Fifty years ago last Saturday, Apollo 10 left for the moon. As you read this, depending when you click in, they are/were part way there. The mission’s big events will have their anniversary Wednesday, and that is when the main post will come.

Meanwhile . . .

From the fifties onward, there were dozens of books by people like Arthur C. Clarke and Wily Ley that explained in great detail how  we would go to space. I read most of them — at least every one I could get my hands on. There were a lot of people like me then. A lot of them spent the last decades working for NASA.

Now our knowledge of the universe is vastly greater, and most kids today know more than the best informed knew then. Still, some basic things get missed, because “everybody already knows them”.

Actually, they don’t. Here is an example, which I need to get out of the way before I talk about Apollo 10 on Wednesday.

Imagine, a rocket leaves Cape Canaveral with rocket engines flaming. The engines only burn for a  fairly short time, for reasons of efficiency; then the rocket coasts upward into orbit.

Right. And wrong. There is one more important thing that happens, but rarely get mentioned any more. A second critical burn has to happen at the high point of the initial orbit (apogee).

A rocket heading for orbit leaves the pad vertically, but it immediately begins to roll over. It needs to gain altitude to get above the atmosphere, but it also needs to gain velocity horizontally, so its upward path is a precisely programmed curve that begins vertically and ends horizontally (i.e. tangent to the surface of the Earth). This tangent is reached long after the rockets have ceased firing, at apogee, roughly half way around the Earth.

Caveat: everything in orbital mechanics is more complicated that the explanation you will get from someone like me. Nevertheless, this should be close enough for our purposes.

Let’s assume that the orbital insertion burn did not happen at apogee. Our craft would have achieved enough speed to reach its orbital altitude, but not enough speed to remain there. It would immediately begin to descend.

Think of a high fly ball to center field — up, then down again. Same Earth, same gravity, same result.

Such a rocket leaving Earth would burn up on reentry. If it were launched from an airless body like the moon, it would end up in an elliptical orbit with its low point very near the surface.

Instead, if all went well and the secondary burn took place when the rocket had reached its orbital altitude, it would change from a sharply elliptical orbit to a more nearly circular one. This is the normal way things get done.

You’ll need to have this in mind when we look at Apollo 10’s “dress rehearsal” on Wednesday.