What is Space?

Space, most generally, might be described as the boundless container of the universe. Its contents are all physical things that we know of, and more. To describe the contents of space, we use terms of distance, mass, force, motion, energy, and time. The units we use depend on the scale we are considering. Units useful to us on the scale of human life become difficult to use for much smaller domains (such as atoms), and as we describe space beyond our planet Earth.

Consider a distribution of mass at very large distances throughout space and the motion and energy transformation processes going on all the time. While these masses are very distant from each other, they do interact in various ways. These include gravitational force attraction; emitting, absorbing, or reflecting energy; and sometimes, though statistically very seldom, colliding.

Interplanetary Space

Interplanetary space refers to that region of our container that holds the Sun, the nine major planets that revolve about the Sun, and all other mass, distances, force interactions, motions, and transformations of energy within that realm. Distances are often given in terms of the average distance separating the Sun and Earth, in units called astronomical units (AU). Pluto, our most distant planet, is 39 AU from the Sun. Each body in the solar system exerts a gravitational pull on every other body, proportional to their masses but reduced by the separation between them. These forces keep the planets in orbit about the Sun, and moons in orbit about planets. Earth's moon, though relatively small in mass, is close enough to cause tidal changes in Earth's oceans with its pull. The massive planet Jupiter, however, affects orbits throughout the solar system.

Between Mars and Jupiter lies a ring of debris called the main asteroid belt, consisting of fragments of material that never became a planet. Gravitational forces (primarily those of the Sun and Jupiter) pull the asteroids into more defined orbits within this doughnut-shaped region. Some of these asteroids, because of collisions or by gravitational perturbations , leave the main belt and fall into Earth-crossing orbits. These are the ones that are the subject of disaster films and to which craters on Earth and the extinction of the dinosaurs are attributed. In addition to these asteroids, comets (which are essentially large, dirty snowballs) pass through the solar system, leaving dust trails. As Earth travels about the Sun, it collides with some of these dust trails, giving rise to meteor showers.

As we peer into space from our home planet, whether with our naked eye or with the most powerful telescope, we are looking back through time. Light arriving at our eyes carries information about how the source looked at a time equal to the travel time of the photons of light. For example, our

nearest star neighbor is a three-star system called Alpha Centauri, which is 4.3 light years away. This means that, as we look at this star system, we can know only how it looked 4.3 years ago, and never as it looks right now. We see the light from more distant stars that may have died and vanished many, many years ago.

see also Space Environment, Nature of (volume 2).

David Desrocher

Bibliography

National Geographic Atlas of the World, 7th ed. Washington, DC: National Geographic,1999.

Shirley, James H. Encyclopedia of Planetary Sciences. New York: Kluwer Academic Publishers, 2001.

Weissman, Paul. Encyclopedia of the Solar System. San Diego, CA: Academic Press,1998.

Internet Resources

Solar System Dynamics. NASA Jet Propulsion Laboratory, California Institute of Technology. <http://ssd.jpl.nasa.gov/>.

Solar System Exploration Home Page. National Aeronautics and Space Administration.<http://solarsystem.nasa.gov/>.

Solar System Simulator. NASA Jet Propulsion Laboratory, California Institute of Technology. <http://space.jpl.nasa.gov/>.

Virtual Solar System. National Geographic. <http://www.nationalgeographic.com/solarsystem/splash.html>.