## Vacuum

A pure vacuum—a volume devoid of any material whatsoever—is something not yet encountered by scientists. As we found in Chapter 3, even in Earth orbit at altitudes up to around 1000 km (620 miles) there is a residual atmospheric density, and even in the spaces between the planets in the solar system there is material, called the interplanetary medium (mainly emanating from the Sun, as we have seen). However, as far as people and spacecraft are concerned, the degree of vacuum is fairly academic. Over millions of years, we and our ancestors have adapted to an environment where every square centimeter of our bodies is exposed to an atmospheric force of about 10 Newtons, which translates into imperial units as the familiar sea-level pressure of about 15 pounds per square inch. If we remove this pressure by foolishly stepping out of a spacecraft without protection, then surprisingly tests show that we don't explode, or anything dramatic like that. The main problem is that there is no oxygen to breath, and consciousness is lost after a few tens of seconds, followed by death after a couple of minutes. Precisely what happens, and when, is not well known, as scientists are understandably reluctant to do too many experiments! For spacecraft, the effects of high vacuum are rather less spectacular, but nevertheless the spacecraft designer needs to know something about it to avoid using the wrong materials in the spacecraft's construction.

At about 800-km altitude in Earth orbit, the atmospheric pressure is tiny (of the order of 0.000 000 000 001 Newtons per square centimeter), and at these low pressures materials suffer an effect called outgassing. This is related to what happens to water when heated—the surface water molecules escape the body of the liquid, and if the process continues, all of the water will vaporize into gas. Similar things happen to metals in high vacuum, where the low pressure causes the surface atoms to outgas. For example, at temperatures of around 180°C, a surface composed of zinc will recede at a rate of around 1 mm per year. However, for a material like titanium—one much more commonly used in spacecraft construction—a temperature of 1250°C is required to achieve the same rate of recession. Thus, as long as the designer chooses the construction materials appropriately, outgassing will not be an issue as far as the strength of the structure is concerned. But sometimes there is a concern over the outgassing material contaminating the spacecraft's surfaces; for example, the performance of a space telescope may be compromised if outgassed material is deposited onto the system's optics.

A related problem for spacecraft is the effect of vacuum upon commonly used terrestrial lubricants. The highly volatile oil-based lubricants we use in our machines down here would outgas (or boil away) in no time at all in the vacuum of space, which has given rise to a whole new science of space tribology. To overcome this problem, engineers have had to develop solid lubricant coatings for use in spacecraft bearings and mechanisms. Interested readers can 'google' the term molybdenum disulfide which is commonly used as a solid lubricant.

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