In April 1977, after more than 10,000 hours of work, a group of physicists, engineers and astronomers of the British Interplanetary Society designed on paper an unmanned nuclear-powered spaceship capable of making a one-way journey to a nearby star within a human lifetime. The ambitious enterprise was known as Project Daedalus.
The 191 metre long spaceship would weigh an incredible 68,600 tons including 50,000 tons of fuel. The engines that would provide the propulsion to Daedalus operates on the principle of nuclear fusion whereby atoms of deuterium (a heavy isotope of hydrogen) and helium-3 (a light isotope of helium) are fused together resulting in the emission of an immense amount of energy. Deep inside the main engine chamber of diameter 100 metres, small fist-sized thermonuclear pellets containing frozen deuterium and helium-3 are compressed and heated to detonation point via powerful electron beams or lasers. When the pellets explode, the force of the nuclear explosion pushes against a flexible, but strong molybdenum plate, causing the ship to accelerate.
The main engine of Daedalus must ignite some 250 thermonuclear pellets each second for more than two years to accelerate the ship up to 7 percent the speed of light. After reaching this speed, the main engine is discarded, allowing a second stage to take over for another 1.8 years using a reaction chamber only 44 metres in diameter. The final speed Daedalus will reach by the time all the fuel tanks are empty would be 13.8 percent the speed of light.
Despite the large, cumbersome nature of this spacecraft design with its lack of energy recycling, Daedalus can reach Alpha Centauri in roughly 46 years or Barnard's Star the star of prime interest for this mission lying 6 light years away in about 60 years. However, Project Daedalus is just a design and may never be built at least by us because of the enormous engineering difficulties and monetary costs. As the report on Project Daedalus stated:
"Although many engineering difficulties have been found to exist in the design of this type of interstellar probe, none of these so far appear to belong to the class of insoluble problems. It is true the cost would be very great, but in terms of a fraction of the anticipated gross world product by the end of the present century, it may not be too costly as an international venture.
"Interstellar flight would appear at the moment to be a feasible proposition both to the human race and, by implication, to any other civilization in the Universe with at least our own abilities."
Since interstellar travel is technically feasible for any civilization with at least our own abilities as of the 1970s, is there a quicker way to travel to the stars?
Yes there is.
The first step one needs to take in the quest to reach the stars is to reduce the mass of the probe, payload, engines, fuel tanks and the fuel itself. The importance of lowering the mass is clearly elucidated by Dr Paul C. W. Davies:
"A light particle is more easily moved by a given force than a heavy one. If a particle becomes exceedingly light it will be accelerated by any stray forces, and so will tend to travel very fast. In the limiting case that the mass dwindles away to nothing, the particle will always travel at the fastest possible speed, which is the speed of light." (1)
As for the fuel itself, it must be massless. As John R. Pierce of Bell Telephone Laboratories said:
"Clearly it is impossible to attain a velocity close to that of light by using interstellar [or terrestrial] matter as [solid] fuel."
And the second step must involve some way of recycling this fuel or at least obtain it easily from the surrounding environment during space travel. Because if there is any way we can recycle or freely obtain this fuel, there would be nothing in the laws of physics to restrict us from approaching the speed of light using the right technology. It all depends on our choice of a fuel.
In that case, what kind of fuel should it be?
Well, how about electromagnetic radiation? We know radiation has the inherent ability to move charged or uncharged matter when it is emitted. So there will be a force exerted on solid matter by this fuel. It is also massless, obtainable anywhere in space, and can be recycled according to the Unified Field Theory.
Unfortunately scientists are not yet aware of how to recycle radiation. Nor do they see how much force is present in radiation as a means of accelerating probes to speeds approaching the speed of light. The technology is not yet known to them how this might be achieved. Until the technology is figured out, scientists have to use radiation in a different way. Given the speed of a typical massless particle like the photon (also called electromagnetic radiation) is about 300,000 kilometres per second (the fastest particle known to science), scientists have pretty much decided to go to the extreme of using this particle for communicating with ETs. But it won't be for propelling any interstellar spacecraft at least in the near future. Instead scientists believe the only piece of technology capable of communicating with ETs is the radio telescope.
The work of Hoyle and Wickramasinghe
Never mind. Let's take one step back for a moment and consider the conclusion from the British Interplanetary Society. Interstellar travel is feasible using nuclear power. Build the nuclear-powered spaceship and it can reach the nearest stars within a lifetime by any civilisation with at least our own technical abilities. If we don't try it out ourselves, what happens if another civilisation in the Milky Way decides to make the attempt. What then?
According to Sir Frederick Hoyle, a former professor of astronomy at Cambridge University, England, and Professor Chandra Wickramasinghe of University College, Cardiff, if one civilization could develop a technology capable of traveling interstellar space at a top-speed of one-tenth the speed of light, to colonise a habitable planet, it would take that civilization about two million years to cover the entire galaxy.
They assumed each habitable planet were 50 light years apart, took 500 years for the colonists to travel to those planets at the speed given above, and another 500 years to set-up a settlement before going on to the next planet. Since the diameter of our galaxy is about 100,000 light years, Hoyle and Wickramasinghe estimated it would take two million years to colonise it.
But why live on a habitable planet and so interfere in the development and evolution of life native to that planet? Why not live on the surface of a dead world like the Moon or in space?
Eric M. Jones of the Los Alamos Laboratory noted this possibility and made a more extensive study using computers on this question of galactic colonisation. Jones added other factors to his model, including the time it would probably take for unmanned probes to make preliminary exploration of potential planetary systems.
The result he attained from his study was this: it would take the colonists about five million years to fully explore and colonise the entire galaxy! Even if it took a thousand times longer for the colonists to achieve their goals, the time would have been ample enough to do so.
The great Fermi paradox
What an incredible discovery. This brings us back to the original problem. As the Italian-born U.S. physicist, Dr Enrico Fermi (1901-1954), said in 1943:
"If there are so many people out there, where are they?"
If they are already here, as the mathematics of galactic colonisation would suggest, or there are many more seeded life-bearing planets that have evolved civilizations, then we should have seen them by now or at least heard from them with our radio telescopes. But for some reason we have not. Not even Mars or the Moon are peppered with aliens carving out an existence on these inhospitable worlds.