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flight in the universe, whereas 100 years go by for those who stay at home. The difference in time between the space travellers and the people on earth can be calculated by the following formula, given | 1 — (ofc — (vje)® The speed of the space-ship's flight can be calculated by the bask rocket equation worked out by Professor Ackeret: 1 — (1 — f)*p/e = wle[r(1 — **w/c] At the moment when our space-ship is approaching the star which is its target, the crew will undoubtedly examine planets, fix their position, undertake spectral analyses, measure forces of gravity and calculate orbits. Lastly they will choose as landing-place the planet whose conditions come closest to those of our earth. If our space-ship consists solely of its pay load after a journey of shall we say eighty light years, because all the energy supplies have been used up, the crew will have to replenish the tanks of their space-craft with fissionable material at their goal. Let us assume, then, that the planet chosen to land on is similar to the earth. I have already said that this assumption is by no means impossible. Let us also venture the supposition that the civilisation of the planet visited is in about the same state of development as the earth was 8,000 years ago. Of course, this would all have been confirmed by the instruments on board the space-ship long before the landing. Naturally our space travellers have also picked on a landing site that lies close to a supply of fissionable matter. Their instruments show quickly and reliably in which mountain ranges uranium can be found. Our space travellers see beings making stone tools; they see them hunting and killing game with throwing spears; flocks of sheep and goats are grazing on the steppe; primitive potters are making simple household utensils. A strange sight to greet our astronauts! by the Lorentz transformations: (t = spacet-travellers' time, T=time on earth, v= speed of flight, c= speed of light). (v — velocity, w = speed of jet, c = speed of light, t = fuel load at launching). The landing is carried out according to plan. But what do the primitive beings on this planet think about the monstrosity that has just landed there