Space Environment, Nature of the

Near-Earth space is a complex, dynamic environment that affects not just objects in space, but our everyday lives as well. It exists as the interaction of energy and mass from a variety of sources. Earth, with its magnetic field and its atmosphere, interacts with the Sun to form the solar-terrestrial system, which accounts for most of the effects of the near-Earth space environment. Deep space sources (e.g., other stars and galaxies) contribute particle and electromagnetic radiation that also interacts with Earth. Finally, there are solid bodies within and passing through the solar system that can and do interact with Earth. All of these systems affect orbiting artificial satellites and also have more direct effects on life on Earth, right down to the planet's surface.

The Sun's Interactions with Earth

The Sun is the greatest source of energy in the solar system. It drives most of the activity of the near-Earth space environment. Solar energy couples with Earth's atmosphere and surface, giving rise to terrestrial weather. In a similar way, solar radiation interacts with near-Earth space to give rise to space weather. The Sun continuously emits radiation in two primary forms: electromagnetic and particle.

The electromagnetic radiation emitted by the Sun spans the spectrum, from radio waves up through infrared and the visible light wavelengths to the ionizing energies of extreme ultraviolet, X-ray, and gamma radiation. Ultraviolet radiation is the familiar radiation that can burn human skin and fade curtains. Fortunately, the gases in Earth's atmosphere shield us from most ultraviolet radiation. It is the interaction of intense radiation, such as extreme ultraviolet radiation, that strips electrons from (or ionizes) the gases in the upper atmosphere, creating what is called the ionosphere . One example of how the ionosphere is affected by direct radiation by the Sun and by nighttime shielding by Earth is AM radio. At night, the thickness of the ionosphere shrinks. Radio waves then bounce off the bottom of the ionosphere at a higher altitude, giving these waves longer pathways to follow. This leads to the signals of certain AM stations reaching much larger areas at night than they do during the day.

Particle-type radiation from the Sun, referred to as the solar wind , consists primarily of electrons and protons that are thrust from the Sun's surface at speeds of hundreds of kilometers per second. These flowing charged particles constitute and interact with an interplanetary magnetic field. When these particles stream past Earth, they change the shape of Earth's magnetic field (called the geomagnetic field), creating a region called the magnetosphere, and affecting currents that flow about the planet. Charged particles are accelerated along the concentrated field lines at Earth's magnetic polls, generating eerie and beautiful auroral displays.

Satellites are affected by the harsh radiation environment more directly. Penetrating charged particles can cause upsets in sensitive electronics components. Surface charge buildup and discharge can cause a wide variety of failures. Intense radiation can reduce the effectiveness of solar power arrays. And thermal expansion and contraction can cause mechanical failures.

Deep Space Contributions

Other sources that contribute to the near-Earth space environment include galactic cosmic ray particles, which originate from outside of the solar system. The higher the altitude, the less atmosphere there is to act as a shield, leading to greater exposure to these cosmic ray particles. A Geiger counter would detect a much higher number of such particles during an airplane flight than it would on the surface of Earth (away from radioactive sources, of course). For astronauts, the radiation hazards from all sources are serious. The National Aeronautics and Space Administration (NASA) monitors many different sources of information on the radiation environment to keep astronauts safe. Significant (though not complete) shielding can be afforded to spacewalking astronauts by simply having them go back inside their shuttle or space station.

Interactions with Comets and Asteroids

Comets and asteroids also contribute to the near-Earth space environment. Comets pass through the solar system, sometimes repeatedly because of their orbits. Both forms of solar radiation act on these dirty snowballs in space. As comets come near the Sun, the absorbed heat and solar wind pressure cause particles to come loose from the comet.^* Solid particles of ice and rock (meteoroids ) blown off by the Sun stay suspended in the comet's orbital path. When that path is close to our own orbit around the Sun, Earth will collide with these particle streams, giving rise to our annual meteor showers. While most of these particles are very small and carry very little mass, they pose yet another hazard to our satellites. Though immediate failure from meteoroid impacts seldom occurs, the continuous bombardment of these grains of sand have a degrading effect on satellite surfaces. For example, they chip and crack the cover glass on solar panels, making them less efficient. Pitting allows atomic oxygen, present in low Earth orbits, to react with an exposed surface, causing corrosion and reducing the serviceable lifetimes of satellites.

Meteoroids can also come from outside the solar system (sporadics) or from other Sun-orbiting bodies, such as asteroids. The main asteroid belt lies between Mars and Jupiter, and may be the remnants of what would have been another planet that never formed in the solar system. While most of these stay in safe orbits away from Earth, some have made their way into Earth-crossing orbits, perhaps through collisions and gravitational perturbations . Such objects have been known to collide with Earth over time and are expected to do so in the future. An asteroid as small as 0.5 to 1 kilometer (0.3 to 0.6 mile) in diameter impacting Earth can cause significant immediate and long-lasting damage. It is believed that a somewhat larger event may be responsible for the extinction of the dinosaurs and the destruction of perhaps one-fourth of all life on Earth about 65 million years ago. Another major impact event occurred in Siberia in 1908. What may have been a small asteroid exploded over the Tunguska forestlands, laying flat hundreds of square kilometers of trees. Evidence of these large impact events exists in the form of the craters they have left on Earth.

Recognizing that such events are very rare but may occur and cause great catastrophe, in the early 1990s the U.S. Congress formalized a scientific effort called Planetary Defense to look for near Earth objects (NEOs) that might collide with Earth. Since that time, many NEOs have been discovered. Major impacts are certain to occur in the future, but it is hard to say when.

Space Debris: A Growing Concern

Finally, humankind itself contributes to our near-Earth space environment through launch and space activities over the years. Space debris from such activities is a growing concern. Windows on the space shuttle are replaced regularly because of chipping caused by collisions with small pieces of space junk or by natural meteoroid strikes. Collision risk during launch, orbit, and reentry operations continues to rise. Larger objects in low Earth orbits (rocket boosters, space stations, etc.) eventually fall back through Earth's atmosphere, posing a small, yet real risk to human life. Debris objects actually reenter the atmosphere quite frequently. Observers often mistake these reentering objects for meteors or UFOs. A woman named Lottie Williams may have the distinction of being the first person to be hit by reentering space debris. While her claim of being hit on the shoulder by a small piece of a Delta II rocket may be difficult to verify, the piece of debris that she claims hit her has been verified to be from just such an object.

see also Asteroids (volume 2); Close Encounters (volume 2); Comets (volume 2); Cosmic Rays (volume 2); Solar Particle Radiation (volume 2); Space Debris (volume 2); Sun (volume 2); Weather, Space (volume 2).

David Desrocher

Bibliography

Gombosi, Tamas I. Physics of the Space Environment. Cambridge, UK: Cambridge University Press, 1998.

Johnson, Francis. Satellite Environment Handbook. Sanford, CA: Stanford University Press, 1999.

Steele, Duncan. Rogue Asteroids and Doomsday Comets: The Search for the Million Megaton Menace that Threatens Life on Earth. New York: John Wiley & Sons, 1997.

Tascione, Thomas F. Introduction to the Space Environment. Malabar, FL: Krieger Publishing Company, 1988.

Internet Resources

Center for Orbital and Reentry Debris Studies. Aerospace Corporation. <http://www.aero.org/cords/>.

Hamilton, Calvin J. "Terrestrial Impact Craters." <http://www.solarviews.com/eng/tercrate.htm>.

Living with a Star Program. National Aeronautics and Space Administration. <http://lws.gsfc.nasa.gov/lws.htm>.

Planetary Defense Workshop. Lawrence Livermore National Laboratory.<http://www.llnl.gov/planetary/>.

Primer on the Space Environment. National Oceanic and Atmospheric Administration,

Space Environment Center. <http://sec.noaa.gov/primer/primer.html>. SpaceWeather.com Daily Update Page. National Aeronautics and Space Administration.<http://www.spaceweather.com/>.

What is the Magnetosphere? Space Plasma Physics Branch, NASA Marshall Space Flight Center. <http://science.nasa.gov/ssl/pad/sppb/edu/magnetosphere/>.

^*Solar wind is responsible for the direction of a comet's tail, which always points away from the Sun.