Note to Reader

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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