1835

From The Space Library

German astronomer Joseph Johan von Littrow writes Geschichte der Endeckung der allgemeinen Gravitation durch Newton (History of the Discovery of General Gravitation by Newton) in which he includes Kuntsliche Monde, die wir uns vieleicht einmahl selbst machen konnen (Artificial moons, which we may one day be able to do ourselves). In this short article he describes how an artificial satellite is possible but will have to wait until the force required for the launch becomes available. He also calculated the exact velocity required to reach orbit around both the Earth and the moon. This essay was republished by his son in his book "Die Wunder des Himmels".

"Applying the foregoing to artificial moons. We all know when a ball is shot to the horizon, the shot describes a curved line, at the end of which it falls back to earth. The greater the force with which the bullet is driven from the mouth of the gun, the greater is the curve which it describes above the earth, and it is clear that force, the charge of the cannon, might at last become so great that the ball does not fall back to the earth, but that it would have to describe a curved line around the earth. But if it did the same thing that the moon has done for a long time, we would in fact obtain a little moon in this way, so that in the end we could raise these moons in any quantity, like our balloons or soap-bubbles we would only share with our guns the necessary strength.

"And how big should this force be, what is the initial speed to reach that end? The answer to this question is very easy for those who are familiar only with the first principles of mechanics. If one multiplies the height of the body by the diameter of the circle in the first second, and takes the square root of the number thus obtained, one has the sought-after initial velocity of the sphere in question.

"For the earth, for example, that fall height is 15 feet. But its radius is 19632000 feet. So if the last number is taken twice, and multiplied by fifteen, we get 588960000, and of that number the square root is 24268. The cannon would therefore have to receive a charge through which the ball made a distance of 24,268 feet in the first second of its course. However, our cannonballs, which travel a maximum of 700 feet in the first second, are still very far away.

"The moon-dwellers, on the other hand, provided that they are equipped with better guns, could carry out such an attempt, which is still impossible for us, much easier; for with them the heaviness of the bodies is only the fifth part of our heaviness, so will the fifth part of that charge suffice, which make the shot a satellite of the moon, a second-ranked moon. An initial speed of 5000 feet would drive this ball around the moon. But since this speed is also more than seven times greater than that produced by our guns, the Selenites, too, will have some difficulties to overcome in carrying out this experiment, especially if, as may be presumed, they are not as advanced as we are, have advanced in ballistics, or if at the end they should still have no guns at all." - Joseph Johan von Littrow