Already much effort has been devoted to finding out the conditions, which would have to be overcome on a planetary journey, The Russians have organized "sampler" expeditions with unmanned, radio-controlled space vehicles. At the same time Americans aboard the "sky laboratory" in orbit round the earth have concentrated on the problems of enduring long periods of space travel.

     In these twenties, Newton sent a paper on light and colours to the Royal Society of London, and devised a telescope in which the principle component was a concave or magnifying mirror. He enquired into how light was produced and developed what was known as the "emission or corpuscular theory of light", according to which light is the product of a luminous body of tiny particles.

     The theory was also used to explain the colours seen when light is reflected from a thin film (e.g. a film of soap) and a series of dark and light rings circling round a central black spot is seen.

     Newton said that some of the particles of light were reflected and others were refracted.

Rings of colour from the reflected and refracted light were known as "Newton’s Rings".

     When different liquids mix, it is because they have similar types of molecules which reading link up with each other, like milk and water. In the case of oil and water the groups of molecules prefer to stay apart.

     The patches of oil floating on top of the water are usually circular because of another characteristic of molecules, which produces what scientists term surface tension. This is a cohesive force caused by the attraction of the molecules to each other. They cling so tightly that they produce a surface layer which acts like an elastic skin or the rubber envelope of a balloon. The molecules are trying to pull the liquid into as small a space as possible. As well as producing a circular shape, this tension makes the surface area of each oil patch as small as possible.

     Stars are not fixed in space, but are travelling in different directions at different speeds. Seen from the earth, these movements appear to be so small that groups of stars, or constellations, seem to have a permanent relationship. The star patterns we see in the sky are almost the same as those seen by our ancestors hundreds, or even thousands of years ago.

     The sizes of stars vary tremendously, from about a tenth of the diameter of the sun to 20 times its diameter. Most stars appear to be white when looked at with the naked eye, but some are bluish-white, yellow, orange and red. The varied colours are due to differences in surface temperature. The brilliant, white stars are the hottest with temperatures of several hundred thousand degrees. The less brilliant, orange and red stars have a temperature of about 2,000 degrees.

     There are exceptions, however. The red giant, Betelgeuse, in the constellation (or group) of Orion, appears to be brilliant because of its size. Its diameter is 250 million miles, which is greater than the diameter of the earth’s orbit round the sun.

Shooting stars which the sometimes seen moving across the night sky for a few seconds are really meteors. These small particles flare up as they strike the earth’s atmosphere but soon burn out.

     The spiral arms of the Milky Way are rich in hot, bright stars, interstellar clouds of gas (mainly hydrogen) and dust. The American astronomer obtained the first evidence of spiral arms in 1951. W.W. Morgan, who identified three.

     Our own system of sun and planets appears to be situated towards the inner edge of one of the arms, which are about 1,300 light-years away. The Andromedanebula, a vast mixture of gaseous and solid matter, is visible as a small luminous patch in our sky. But it is comparable in size to the Milky Way and seems remarkably similar to our own galaxy.

     The Palomar telescope, 200 inches in diameter, situated on Mount Palomar in California, has perhaps 1,000 million galaxies within the scope of its vision.