# Astrodynamics 2.0/Introducing Space

## Introducing SpaceEdit

### IntroductionEdit

Space. Most people generally define it as "The stuff that's out there." When asked what was meant by "out there," most will reply, "Not on Earth." And, when you add in the fact that space is actually very empty, the definition grows up to be, "The stuff that's not stuff that's out there and not on Earth." Now there's a very articulate definition.

Here is a slightly more refined definition, "Outer space: any location outside the Earth's atmosphere; 'The astronauts walked in outer space without a tether'; 'The first major milestone in space exploration was in 1957, when the USSR's Sputnik 1 orbited the Earth' "

Hmmm. So they were actually pretty correct. Space is the area outside of the atmosphere of the earth. Outside generally meaning past about 250 kilometers up or so, because the atmosphere actually doesn't stop on a fine line, rather it fades out until it becomes indistinguishable from the surrounding material.

Wait, surrounding material? What material, space is supposed to be a vacuum, with no nothing ever anywhere! That's not entirely true. Space actually has more stuff than nothing. Not too much, but it's stuff. A rather high estimate puts the maximum density at about 100 particles per square meter. That's pretty good. Not much, but stuff.

This is the space that this book will deal with, or rather the motion of objects through it.

### Introducing SpaceEdit

You're probably thinking, "But isn't that the title of the chapter?" Right here is going to talk about a different kind of space. "Ordered space."

When you learned about math, you probably came across some problem where you had to graph some function or other. How did you do it? "I got a piece of graph paper, and I..." What's graph paper? "Graph paper is a piece of paper with lines on it that are arranged in a grid." Why were you using a grid? "Because it had numbers on it that let me pick points and connect them to draw the function."

You were using the graph paper because the grid represented something called the cartesian coordinate system. This system allowed you to use numbers as a kind of "GPS" on your paper.