% title ISSDC: Space Characteristics

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<H2>Space Characteristics and Resources</H2>

<p>Space is not a friendly place. Mostly, it is nothing: vacuum,
and apparent lack of gravity. What little there is in space is
mostly hazardous, even the things that are assets.

<p>One asset abundant in space, at least out to Mars orbit, is
power, in the form of energy from the Sun that can be converted
into electricity. The supply of power from the Sun is continuous
and uninterruptible, and it is 6 to 15 times as intense in Earth
orbit as on the Earth's surface (solar energy intensity is absorbed
by Earth's atmosphere). Solar energy does, however, present hazards,
in that it is harmful to humans who are not shielded from it. The
sun is also the source of flares producing intense radiation deadly
to unprotected humans and equipment.

<p>The Sun is not the only source of hazardous radiation in space:
cosmic rays, gamma rays, and X-rays are also part of the ambient
space environment, at sufficient intensities to be damaging to
humans. Earth's magnetic field prevents most of this radiation from
reaching low earth orbits and the planet's surface; it also, however,
traps dangerous levels of protons and electrons in the Van Allen
radiation belts, which begin at 250 to 750 miles altitude, and
extend to between 37,000 and 52,000 miles altitude.

<p>The primary law of space is Orbital Mechanics, which determines
where things can stay and where they can go. Everything in space
is moving, attracted by gravitational fields (usually interacting)
of every major object in the vicinity. Which orbit an object is in
depends on its position and velocity at any particular time. Orbits
are changed with acceleration due to thrust, usually from rockets.

<p>Satellites are constrained to be far enough above most of any
atmosphere that an orbit can be sustained for a few days before
decaying through friction with the atmosphere (about 100 miles above
Earth's surface; a 150-mile Earth orbit can be maintained for about
five years, a 250-mile orbit for about ten, with variations depending
on solar activity and other factors that can cause the atmosphere
to expand or contract).

<p>Objects with bigger cross-sections experience faster orbital
decay. Higher velocities put objects in higher orbits; an object
achieving ``escape velocity'' (25,000 mph for Earth) will leave the
influence of its ``host'', and go into orbit around something else.
Orbits are also not perfectly stable, requiring that satellites use
small rockets for ``station-keeping'' to stay where desired. Some
quirks of orbital mechanics cause gravitational forces due to two large
bodies (e.g., moon and Earth, or Earth and Sun) to balance in some locations, creating ``Libration Points''
that are either unusually stable (L4 and L5) or are unstable but
provide opportunities for maintaining spacecraft in otherwise
impossible locations (L1, L2, and L3).  All five libration points can host multiple small objects in local orbits:  around stable L4 and L5, these orbits are large and apparently elliptical; around unstable L1, L2, and L3, orbits are convoluted but viable with station-keeping propulsion.

<p>Solid objects in space are also both hazards and assets. Debris
is a nuisance: at orbital velocities, even paint flecks can do
damage, and small bolts or rocks can puncture spacecraft cabins.
Most human-caused debris is in Low Earth Orbit (LEO), Geosynchronous
Earth Orbit (GEO), and Molniya orbits used for Russian communications
satellites; all objects larger than three inches in diameter are
tracked by the United States Air Force, and warnings are provided
when collisions are predicted.

<p>Natural debris is primarily dust, which can degrade materials
that stay in orbit for long periods of time. The Moon and near-Earth
asteroids, however, represent vast reserves of relatively easily
accessible resources; getting to them requires much less energy
than launching materials from the Earth. The lunar surface is 99%
oxides, of silicon, iron, aluminum, calcium, magnesium, and titanium.
Asteroids are of several types, and can contain nickel, iron, water,
carbon, and carbon compounds.

<p>The space environment near Mars is even less hospitable than near
Earth.  With no magnetic field, Mars lacks the equivalent of Van Allen
radiation belts that provide some protection for human operations in GEO.
Solar radiation in Mars orbit is only 36% of the intensity near Earth,
requiring larger solar panels to produce the same amount of electricity.
Moons Phobos and Deimos are close to the planet; at 5825 miles altitude,
Phobos is below Mars synchronous orbit.  Neither of the two tiny moons
has enough gravity to produce a Martian equivalent of libration points.

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