http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)
The above Wikipedia article introduces Project Orion, which in the 1950s and the 1960s studied the design of a rocket that is propulsed by small nuclear explosions. Behind the spaceship is a pusher plate, behind which we detonate small nuclear bombs. Nuclear explosions push the spaceship to ever higher speeds. Theoretically, the spaceship can be accelerated up to 10 % of the speed of light.
Freeman Dyson's Orion design
In 1968, famous physicist Freeman Dyson performed an analysis on alternative designs of a nuclear pulse spaceship. A major problem in the design is how to cool the pusher plate after each nuclear explosion, so that the plate does not melt.Dyson's Momentum Limited Orion rocket would contain a ship whose mass is 100,000 metric tons, and a stockpile of 300,000 hydrogen bombs, each weighing 1 metric ton. The spaceship would be huge by today's standards: the mass of the International Space Station which is currently orbiting the Earth, is only 450 tons.
The pusher plate in the Momentum Limited Orion rocket would contain an ablation coating that would slowly vaporize in nuclear explosions, and take away the excess heat.
Dyson's design would produce a rocket that runs at 3 % of the speed of light. Thus, it would only take 133 years to reach Alpha Centauri.
Dyson estimated the cost of the rocket at $367 billion, which in current dollars is roughly $3,000 billion.
Nuclear fallout is not a big problem
http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)#Potential_problemsFreeman Dyson estimated that each launch would cause only at most 1 extra lethal case of cancer for humans. That is clearly negligible, and constitutes no reason to refrain from using nuclear pulse propulsion in rockets.
The fallout can be reduced by performing the first explosion with conventional explosives. Also, we can design hydrogen bombs where the energy from fission, and consquently the fallout, is very small.
Finding habitable exoplanets
Recent results from the Kepler space probe show that there are probably billions of Earth-like planets in the habitable temperature zone in the Milky Way. Locating such planets requires better instruments than Kepler. And we still need better instruments to determine the spectrum of the atmosphere of a potentially habitable planet.Maybe in 20 years we will know if there are habitable planets in the vicinity of the Earth, that is, at most 15 light years away. And we will know if their atmosphere contains carbon dioxide or oxygen. If the atmosphere contains oxygen, then it might be that there is actually vegetation on the planet.
Finding a planet with vegetation would be very exciting news. With good luck, the atmosphere might be such that humans can readily breathe the air on the planet! But if we do not find any oxygen, then we need to pick a planet that contains a suitable amount of carbon dioxide in its atmosphere and "terraform" such planet for it to be easily colonizable by humans.
Terraformation of planets
It may be that the Earth is the only planet in the Milky Way that carries life. If that is the case, we will not find any planets where the atmospere contains enough oxygen for humans to breathe. But if we find a planet with water and a suitable amount of carbon dioxide in its atmosphere, we can slowly transform the planet to be suitable for humans.The idea is that we send an unmanned spacecraft to the planet to seed there cyanobacteria and maybe also plants. If we are able to create a vegetation similar to the Earth, then it will take on the order 40,000 years to produce enough oxygen to the atmosphere of the planet, so that humans can breathe the air there.
The travel time to exoplanets is of the order of hundreds or thousands of years, and terraformation of planets takes tens of thousands of years. We are talking about really long-term projects here.
Panspermia: there might well be exoplanets that carry vegetation
The Earth formed 4.5 billion years ago, and it was under a very intense bombardment from asteroids for a few hundred million years after that. The asteroid bombardment has ejected large parts of the early Earth's crust to space, and eventually out of our solar system. Thus, much of the Milky Way has been polluted by rocks from the young Earth.Suppose that there already was life on the Earth during the asteroid bombardment. Let us calculate how many 10 cm size life-carrying rocks from the crust might have been ejected from the early Earth in the asteroid bombardment. Suppose that 2 % of the Earth's area was ejected. Let us assume that life might have been present in the top 10 meters of crust rock. That makes:
10 million square kilometers * 10 billion 10-cm-sized crust rocks / km^2
= 10^17 rocks
The volume of the Milky Way is some 100,000^2 * 10,000 = 10^14 cubic light years. From the Kepler space telescope results, it has been estimated that the Milky Way might contain 10 billion planets that are friendly for life. Thus, there is an average of 10^-4 life-friendly planets in a cubic light year.
Suppose that we have a rock that drifts slowly a distance of one light year past the stars in the Milky Way. The probability of it hitting a life-friendly planet along that distance is:
10^-4 * (0.04 light seconds)^2 / (3 * 10^7 light seconds)^2
= 4 * 10^-21
Above we have used the fact that the diameter of an Earth-like planet is some 0.04 light seconds, and that a light year is 3 * 10^7 light seconds.
If the rock moves at a relative speed of 10 km/s past the stars in the Milky Way, then in a billion years, the rock will cover a distance of 33,000 light years.
We get that the probability for a single rock to hit a life-friendly planet in 1 billion years is roughly 10^-16. Since there were 10^17 such rocks, we get that 10 rocks will hit a life-friendly planet during a billion years.
Life may spread further from these 10 exoplanets if there is sufficient asteroid bombardment of those planets. That way, we may get a chain reaction where life spreads exponentially through the Milky Way. In 10 billion years, most of the 10 billion life-friendly planets in the Milky Way might get seeded with life.
Currently, it is not known if bacteria can survive a billion years in space inside a rock of size 10 centimeters. But it is well possible that bacteria could stand such conditions. Also, we do not know if bacteria can survive the heat when the rock falls through the atmosphere of the planet.
We do not know if life originated on the Earth, or if it started on some exoplanet in the Milky Way, and was seeded to the Earth inside a rock that was ejected from some exoplanet. If life originated on the Earth, and it originated only after the big asteroid bombardment of the Earth, then it may be that the Earth is the only life-carrying planet in the Milky Way.
But if life originated on the Earth or some exoplanet before the asteroid bombardment of that planet, then rocks containing seeds of life have polluted the Milky Way. In that case, it is possible that we will find many exoplanets that already carry life. Furthermore, that life may be related to the life on the Earth. Exoplanets may harbor life where the DNA structure and much of the chemistry of living cells is similar to the Earth. We might even find planets where there are plants and animals that look like ones on the Earth. In that case, sci-fi stories about strange planets with jungle-like vegetation and dinosaur-like creatures might actually be more realistic than we could imagine!
