![]() ![]() ![]() Our whole solar system, together with all the local stars you can see on a clear dark night, orbits the center of our home galaxy, a spiral disk of some 200 billion stars we call the Milky Way. (A light year is the distance light travels in a year, at about 300,000 km per second.) The sun's nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of about 4.3 light years. The sun is the richest source of electromagnetic energy in the solar system. It includes the satellites of the planets, numerous comets, asteroids, meteoroids, and the interplanetary medium, which permeates interplanetary space. The solar system consists of an average star we call the sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. In the 21st century, knowledge of the solar system is advancing at an unprecedented rate. At last, with interplanetary travel, instruments can be carried to many solar system objects, to measure their physical properties and dynamics directly and at very close range. Starting with the emergence of space flight in 1957, instruments operating above Earth's obscuring atmosphere could take advantage not only of light waves and radio waves, but virtually the whole spectrum. Radio waves, received here on Earth, have been used since 1931 to investigate celestial objects. Then in the 20th century people discovered how to use additional parts of the spectrum. For all of human history and pre-history, observations were based on visible light. For nearly all that time, people have had to rely on long-range and indirect measurements of its objects. The solar system has been a topic of study from the beginning of history. ![]()
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