Orbital theory begins with the German astronomer Johannes Kepler (1571-1630). He empirically derived three laws that describe planetary motion in our solar system and the orbits of artificial satellites: (This accomplishment is especially impressive because he preceded Sir Isaac Newton and so did not have the benefit of Newton’s physics.)

1) The orbit of a satellite with respect to the earth is an ellipse, the earth being at one of the foci.

2) A line drawn from the earth to a satellite sweeps across equal areas in equal times.

3) The square of the time a satellite takes to complete one orbit is proportional to the cube of its mean (average) distance from the earth.

The first law deals with the shape of an orbit, a circular orbit being a special case of the ellipse. The second law covers the speed of a satellite at various points along the orbit, and the third law has to do with the satellite’s orbital period. The closer a satellite’s orbit is to the earth, the faster it moves and the shorter its orbital period.

The French-Italian astronomer Joseph Louis Lagrange (1736-1813) calculated stable points in the orbit of one body around another where gravitational forces were balanced. His work was relatively forgotten until it was noticed that asteroids in the orbit of Jupiter tended to gather at these Lagrangian points, L1 through L5.

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In 1969 Gerard K. O’Neill, a physics professor at Princeton, asked his students to design a large living environment in space, rather than on a planet or a moon. He published papers at Princeton in 1974 on the construction of space colonies at L5, the most stable position in the lunar orbit, 240,000 miles from the earth.

In this plan solar-powered magnetic launchers would propel material from the moon to L2 where they would be caught by a large “catcher’s mitt.” Then the material would be moved to L5 where it would be used to build large space habitats for thousands of people.

One design is two counter-rotating cylinders (with a net spin of zero) pointed always toward the sun. Each cylinder would be built of strips of land and glass and would have a diameter of up to four miles and a length of up to twenty miles. A cylinder this size would rotate once every two minutes to provide one G of centrifugal force and would house up to one million people.

Lunar ore is rich in aluminum and titanium, and oxygen would be a plentiful by-product of the solar-powered refining process. Carbon, nitrogen, and hydrogen are scarce on the moon, so these would have to be transported from the earth. In particular, liquid hydrogen would be carried in large tanks to make thousands of gallons of water. Alternatively, these raw materials might be obtained in a low-gravity environment from asteroids.

There are three kinds of asteroids, located between Mars and Jupiter. Collectively, they are rich in iron, nickel, magnesium, water, hydrocarbons, and nitrogen. They could also provide raw materials for the construction of space colonies and space ships. They might be moved from their orbits by rockets and placed in orbits near the Earth, or a solar-powered manufacturing plant might be constructed at the Martian Lagrangian point, L5.

After the habitats are constructed, the space colonists could earn their living by manufacturing solar-to-electric power plants to be placed in geosynchronous orbit at an altitude of 22,323 miles in the equatorial plane. (The geosynchronous orbit, proposed by Sir Arthur C. Clarke in 1947, is the orbit most commonly used by television and other communications satellites today. It has the advantage of serving fixed-dish, rather than tracking, antennas.)

These satellite power plants would collect solar energy and beam it as microwave energy to receiving stations on earth where it would be sold as electricity at commercial rates. Critics of this plan say the geosynchronous orbit is already crowded and the microwave energy is likely to be disruptive to communications and also ecologically harmful.

However, the idea of mining the moon for raw material, using solar power to smelt metals, and manufacturing large living environments and even planet-sized space ships in space remains tenable. The essential ingredients for living: light, air, gravity, land, and food could be easily supplied to a living environment in space.

The far side of the moon would be an especially good environment for radio astronomy because antennae and other receiving equipment would be shielded from radio-frequency interference from the earth. Large parabolic reflectors might be constructed in craters or mounted on poles above the moon’s surface. Received signals could be combined to create a very large virtual “dish” with great focusing power. Data gathered from this equipment could be transferred to the earth by lunar-orbiting satellites.

Radio-frequency signals from the earth to the moon or to space colonies at L4 or L5 take about one and one-quarter seconds to travel one way, not long enough to stifle interactive communication. In space-to-space transmissions there is no atmosphere to absorb the shorter wavelengths, so transmission frequencies above the radio frequency range, including laser light, x-rays, and positrons could be used for communication. Optical lenses, small parabolic reflectors, and electrostatic and magnetic fields might focus these shorter wavelengths and particles.

Plans for colonizing Mars in the next two hundred years might include the construction of a “shuttle bus” placed in an elliptical orbit around Earth and Mars with a period of two years. Signals from Earth to Mars take between three and twenty-two minutes to travel in one direction, depending on the relative position of the two planets, and Mars would be eclipsed by the Sun at its greatest distance from the earth every two years.

Because of these time delays, communication would be less interactive, giving each party time to compose letters of information rather than sentences. A radio-controlled robot on Mars would necessarily be intelligent enough to make decisions while awaiting further communication from Earth.

To travel to Alpha Centauri, 4.3 light years away, would require a voyage of at least one hundred years, at speeds attainable by the present liquid hydrogen and liquid oxygen yields water plus energy travel technology. This implies a robot-only crew, generations of people, or folks cooled in suspended animation.

Travel distances will be the major restriction for interstellar people-to-people communication. There are eleven stars within 11.1 light years from us, a possible limit for human migration, using present travel technology. At greater distances than these, we might communicate with non-human intelligence.

Fusion power, converting hydrogen to helium plus energy, will be the breakthrough in interstellar travel. Traveling near the speed of light, the passengers will experience time-dilation effects, extending travel distances and human migration throughout the galaxy.

According to relativity theories, developed by Max Planck and Albert Einstein in the early 1900s, a distance like 100 light years would take much less than 100 years to travel, as measured on the ship. As the ship approaches the speed of light, the travel time becomes almost instantaneous.

There are two major designs that can provide artificial gravity for interstellar travel: One is a flat ship or an apartment building that accelerates and decelerates at one G, a space elevator. The other is the rotating Stanford torus, living in a doughnut.

The front part of the interstellar space ships will be a huge scoop to collect free hydrogen in space to use as fuel. The middle part will either be an apartment building or some variant of a rotating wheel or cylinder. The rear part of the space ship will be the fusion engine, using the controlled power of a hydrogen bomb for propulsion.

Interstellar space ships will be like small traveling stars. They will communicate throughout the galaxy by modulating their fusion reactions and exhaust. The output of entire stars might also be modulated by gravity or by huge plates in space with electrostatic or magnetic fields for communication at maximum distances.

Interstellar communication will be slowly interactive. Requests for physical supplies will not be an important item of conversation, since the travel time will be so great. Scientific data, music, philosophy, and the visual arts might be the major subjects of conversation for mutating gene pools of divergent civilizations.

These red-shifted long-distance messages will become historical records, traveling backwards in time through an expanding universe. Such communication would tend to be one-way and documentary in nature. It could be made more effective by including if-then-else or case a, b, c,… type logic in questions that anticipate long-awaited replies. The problem of administrators hogging the mikes might be overcome by sorting information contained in the conglomerate of many individual data packets at the receiver.

There are one hundred billion stars in our Milky Way galaxy, which rotates once every 200 million years. It has a diameter of 100,000 light years and is 5 to 10,000 light years in thickness.

Human transportation and communication will be forever limited to our galaxy, but conversations with non-human intelligence beyond our galaxy are possible. The Local Group, the half-dozen galaxies of which we are a part, is a further boundary.

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