This book is being marketed to potential readers in the Americas, Europe, and around the world. Because of the differing engineering units used in the aerospace industry on either side of the Atlantic, both sets of numbers will be used throughout the book, choosing whichever system seems to suit the particular example. Thus, discussions of the Apollo program will use US customary units, while those of the Soyuz or Ariane programs will use strictly metric units. For the benefit of all readers, it is good to be familiar with a few basic values common in the aerospace field. These are given in the following table, in both their US customary and international metric system equivalents:

Light plane altitude |
10,000 ft |
3 km |

Airliner altitude |
50,000 ft |
15 km |

Space altitude |
330,000 ft (62 miles) |
100 km |

Orbital velocity |
17,500 mph (5 miles/s) |
8 km/s |

Escape velocity |
25,000 mph (7 miles/s) |
11 km/s |

Low earth orbit |
100-500 miles |
200-800 km |

Earth to Moon |
238,000 miles |
383,000 km |

Sun to Earth |
93,000,000 miles |
150,000,000 km |

Sun to Mars |
142,000,000 miles |
229,000,000 km |

Earth to Mars |
370,000,000 milesa |
600,000,000 kma |

Galaxy diameter |
100,000 light-years |
30,000 parsecs |

a Assuming Hohmann transfer orbit, which takes the spacecraft halfway around the Sun.

a Assuming Hohmann transfer orbit, which takes the spacecraft halfway around the Sun.

As a rule of thumb, it is a good idea to memorize the "easy" numbers. An educated space-savvy resident of Planet Earth should know that space officially begins at 100 km, that low Earth orbit is centered at about 200 miles - or 300 km - above sea level, and that the diameter of the Galaxy is 100,000 light-years (74 trillion Earth diameters!). It is also useful to know that the speed of light is 300,000 km/s, and that a light-year is the distance that light travels in one tropical Earth year. This amounts to just under six trillion miles. It is also vital to know that the difference between escape and orbital velocities is a much smaller value than that of orbital velocity itself, and that planetary atmosphere entry speed is roughly the same as the escape velocity, if the spacecraft is entering from interplanetary or cislunar space.

When numerical values are spelled out, as in the cases just cited, standard American usage will be employed, so that progressing by factors of 1,000, the sequence is thousand, million, billion, trillion, quadrillion, quintillion, etc.

Since the kilogram is technically a unit of mass, while the pound is a unit of weight, this book will typically use these terms in this way, to avoid ambiguity. The corresponding units of weight and mass in the respective systems are newton and slug, which are rarely used in common parlance. For those who seek further information, please see the mathematical appendix.

Chapter 1

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