrive.

What about propulsion? Getting a starship to a significant fraction of the speed of light requires something considerably better than chemical rockets. A number of proposals have been made and analyzed.¹² One of my favorites starts with a form of propulsion proposed some decades back for interplanetary flight and currently being experimented with: sails. There is no air in the vacuum of space¹³ but there is quite a lot of light, and light has pressure. A light sail is a thin film of reflective material with an area of many square kilometers, perhaps many thousands of square kilometers. The ship attached to the sail controls its angle to the sun, rather as an ordinary sail is controlled on Earth.

The sunlight is all going one way – out. Its pressure can be used to accelerate away from the sun, but how do you get back? One answer is gravity. Just as an ordinary sailboat combines the pressure of its keel against the water with the pressure of the wind against its sail, a solar sailboat combines the pressure of light with the pull of solar gravity. To accelerate at right angles to the direction of the sun, you tilt the sail so that the combination of light pressure and gravity adds up to the vector you want. To accelerate toward the sun you furl your sail or angle it edge-on to the light, and wait for the sun to pull you in.

The great advantage of a light sail is that it requires no fuel. One problem is that the farther you are from the sun, the less there is to push you; for most of an interstellar voyage the sun is just one more star. The solution is to provide your own sunlight. Build a very powerful laser somewhere in the solar system, aim it at the sail of your interstellar ship, and blow it across space. The ship goes; the power source remains behind.¹⁴

A solar sail backed by a very large laser cannon is an elegant solution to the problem of getting to the stars, but one problem remains: stopping. Unless the star you are going to is also equipped with a laser cannon, you are flying a ship without brakes.

My favorite solution was offered by Robert Forward.¹⁵ His ship has two solar sails, a circle inside a larger circle. When you approach the target system you cut loose the outer ring and angle everything so that the laser beam misses the sail still attached to the ship, hits the other, bounces off it, and is reflected back into the first sail. The detached sail accelerates into space, driven by the beam, while the spaceship is slowed by the reflected beam hitting the sail still attached.

Before the second ship arrives, the first builds a second laser cannon to provide brakes. Nobody could expect a maneuver that complicated to work twice. Once you have a laser at each end of things, traveling back and forth gets a lot easier.

The sunside armor’s peelin’, and the reels have too much slack,
And it takes a week to get her up to speed.
The mainsail’s full of pinholes, and the coffeemaker’s cracked,
But I can’t think of anything I need.

As twelve hundred klicks of sail begin to furl, and keel jets hiss,
And the pickup point sends grapnels out to roam,
I fin’ly have the chance to think of one I love and miss,
Running down the windward passage to our home.
Running down the windward passage to our home.

You’ll never call me wealthy, and I haven’t come too far,
And there’s folks I know would make me out a fool.
But they’ve never cruised the system, and they’ve never sailed the stars,
With ten million miles of sunlight for their fuel.

FIFTY YEARS AFTER THEY STOP LAUGHING

How likely is any of this to happen? Nanotechnology would make a space elevator technically possible but there would still be political problems getting a project on that scale built. Whether they will prove insurmountable depends on the climate of opinion thirty or forty years from now, which is hard to predict. Even without a space elevator, nanotechnology should make possible much stronger and lighter materials, which would strikingly lower launch costs, perhaps by enough to establish a real human presence in space. Defense against near-Earth objects is one reason to do it, a reason that could become urgent if we spot something big on a collision course.

Interstellar travel is a harder project. It may happen, and it is interesting to think about, but I do not expect anyone to arrive at another star any time in the next fifty years, which is about as far forward as we have any reasonable hope of predicting future technology. If it does happen, Forward’s magnificent kludge, the sail within a sail, is as likely as anything else.

I’ll adapt the reply that Arthur Kantrowitz gave, when someone asked a similar question about his laser propulsion system. The Space Elevator will be built about 50 years after everyone stops laughing.

Arthur C. Clarke