It seams likely now that men will one day try to travel to the stars. They will go in search of new planets to live on, away from the overcrowded Earth. They will go in search of knowledge and adventure.
From what we know, a journey to a planet outside our Solar System will take a very long time. The nearest stars with planets may be much farther away than the nearest ones without planets. The Apollo spacecraft reached a speed of 25,000 miles a hour on the journey to the Moon. At this speed it would take 280 million years to reach Proxima Centauria, the nearest star.
The first starships to be built would be much faster than the Apollo craft. But they are not likely to go at anything like to speed of light. So they will take hundreds of years to reach stars with planets on which people can settle.
This does not matter if the spaceships are built to last long enough, and if the people on board are prepared to live, have children and die on the spaceship.
A 'multi-generation' expedition like this, planning to start a colony at the end of its long journey, would have to be very large - at least 2,000 people. If there were fewer the colony would not be able to produce enough strong children. Several starships could travel in a fleet to make up the numbers.
Starships will have to be very big to hold from 200 to 2,000 people each. One possible model would be powered by nuclear engines using a hydrogen fuel. Hydrogen is the most common element in the Universe. The star travellers would be able to refuel from hydrogen-rich planets like our Jupiter on their journey.
The star travellers might sleep during the long journey, rather as an animal hibernates in the winter. An autopilot would control the spaceship an wake up the passengers when a likely planet had been reached. Or there could be some passengers asleep and some awake.
The spaceships of the future would be assembled in space. They would be made of materials invented for use in space. They will use engines that work best in space. These would not need to be as massive as those needed to lift the Apollo spacecraft off the Earth.
One engine that has been projected would use nuclear fission the create great heat. Hydrogen heated in this way would provide enormous thrust.
Another sort of engine using nuclear power might explode tiny hydrogen bombs behind the spaceship, creating thrust directly. There would be a rapid chain of small explosions and each one would push the spaceship forward.
Scientists have thought of other ways of powering starships, using the knowledge they have at the moment. Some think that thrust could be obtained by using photons - particles of light - in huge 'photon-sail' spaceships. An ion-powered rocket might also be used. Electrically charged atoms would provide the thrust as they were electrostatically repulsed from the engine.
The asteroids in the Asteroid Belt might be used as spaceships. The asteroids could be hollowed out and fitted with motors and life support systems for hundreds of people to live in.
Space travel to the stars is probably a long way off. There are no pressures on man at the moment so great that he has to leave the planet. But at some time in the future he may want or need to go somewhere else. Then the first starships will be built - to make the most adventurous journey of all.
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