Page 112 - Intro to Space Sciences Spacecraft Applications

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Spacecraft Environment
particles moving at relatively fast velocities. Some of these particles are
the monatomic oxygen atoms created by photodissociation. Being a high-
ly reactive element, when these atoms strike a spacecraft they tend to
combine with the spacecraft materials, creating a potentially damaging
corrosion over long periods of time in space.
The major problems associated with operating in the near-vacuum
environment of orbits around the earth concern the materials used. Some
materials may change their physical properties over time through out-
gassing-the release of trapped gases within the materials-or from the
radiation and/or corrosion damage discussed earlier. Materials in contact
in a vacuum can actually transfer atoms across their surfaces and eventu-
ally “cold weld” to each other. This may cause severe problems in space-
craft with moving parts. Special substances must be used to separate and
lubricate moving parts in space, because lubricants commonly used on
earth would simply boil away in a vacuum.

Micrometeorites

The earth is constantly being bombarded by small particles from space.
Meteor showers occur when these particles encounter the atmosphere and
burn up with the friction of reentry. Though extremely small, these parti-
cles may have enormous velocity differences with an orbiting spacecraft.
Collision with these micrometeorites can pit the surface of a spacecraft
and degrade the performance of sensors or systems (such as solar power
cells and thermal coatings) and perhaps cause more serious damage.

Man-made Debris

In the short time that man has been able to place objects in space, the
debris created by launch vehicles and spacecraft operations has already
created a situation of some concern. The U. S. Space Command has iden-
tified more than 7,000 artificial space objects of 10 cm diameter or larger
orbiting the earth. Almost 6,000 of these are in low-earth orbit, and about
half of the count are spent rocket stages and active and inactive satellites.
Computer simulations predict approximately 17,500 objects between 1 and
10 cm in diameter along with millions of smaller particles of space debris.
The total mass of man-made debris is about 15,000 times the naturally
occurring mass concentration around the earth. The relative velocity
between objects that may collide in low-earth orbits is around 10 km/sec