The 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.
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."
The 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.
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.
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
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.
If 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
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
Mercury has been known since ancient times. Its elusiveness
generated the name Hermes, given by the Greeks, later translated to Mercurius
by the Romans.
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 beauty
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.
While 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).
The 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.
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.
The 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.
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.
How the Moon's phases change
Because it takes 27.3 days both to rotate on its axis and to
orbit Earth, the Moon always shows us the same face. We see the Moon because of
reflected sunlight. How much of it we see depends on its position in relation
to Earth and the Sun.
The 27.3-day number is what scientists call a sidereal month,
and it is how long it takes the Moon to orbit the Earth in relation to a fixed
star. Another measurement, called a synodic month, is measured between in
relation to the Sun and equals 29.5 days. Full moons and new moon are measured
by the synodic month.
Earth's gravity keeps the Moon in orbit, while the Moon's
gravity creates tides on our oceans
On the moon
Like the four inner planets, the Moon is rocky. It's
pockmarked with craters formed by asteroid impacts millions of years ago. Because
there is no weather, the craters have not eroded.
The Moon has almost no atmosphere, so a layer of dust -- or
a footprint -- can sit undisturbed for centuries. And without an atmosphere,
heat is not held near the planet, so temperatures vary wildly. Daytime
temperatures on the sunny side of the Moon reach 273 degrees F; on the dark
side it gets as cold as -243.
In June of 1999, reserchers discovered by accident that a
huge cloud of sodium gas trails behind the Moon.
The Lunar Prospector in 1998 provided evidence of ice near
the Moon's poles, perhaps as much as 6 billion tons of it.
The Moon travels around the Earth at a little more than half
a mile per second; its speed is slowing and the satellite is gradually moving
away from Earth.
The fourth planet from the sun has always captivated our
imagination, and while scientists haven't proven there's any life, not even the
microscopic variety, the dusty red planet still commands our attention (and a
lot of space missions).
On the planet
The surface of
Mars is more interesting than most planets. Like Mercury, Venus and Earth, Mars
is mostly rock and metal. Mountains and craters scar the rugged terrain. The
dust, an iron oxide, gives the planet its reddish cast. A thin atmosphere and
an elliptical orbit combine to create temperature fluctuations ranging from
minus 207 degrees Fahrenheit to a comfortable 80 degrees Fahrenheit on summer
days (if you are at the equator). Researchers have recently monitored huge
storms swirling on Mars. The storms are very similar to hurricanes on Earth.
Mars has two moons, Phobos and Deimos.
Is there water?
Mars was most likely warm and wet about 3.7 billion years
ago. But as the planet cooled, the water froze. Remnants exist as ice caps at
the poles (as shown here). A recent image of Mars taken by the Hubble Space
Telescope shows evidence of water-bearing minerals in large amounts, and
scientists say the deposits may provide clues to the planet's water-rich
Is there life on Mars?
It has not yet been proven that there is life on Mars. A
NASA announcement in 1996 about microscopic life found in a meteorite has
failed to convince skeptics, and the search continues.
The apparent odd motion of Mars as seen from Earth stumped
scientists for centuries, finally leading in the early 1600's to the notion
that planets orbited the sun in an elliptical pattern. Percival Lowell, an
amateur astronomer who studied Mars into the early 1900s, thought he saw canals
that must have been dug by inhabitants. Upon closer examination with modern
telescopes and planetary probes, they turned out to be optical illusions.
In 1938, Orson Welles broadcast an Americanized version of a
40-year-old British novel by H.G. Wells -- The War of the Worlds. The radio
drama was perceived by many as a real newscast about a Martian invasion near
Princeton, New Jersey.
The fifth planet
from the sun is a huge ball of gas so massive it could hold all the other
planets put together. What we can see of the planet are bands of the highest
clouds in a thick atmosphere of hydrogen and helium. Traces of other gases
produce the bright bands of color.
The Red Spot
Jupiter's most familiar feature is swirling mass of clouds
that are higher and cooler than surrounding ones. Called the Great Red Spot, it
has been likened to a great hurricane and is caused by tremendous winds that
develop above the rapidly spinning planet. Winds blow counterclockwise around
this disturbance at about 250 miles per hour. Hurricanes on Earth rarely
generate winds over 180 miles an hour.
The Red Spot is twice the size of Earth and has been raging
for at least 300 years. It is one of several storms on Jupiter.
At Jupiter's center is a core of rock many times the mass of
Earth. But the bulk of the planet is a thick gaseous murk that appears smeared
through a telescope because the planet moves so rapidly beneath. Jupiter's
rapid rotation causes it to bulge, making the diameter 7 percent greater at the
equator than at the poles.
Jupiter has thin, barely perceptible rings and at least 16
satellites. The four largest-- Io, Europa, Ganymede and Callisto -- are called
the Galilean moons. They orbit in the same plane and are all visible in a
Jupiter: ruler of the roman gods, also jove
Jupiter was believed by Mesopotamians to be a wandering star
placed in the heavens by a god to watch over the night sky. In 1610, Galileo
Galilei used a 20x telescope to observe three "stars"
Jupiter. Over several nights he observed these "stars," but each
night they were in different positions, leading to his conclusion that they
were bodies orbiting the giant planet.
In 1994, astronomers around the world watched as the
fragments of comet Shoemaker-Levy 9 struck Jupiter -- an event that had been
forecast. This image shows a bright cloud more than 8,600 miles in diameter caused by the impact.
You could stuff 1,300 Earths into Jupiter
Much like its
neighbor Jupiter, the sixth planet from the sun has a rocky core and a gaseous
surface. But Saturn is chiefly known for its intricate series of rings that
encircle it. The mile-thick rings are made of countless orbiting ice particles,
from less than an inch to several feet in size.
Up close, it's clear that Saturn has more rings than we can
count. But though you can't see all of them from Earth, you can spot three of
them with a good telescope,.
The two outermost rings are separated by a dark band called
the Cassini Division, named for the astronomer who discovered it in 1675. The
Cassini division isn't empty, but it has less material in it. The middle ring
is the brightest, and just inside it is a fuzzy one that can be difficult to
Saturn has 18 known satellites, made mostly of ice and rock.
The largest, Titan, orbits Saturn every 16 days and is visible through a
good-sized amateur telescope. Titan, which is larger than the planet Mercury,
has a thick atmosphere that obscures its surface. Though researchers aren't
sure how many moons Saturn has, the total is likely at least 20, and may be
When Galileo Galilei first studied Saturn in the early
1600s, he thought it was an object with three parts. Not knowing he was seeing
a planet with rings, the stumped astronomer entered a small drawing -- a symbol
with one large circle and two smaller ones -- in his notebook, as a noun in a
sentence describing his discovery. Debate raged for more than 40 years about
these "ears," until Christiaan Huygens proposed that they were rings.
Giovanni Domenico Cassini later discovered a gap between the rings, which
gained his name, and he also proposed that the rings were not solid objects,
but rather made of small particles.
planet from the sun is much like its gaseous neighbors, with a cloudy surface,
rapid winds, and a small rocky core.
Uranus: personification of heaven in ancient myth
Perhaps because of a collision with a large object long ago,
Uranus orbits at an extreme tilt of 98 degrees -- sort of on its side. This
causes one pole to point toward the sun for decades, giving the planet strange
Uranus has numerous satellites and a faint set of rings. If
all the possible satellites being studied are confirmed, Uranus would have 16
regular and five irregular moons, making it the most populated planetary satellite
system known. Saturn is known to have 18 satellites (there may be more, but
they have not been well-documented).
Uranus was thought to be a star until William Herschel
discovered in 1781 that it orbited the Sun.
planet from the Sun -- well, some of the time it's eighth, but more on that
later -- has a rocky core surrounded by ice, hydrogen, helium and methane.Like
the other gas planets, Neptune has rapidly swirling winds, but it is thought to
contain a deep ocean of water. Its quick rotation fuels fierce winds and myriad
storm systems. The planet has a faint set of rings and 8 known moons.
Because of Pluto's strange orbit, Neptune is sometimes the
most distant planet from the Sun. Since 1979, Neptune was the ninth planet from
the Sun. On February 11, 1999, it crossed Pluto's path and once again become
the eighth planet from the Sun, where will remain for 228 years.
Neptune: roman god of water
Neptune was discovered in 1846 after mathematical
calculations of Uranus' movements predicted the existence of another large
Pluto, which is
only about two-thirds the size of our moon, is a cold, dark and frozen place. Relatively
little is known about this tiny planet with the strange orbit. Its composition
is presumed to be rock and ice, with a thin atmosphere of nitrogen, carbon
monoxide and methane. The Hubble Space Telescope has produced only fuzzy images
(above) of the distant object.
Pluto's 248-year orbit is off-center in relation to the sun,
which causes the planet to cross the orbital path of Neptune. From 1979 until
early 1999, Pluto had been the eighth planet from the sun. Then, on February
11, 1999, it crossed Neptune's path and once again became the solar system's
most distant planet. It will remain the ninth planet for 228 years.
Pluto's orbit is inclined, or tilted, 17.1 degrees from the ecliptic
-- the plane that Earth orbits in. Except for Mercury's inclination of 7
degrees, all the other planets orbit more closely to the ecliptic.
Interestingly, a similar thing happens with Jupiter's moons:
Many orbit on the ecliptic, but some are inclined from that plane.
Did you wonder: Will Pluto and Neptune ever collide? They
won't, because their orbits are so different. Pluto intersects the solar
system's ecliptic, or orbital plane, twice as its orbit brings it
"above," then "below" that plane where most of the other
planets' revolve -- including Neptune. And, though they are neighbors Pluto and
Neptune are always more than a billion miles apart.
Is it a planet at all?
Some astronomers think Pluto may have wandered into the
system of planets from a more distant region known as the Kuiper belt -- a
region beyond the orbit of Pluto thought to contain Pluto-like objects and comets
that orbit the sun in a plane similar to the planets of the solar system.
If that's the case, Pluto is not a planet at all, but is
probably more like a large asteroid or comet. Some have also suggested that it
may have once been a moon of Neptune and escaped.
The International Astronomical Union, the organization
responsible for classifying planets, gives these reasons for questioning
Pluto's status as a planet:
All the other planets in the outer solar system are gaseous,
giant planets whereas Pluto is a small solid object
Pluto is smaller than any other planet by more than a factor
Pluto's orbit is by far the most inclined with respect to
the plane of the solar system, and also the most eccentric, with only the
eccentricity of Mercury's orbit even coming close
Pluto's orbit is the only planetary orbit which crosses that
of another planet (during 1999 Pluto will again cross Neptune's orbit, thus
regaining its status as the most distant planet)
Pluto's satellite, Charon, is larger in proportion to its
planet than any other satellite in the solar system.
Pluto has one moon, Charon, which was discovered in 1978. The
satellite may be a chunk that broke off Pluto in a collision with another large
Pluto: hades in ancient myth, roman god of the underworld
Pluto was not discovered until 1930, by amateur American
astronomer Clyde Tombaugh. Since Tombaugh's death in 1997, many astronomers
have increasingly urged the International Astronomical Union, which names
celestial objects, to strip Pluto of its status as a planet.
After a news report generated a flurry of irate e-mails
about the possible change, officials assured the world that Pluto would remain
a planet. But it will also likely become the first in a new class of celestial
object known as a TNO, or Trans-Neptunian Object. It seems Pluto may then have
a sort of dual citizenship.
Made of dust, ice, carbon dioxide, ammonia and
methane, comets resemble dirty snowballs. You may remember them as blurry
smudges in the sky. Comets orbit the Sun, but most are believed to inhabit in
an area known as the Oort Cloud, far beyond the orbit of Pluto. Occasionally a
comet streaks through the inner solar system; some do so regularly, some only
once every few centuries.
Heads and tails
As a comet nears the Sun, its icy core boils off, forming a
cloud of dust and gas called a head, or coma. Comets become visible when
sunlight reflects off this cloud. As the comet gets closer to the sun, more gas
The gas and dust is pushed away by charged particles known
as the solar wind, forming two tails. Dust particles form a yellowish tail, and
ionized gas makes a bluish ion tail. A comet's tails, like these on comet
Halley, always points away from the Sun.
When Earth crosses the path of a comet, even if the comet
hasn't been around for a few years, leftover dust and ice can create increased
numbers of meteors.
Quick quiz: How
many planets orbit our Sun? If you said nine, you're shy by several thousand. Scientists
consider asteroids to be minor planets - some are hundreds of miles wide (and
Most, but not all, orbit the sun in an asteroid belt between
Mars and Jupiter. The huge gravitational pull of Jupiter accelerated these
asteroids to more than three miles per second -- too fast to prevent violent
collisions. Otherwise, they might have joined up to form "real"
planets. When asteroids collide, fragments sometimes are sent on a collision
course with Earth and become meteors.
Size and makeup
The vast majority of asteroids are small, compared with a
large one like Ida, this 32-mile-long chunk of stone and iron that was
photographed in 1993 by the Galileo spacecraft. Though we normally think of
asteroids as crater-makers, they are typically pockmarked with their own impact
Scientists divide asteroids into two groups, based on how
they appear in infrared images: light and dark. The lightest-looking asteroids
are rocky bodies with lots of iron and nickel, and they resemble lunar rocks. The
darkest asteroids have high quantities of hydrated minerals and carbon.
In the early days of the solar system (some 4.6 billion
years ago) asteroids had metallic cores, middle regions of stone and iron, and
surfaces of stone. Over time, many of them collided with others and broke
apart. The fragments, which became many of today's asteroids, are therefore
classified as irons, stony-irons or stony.
When an asteroid, or a part of it, crashes into Earth, it's
called a meteorite.
There are two hypotheses about how most of the asteroids
formed. One says they broke off of a mother planet that existed between Mars
and Jupiter. More likely, however, they represent what space was like before
the planets formed, and they are the remnants of that process -- bits and
pieces that never quite joined together.
The threat of impact
Since the Earth was formed more than four billion years ago,
asteroids and comets have routinely slammed into the planet. The most dangerous
asteroids are extremely rare, according to NASA.
An asteroid capable of global disaster would have to be more
than a quarter-mile wide. Researchers have estimated that such an impact would
raise enough dust into the atmosphere to effectively create a "nuclear
winter," severely disrupting agriculture around the world. Asteroids that
large strike Earth only once every 1,000 centuries on average, NASA officials
Smaller asteroids that are believed to strike Earth every
1,000 to 10,000 years could destroy a city or cause devastating tsunamis.
More than 160
asteroids have been classified as "potentially hazardous" by the
scientists who track them. Some of these, whose orbits come close enough to
Earth, could potentially be perturbed in the distant future and sent on a
collision course with our planet.
Scientists point out that if an asteroid is found to be on a
collision course with Earth 30 or 40 years down the road, there is time to
react. Though the technology would have to be developed, possibilities include
exploding the object or diverting it.
For every known asteroid, however, there are many that have
not been spotted, and shorter reaction times could prove more threatening. NASA
puts the odds at 1 in 10,000 of discovering an asteroid that is within 10 years
Two programs have been set up to actively search for
Near-Earth Objects (NEO's): NASA's Near Earth Asteroid Tracking (NEAT) program,
and Spacewatch at the University of Arizona.
Also, the Spaceguard Foundation was established in 1996 in Rome. The international organization's goal is to protect Earth from the impacts by promoting
and coordinating discovery programs and studies of NEOs. A January report shows
that NEOs 1 kilometer or larger are being discovered at the rate of about five
a month. The combined goal of these agencies is to find 90 percent of all NEOs 1 kilometer or larger within the next decade.
- “Astronomy” , B. A. Vorontsov-Veliaminov, Moscow 1991.
- “English for success”, Margareta Dushciac, “Teora” 2000.
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