One day, about 5 billion years from now, the last perfect day will dawn. It is at about this time that scientists estimate the nuclear fuel of the Sun will begin to run out. As we discussed in Chapter 6, the Sun is powered by a nuclear fusion reaction at its core, with hydrogen atoms being fused together to form heavier atoms, and in the process producing the energy that has made the
Sun shine steadily for the last 5 billion years or so. The Sun has shone stably for all that time because of the balance between the huge amounts of energy being generated at its center, tending to blow it apart, and the force of gravity tending to hold it together. On this last perfect day, 5 billion years in the future, the nuclear fuel at the Sun's center will be just about depleted, and the stable balance between energy generation and gravity will be disturbed. The consequences for the Sun will be dramatic as far as the inhabitants of Earth (or indeed any other planet in the solar system) are concerned. I'm not sure who those inhabitants will be; people have been around on Earth for only about a million years, and it seems strange to think of them still being here 5 billion years in the future. Perhaps some other species, directly descended from humans, will exist then, but that's a different story.
What will happen to the Sun when its hydrogen fuel begins to run out? According to our best theories, it will evolve into a red giant star, expanding to a sphere about the same size as Earth's current orbit. In this process the Sun will lose a significant amount of mass, so Earth's orbit radius is predicted to increase to about one and a half times its current radius. Thus Earth will probably escape being engulfed by the Sun. But the surface environment on Earth will be transformed into a blazing desert, with all the oceans' water having boiled away. Put simply, Earth will no longer be able to support life, other than perhaps microbial life buried deep within Earth's crust.
This rather bleak picture of the Sun in its death throes tells us that ultimately people will have to leave Earth. This notion of our successors having to leave home in the distant future has perhaps become a bit of a cliché in contemporary science-fiction literature. Some people believe that other factors, such as climate change, may be more important in forcing the evacuation much sooner. However, the bottom line is still the same: to ensure our ultimate survival as a species, we have to learn how to live and work in space. More importantly, we need to develop and master the technologies required to travel across the cosmos, that is, to transform Hollywood space engineering into reality! How are we going to do this? The technical challenge is huge, simply because the universe is huge. This is expressed rather eloquently by Douglas Adams in his book, The Hitchhiker's Guide to the Galaxy: "Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is.'' To describe the sorts of distances we are talking about, the nearest star (apart from the Sun) is about four light years away. This is the distance that light travels in 4 years at the enormous speed of 300,000 km/sec (186,000 miles/sec). A quick calculation gives this as about 38,000,000,000,000 km (23,000,000,000,000 miles), a distance probably too large for our minds to comprehend. Using our current spacecraft technology, we know that it takes many years to reach Pluto, the most distant outpost of our solar system. However, the nearest star outside the solar system is about 6500 times more distant. This demonstrates the magnitude of the challenge, without even addressing the prospect of traveling across our home galaxy, the Milky Way, which is estimated to be about 100,000 light years across!
How are we to achieve travel across such vast distances? Well, going very fast is obviously a good idea, but our currently accepted laws of physics, due to Einstein, set a speed limit equal to the speed of light (see Chapter 1). Although I think that Einstein might not be the last word in our understanding of the universe, nevertheless in this discussion I will stay within the boundaries of his theories, and accept the light speed limit. To cross huge distances, it would be good to be able to travel at a speed that is a good percentage of light speed, or alternatively find clever loopholes in the laws of physics that will allow us to effectively travel faster than light speed without strictly violating the speed limit. Although the latter sounds like a bit of a contradiction, nevertheless we will see that there are some interesting ideas along these lines (see Exotic Systems, below). Let's take a brief look at some of these ideas for achieving interstellar travel, starting with some less exotic but still futuristic rocket systems.
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