Solar System
Solar systemThe sun and the planets, the moon and the satellites of the other planets, the comets, asterois, and meteoroids make up the solar system. The solar system is located in the Milky Way Galaxy. Almost the whole galaxy is made of stars. Astronomers believe there are at least 100 billion stars. If you counted one star a second it would take you more than thirty thousand years to count 100 billion. And each star has planets, like the sun.Sun.The big burning ball of gas that holds nine major planets in orbit is not unlike many stars in the universe. The Sun makes up 99.86 percent of the solar system's mass and provides the energy that both sustains and endangers us. Scientists have lately begun calling its tremendous outpouring of energy "space weather." Massive energyThe Sun can be divided into three main layers: a core, a radiative zone, and a convective zone. The Sun's energy comes from thermonuclear reactions (converting hydrogen to helium) in the core, where the temperature is 15 to 25 million degrees. The energy radiates through the middle layer, then bubbles and boils to the surface in a process called convection. Charged particles, called the solar wind, stream out at a million miles an hour. Sunspots Magnetic fields within the sun slow down the radiation of heat in some areas, causing sunspots, which are cool areas and appear as dark patches. Sunspot activity peaks every 11 years. The next peak is due in 2000.During this so-called solar maximum, the sun will bombard Earth's atmosphere with extra doses of solar radiation. The last peak, in 1989, caused power blackouts, knocked satellites out of orbit and disrupted radio communications. (See our special report on Sunspots.)Though NASA scientists aren't predicting any record-setting space weather in 2000, the peak is expected to be above average. "It's like saying we're going to have a mild or cold winter," says Dr. David Hathaway at NASA's Marshall Space Flight Center. But as communications rely increasingly on satellites, there are more targets in the sky and more significant consequences to any disruptions.And there may be more to sunspots than disrupted communications. An active sun, known to heat the Earth's outer atmosphere, may also affect our climate. Scientists say a small ice age from 1645 to 1715 corresponded to a time of reduced solar activity, and current rises in temperatures might be related to increased solar activity.Solar flares The Sun frequently spews plumes of energy, essentially bursts of solar wind. These solar flares contain Gamma rays and X-rays, plus energized particles (protons and electrons). Energy is equal to a billion megatons of TNT is released in a matter of minutes. Flare activity picks up as sunspots increase.Effect on Earth The Sun's charged, high-speed particles push and shape Earth's magnetic field into a teardrop shape. The magnetic field protects Earth from most of the harmful solar radiation, but extreme flares can disable satellites and disrupt communication signals. The charged particles also excite oxygen and nitrogen in the atmosphere to create the aurora borealis, or northern lights. More solar radiation during the upcoming solar maximum means an increase in the aurora.Coronal mass ejections Similar to a solar flare, a coronal mass ejection is a bubble of gas and charged particles ejected over several hours. It can occur with or without solar flares, and can also threaten Earth's atmosphere.Final factIf you stood on the Sun, its gravity would make you feel 38 times more heavy than you do on Earth. But it's kind of hot, so please don't try it.MercuryThe innermost planet is rarely seen because of the Sun's glare. With less than half Earth's gravity, Mercury retains only a wisp of an atmosphere (presumed to be helium). The lack of a significant atmosphere allows temperatures to fluctuate from 750 degrees Fahrenheit during the day to minus 320 Fahrenheit at night.Like the other terrestrial planets -- Venus, Earth and Mars -- Mercury is made mostly of rock and metal. This small world is scarred by craters and looks somewhat like our Moon. MERCURIUS: ROMAN WINGED MESSENGER OF THE GODS Historical notes Mercury has been known since ancient times. Its elusiveness generated the name Hermes, given by the Greeks, later translated to Mercurius by the Romans.Venus.The second planet from the sun bakes under twice as much solar radiation as Earth and reaches temperatures of 895 degrees Fahrenheit (480 degrees Celsius). Pressure from the dense atmosphere of sulfuric acid gas is about 95 times greater than Earth's and would crush a human.The thick cloud cover around Venus rotates much faster than the planet itself -- once every four days. After the moon, Venus is the brightest object in the sky.The surface of Venus is mostly a rocky desert (this computer-generated view shows lava flows around Sif Mons). Like Mercury, Earth and Mars, Venus is composed of mostly rock and metal.VENUS: ROMAN GODDESS OF LOVE AND BEAUTYHistorical notes The Greeks believed Venus was two separate objects -- one in the morning sky and another in the evening. Because it is often brighter than any other object in the sky -- except for the sun and moon -- Venus has generated many UFO reports.Final factsWhile all of the planets orbit in an ellipse, Venus' orbit is the closest to a perfect circle. It is the only planet in the solar system whose day (241 Earth days) is longer than its year (225 Earth days).EarthThe third planet from the sun is, in scientific terms, quite similar to the first two. In fact, the four planets of the inner solar system (Mercury, Venus, Earth and Mars) all share rock and metal as their primary ingredients. Each of these so-called terrestrial planets has a solid surface, unlike the gaseous planets of the outer solar system. Perhaps Earth's most distinguishing factor, at least from our point of view, is the presence of water, which contributed to the formation of life some 3,000 million years ago. Most of us ought also to be fond of Earth's unique atmosphere, rich in life-sustaining nitrogen and oxygen.Final factThe Earth's surface is rotates about its axis at 1,532 feet per second -- slightly over 1,000 miles per hour -- at the equator, and the planet zips around the sun at more than 18 miles per second.The MoonThough a satellite of Earth, the Moon is bigger than Pluto. Some scientists think of it as a planet (four other moons in our solar system are even bigger). There are various theories about how the Moon was created, but recent evidence indicates it formed when a huge collision tore a chunk of the Earth away.
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