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Exploring The Heavens
For centuries humanity has studied the heavens, speculating about its
origins and our place in it. Until the 16th century, when Polish astronomer Nicolaus
Copernicus first suggested the earth revolved around the sun, it was widely believed that
the earth was the center of the universe, and the heavens were above and beyond us. Since
then, people have reached toward the stars, first, with telescopes and most recently with
radio astronomy and spacecraft. Humanity has taken its first steps into the heavens, but
the universe is far from yielding all of its mysteries, and our exploration of space
continues as we seek more answers.
Click Here To See NASA's Vision
Click This To Hear Neil Arm Strong Right Before He Walked On The Moon
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free version.
Halley's Comet reappears approximately
every 76 years; this photo, taken in New Zealand in 1986, shows the comet during its most
recent pass by earth. Comets are only visible when near the sun because the intense solar
radiation vaporizes parts of the icy nucleus, forming the comet’s coma and tail.
Barney
Magrath/Science Source/Photo Researchers, Inc.
I INTRODUCTION
Solar System, the sun and the celestial bodies
orbiting the sun, including the nine planets and their satellites; the asteroids, comets,
and meteoroids; and interplanetary dust and gas. The term may also refer to a group of
celestial bodies orbiting another star. In this article, solar system refers to the system
that includes the earth and the sun. The dimensions of this system are specified in terms
of the mean distance from the earth to the sun, called the astronomical unit (AU). One AU
is 150 million km (about 93 million mi). The most distant known planet, Pluto, has an
orbit at 39.44 AU from the sun. The boundary between the solar system and interstellar
space—called the heliopause—is estimated to occur near 100 AU. The comets,
however, achieve the greatest distance from the sun; they have highly eccentric orbits (see
Orbit) ranging out to 50,000 AU or more. This solar system was the only planetary system
known to exist until 1995, when astronomers discovered a planet about 0.6 times the mass
of Jupiter orbiting the star 51 Pegasi. Soon after, astronomers found a planet about 8.1
times the mass of Jupiter orbiting the star 70 Virginis, and a planet about 3.5 times the
mass of Jupiter orbiting the star 47 Ursae Majoris. Since then, astronomers have found
planets and disks of dust in the process of forming planets around many other stars. Many
astronomers think it likely that solar systems of some sort are numerous throughout the
universe. See Astronomy; Galaxy; Star.
II THE SUN AND THE SOLAR WIND
The sun is a typical star of intermediate size and
luminosity. Sunlight and other radiation are produced by the conversion of hydrogen into
helium in the sun’s hot, dense interior (see Nuclear Energy). Although this
nuclear fusion is destroying 600 million metric tons of hydrogen each second, the sun is
so massive (2 × 1030 kg, or 4.4 × 10 30 lb) that it can continue to shine at its
present brightness for 6 billion years. This stability has allowed life to develop and
survive on earth.
For all the sun’s steadiness, it is an extremely
active star. On its surface dark sunspots bounded by intense magnetic fields come and go
in 11-year cycles; sudden bursts of charged particles from solar flares can cause auroras
and disturb radio signals on earth; and a continuous stream of protons, electrons, and
ions leaves the sun and moves out through the solar system, spiraling with the sun’s
rotation. This solar wind shapes the ion tails of comets and leaves its traces in the
lunar soil, samples of which were brought back from the moon’s surface by piloted
U.S. Apollo spacecraft (see Space Exploration; Apollo Program).
III THE MAJOR PLANETS
Nine major planets are currently known. They are
commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and
the outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto). The inner planets are
small and are composed primarily of rock and iron. The outer planets (except Pluto) are
much larger and consist mainly of hydrogen, helium, and ice.
Mercury is surprisingly dense, apparently because it has
an unusually large iron core. With only a transient atmosphere, Mercury has a surface that
still bears the record of bombardment by asteroidal bodies early in its history. Venus has
a carbon dioxide atmosphere 90 times thicker than that of Earth, causing an efficient
greenhouse effect by which the Venusian atmosphere is heated. The resulting surface
temperature is the hottest of any planet—about 477° C (about 890° F). Earth is the
only planet with abundant liquid water and life. Strong evidence exists that Mars once had
water on its surface, but now its carbon dioxide atmosphere is so thin that the planet is
dry and cold, with polar caps of solid carbon dioxide, or dry ice. Jupiter is the largest
of the planets. Its hydrogen and helium atmosphere contains pastel-colored clouds, and its
immense magnetosphere, rings, and satellites make it a planetary system unto itself.
Saturn rivals Jupiter, with a much more intricate ring structure and more satellites,
including one with an atmosphere—Titan. Uranus and Neptune are deficient in hydrogen
compared with Jupiter and Saturn; Uranus, also ringed, has the distinction of rotating at
98° to the plane of its orbit. Pluto seems similar to the larger, icy satellites of
Jupiter or Saturn. Pluto is so distant from the sun and so cold that methane freezes on
its surface.
IV OTHER ORBITING BODIES
The asteroids are small rocky bodies that move in orbits
primarily between the orbits of Mars and Jupiter. Numbering in the thousands, asteroids
range in size from Ceres, which has a diameter of 1000 km (620 mi), to microscopic grains.
Some asteroids are perturbed, or pulled by forces other than their attraction to the sun,
into eccentric orbits that can bring them closer to the sun. If the orbits of such bodies
intersect that of the earth, they are called meteoroids. When they appear in the night sky
as streaks of light, they are known as meteors, and recovered fragments are termed
meteorites. Laboratory studies of meteorites have revealed much information about
primitive conditions in our solar system. The surfaces of Mercury, Mars, and several
satellites of the planets (including Earth’s moon) show the effects of an intense
bombardment by asteroidal objects early in the history of the solar system. On Earth that
record has eroded away, except for a few recently found impact craters.
Some meteors and interplanetary
dust may also come from comets, which are basically aggregates of dust and frozen gases
about 5 to 10 km (about 3 to 6 mi) in diameter. Comets orbit the sun at distances so great
that they can be perturbed by stars into orbits that bring them into the inner solar
system. As comets approach the sun, they release their dust and gases to form a
spectacular coma and tail. Under the influence of Jupiter’s strong gravitational
field, comets can sometimes adopt much smaller orbits. The most famous of these is
Halley’s Comet, which returns to the inner solar system at 75-year periods. Its most
recent return was in 1986. In July 1994 fragments of Comet Shoemaker-Levy 9 bombarded
Jupiter’s dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon
impact, the tremendous kinetic energy of the fragments was converted into heat through
massive explosions, some resulting in fireballs larger than Earth.
Comets circle the sun in two main groups. The Kuiper
belt is a ring of debris that orbits the sun beyond the planet Neptune. Many of the comets
with periods of less than 500 years are members of the Kuiper belt. The Oort cloud is a
theoretical, spherical cloud of comets extending to the edge of the solar system.
Astronomers believe that comets with very long periods reside in the Oort cloud. A chunk
of dust and ice may stay in the Oort cloud for thousands of years. Nearby stars sometimes
pass close enough to the solar system to push an object in the Oort cloud into an orbit
that takes it close to the sun.
Many of the objects that do not
fall into the asteroid belts, the Kuiper belt, or the Oort cloud may be comets that will
never make it back to the sun. The surfaces of the icy satellites of the outer planets are
scarred by impacts from such bodies. The asteroid-like object Chiron, with an orbit
between Saturn and Uranus, may itself be an extremely large inactive comet. Similarly,
some of the asteroids that cross the path of Earth’s orbit may be the rocky remains
of burned-out comets. Chiron and similar objects called the Centaurs probably escaped from
the Kuiper belt and were drawn into their irregular orbits by the gravitational pull of
the giant outer planets, Jupiter, Saturn, Neptune and Uranus.
The sun was also found to be encircled by three
rings of interplanetary dust. One of them, between Jupiter and Mars, has long been known
as the cause of zodiacal light, a faint glow that appears in the east before dawn and in
the west after dusk. The other two rings, one lying only two solar widths away from the
sun, the other occurring in the region of the asteroids, were discovered in 1983.
V MOVEMENTS OF THE PLANETS AND THEIR SATELLITES
If one could look down on the solar system from far
above the North Pole of the earth, the planets would appear to move around the sun in a
counterclockwise direction. All of the planets except Venus and Uranus rotate on their
axes in this same direction. The entire system is remarkably flat—only Mercury and
Pluto have obviously inclined orbits. Pluto’s orbit is so elliptical that it is
sometimes closer than Neptune to the sun.
The satellite systems mimic the behavior of their parent
planets and move in a counterclockwise direction, but many exceptions are found. Jupiter,
Saturn, and Neptune each have at least one satellite that moves around the planet in a
retrograde orbit (clockwise instead of counterclockwise), and several satellite orbits are
highly elliptical. Jupiter, moreover, has trapped two clusters of asteroids (the so-called
Trojan asteroids) leading and following the planet by 60° in its orbit around the sun.
(Some satellites of Saturn have done the same with smaller bodies.) The comets exhibit a
roughly spherical distribution of orbits around the sun.
Within this maze of motions, some remarkable patterns
exist: Mercury rotates on its axis three times for every two revolutions about the sun; no
asteroids exist with periods (intervals of time needed to complete one revolution) 1/2,
1/3, ..., 1/n (where n is an integer) the period of Jupiter; the three inner
Galilean satellites of Jupiter have periods in the ratio 4:2:1. These and other examples
demonstrate the subtle balance of forces that is established in a gravitational system
composed of many bodies.
VI THEORIES OF ORIGIN
Despite their differences, the
members of the solar system probably form a common family. They seem to have originated at
the same time; few indications exist of bodies joining the solar system, captured later
from other stars or interstellar space.
Early attempts to explain the origin of this system
include the nebular hypothesis of the German philosopher Immanuel Kant and the French
astronomer and mathematician Pierre Simon de Laplace, according to which a cloud of gas
broke into rings that condensed to form planets. Doubts about the stability of such rings
led some scientists to consider various catastrophic hypotheses, such as a close encounter
of the sun with another star. Such encounters are extremely rare, and the hot, tidally
disrupted gases would dissipate rather than condense to form planets.
Current theories connect the
formation of the solar system with the formation of the sun itself, about 4.7 billion
years ago. The fragmentation and gravitational collapse of an interstellar cloud of gas
and dust, triggered perhaps by nearby supernova explosions, may have led to the formation
of a primordial solar nebula (see Nova and Supernova). The sun would then form in
the densest, central region. It is so hot close to the sun that even silicates, which are
relatively dense, have difficulty forming there. This phenomenon may account for the
presence near the sun of a planet such as Mercury, having a relatively small silicate
crust and a larger than usual, dense iron core. (It is easier for iron dust and vapor to
coalesce near the central region of a solar nebula than it is for lighter silicates to do
so.) At larger distances from the center of the solar nebula, gases condense into solids
such as are found today from Jupiter outward. Evidence of a possible preformation
supernova explosion appears as traces of anomalous isotopes in tiny inclusions in some
meteorites. This association of planet formation with star formation suggests that
billions of other stars in our galaxy may also have planets. The high frequency of binary
and multiple stars, as well as the large satellite systems around Jupiter and Saturn,
attest to the tendency of collapsing gas clouds to fragment into multibody systems.
Pictures From Hubble
Space Telescope:
Pictures Of Solar
System:
 Mercury orbits closer to the sun than any other planet, making
it dry, hot, and virtually airless. Although the planet’s cratered surface resembles
that of the moon, it is believed that the interior is actually similar to the
earth’s, consisting primarily of iron and other heavy elements. This photograph was
taken in 1974 by Mariner 10, the first probe to study Mercury in detail.
 Venus is the brightest object in our sky, after the sun and
moon. Swirling clouds of sulfur and sulfuric acid obscure Venus’s surface and
inhibited study of the planet from Earth until technology permitted space vehicles,
outfitted with probes, to visit it. These probes determined that Venus is the hottest of
the planets, with a surface temperature of about 460° C (about 860° F). Scientists
believe that a greenhouse effect causes the extreme temperature, hypothesizing that the
planet’s thick clouds and dense atmosphere trap energy from the sun.
 An oxygen-rich and protective atmosphere, moderate
temperatures, abundant water, and a varied chemical composition allow earth to support
life, the only planet to do so. The slightly oblate planet is composed of rock and metal,
which are present in molten form beneath its surface. This photograph, taken by the Apollo
17 spacecraft in 1972, shows Arabia, the African continent, and Antarctica (most of the
white area near the bottem.
 Unpiloted spacecraft from the United States, launched between
1964 and 1976, have supplied the most detailed information on Mars. From this data,
scientists determined that the planet’s atmosphere predominantly consists of carbon
dioxide (CO2), with small amounts of nitrogen, oxygen, and water vapor also present. Due
to the thinness of the atmosphere, daily temperatures often vary as much as 100 Celcius
degrees (190 Fahrenheit degrees). In general, surface temperatures are too cold and
surface pressures too low for water to exist in a liquid state on Mars, so the planet
resembles a cold, high-altitude desert.
 Jupiter is the largest of the planets, with a volume 1400
times greater than that of the earth. Jupiter’s colorful bands are caused by strong
atmospheric currents and accentuated by a dense cloud cover. The massive planet, is shown
here with the four largest of its sixteen satellites: Europa, center, nearest Jupiter,
Io upper left, Callisto lower left, and Ganymede lower right.
Saturn, distinguished by its rings,
ranks as the second largest planet—Jupiter is the largest—in the solar system.
In 1610 the Italian physicist and astronomer Galileo discovered the ring system using one
of the first telescopes ever made. Although the planet formed more than 4 billion years
ago, it continues to settle and contract, generating three times as much heat as it
receives from the sun. The Hubble Space Telescope obtained this image of Saturn on August
26, 1990.
Uranus’s blue-green color comes
from the methane gas present in its cold, clear atmosphere. The dark shadings at the right
edge of the sphere correspond to the day-night boundary on the planet. Beyond this
boundary, Uranus’ northern hemisphere remains in perpetual darkness because of the
way the planet rotates. Scientists compiled this view of Uranus from images returned from Voyager
2 in 1986, when the probe was 9.1 million km (5.7 million mi) away from the planet.
 The 1989 Voyager 2 mission produced this false-color image of
Neptune showing the different components of Neptune’s atmosphere. The red layer shows
scattered sunlight from a haze around the planet, the blue/green indicates methane, and
the white areas are high-level clouds that reflect sunlight above the atmosphere.  Pluto is farther from the sun than the
other planets in the solar system, although it occasionally moves in closer than Neptune
due to an irregular orbit. The small, rocky, and cold planet takes 247.7 years to revolve
around the sun. This artist's rendition depicts Pluto, foreground; its moon,
Charon, background; and the distant sun, upper right.
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