Page:In the high heavens.djvu/328

 this little body is to fall on our earth as a meteorite, it is obviously essential that its orbit shall cross the track followed by the earth. Unless this condition is fulfilled the potential meteorite may pass near the earth on one side or the other, but would not fall down thereon, and we should know nothing about it. We have, therefore, to consider the conditions under which the orbit of the missile shall possess the very fundamental character of crossing the earth's track.

It can be demonstrated by mathematical calculation, as to which there can be no uncertainty, that it would be impossible for a missile projected from the planet Ceres to cross the present track of our earth around the sun, unless at the instant of projection the missile had a velocity, of which the component perpendicular to the radius of Ceres' orbit was about eight miles a second. The actual velocity with which the little body would start on its journey depends partly on the speed with which it was projected, and partly on the speed which the planet has in its orbit. In fact, as a mathematician would say, the velocity with which the object was animated would be the resultant between the velocity of the planet and the velocity imparted by the projecting agent. The velocity with which Ceres moves round in its path is determined by Kepler's law, and it can easily be shown to be about eleven miles a second. It therefore follows from this circumstance alone that the missile will have a speed of eleven miles a second perpendicular to the radius of the orbit of Ceres. The velocity which it receives from the projective force must be compounded with that which it derives from the movement of the planet. We have already seen that it would be utterly impossible for the