Before we attempt to find out the nature of the paths in space followed by meteorites, we must take into account the fact that these bodies are observed from a station—the earth—which is itself in rapid motion. You may have noticed that on a still day, when rain drops fall vertically downward, the streaks they leave on the windows of a swiftly moving car are not vertical but almost horizontal. Obviously, it would be wrong to say the rain drops are falling from left to right or from right to left when they are actually falling almost straight down, and it is only the forward motion of the car that makes them leave horizontal streaks. [Illustration: Diagram showing meteorite moving along a “closed” (elliptic) orbit, e, which intersects the earth’s orbit, E. Held by the gravitational attraction of the sun, the meteorite is a permanent member of the Solar System.] Similarly, neither the apparent speed nor the apparent direction of motion of a meteorite with respect to the moving earth is significant. The important factor is the meteorite’s velocity _with respect to the sun_ at the time the meteorite is picked up by the earth. [Illustration: Diagram showing meteorite moving across the earth’s orbit, E, along an “open” (hyperbolic) orbit, h. The meteorite is traveling at such high velocity that it will pass right through the Solar System and back out into space unless it should chance to collide with the earth or another planet. The sun, however, in any case is able to change materially the direction of motion of the transient visitor to our Solar System.] This factor enables us to determine in which of two possible kinds of path the meteorite was moving _before_ it was “fielded,” as we might say in baseball, by the earth. This factor tells us whether the meteorite was moving about the sun in a relatively short, closed, oval-shaped path or, instead, was following an indefinitely long, open path which began in the depths of space and would have returned there if the collision with the earth had not prevented. Either type of path is technically called an _orbit_. The closed orbits are what the mathematicians term _ellipses_; the open orbits, _hyperbolas_. To scientists, the nature of the orbits followed by meteorites is most important, especially in efforts to determine the mode and place of origin of these bodies. To rocket engineers and astronauts, it also matters a good deal whether the meteorites encountered on flights through space are traveling sedately along closed orbits about the sun or are zipping swiftly along open orbits. The greater the speed of these cosmic “hot-rods,” the more dangerous they are to space travelers. For example, a mere grain of nickel-iron moving at 40 miles per second is quite as lethal as a .50-caliber machine-gun slug, which, relatively speaking, is traveling at only a snail’s pace.