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
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
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.
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.
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.)
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
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.
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
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
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.
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.
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
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.
ROMAN WINGED MESSENGER OF THE GODS
has been known since ancient times. Its elusiveness generated the name Hermes,
given by the Greeks, later translated to Mercurius by the Romans.
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 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
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.
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.
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.
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.
How the Moon's phases change
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.
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.
gravity keeps the Moon in orbit, while the Moon's gravity creates tides on our
On the moon
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.
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.
June of 1999, reserchers discovered by accident that a huge cloud of sodium gas
trails behind the Moon.
Lunar Prospector in 1998 provided evidence of ice near the Moon's poles,
perhaps as much as 6 billion tons of it.
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 background.
Is there life on
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,
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
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
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 telescope.
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" around 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 spot.
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 much higher.
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.
The seventh planet from the sun is much
like its gaseous neighbors, with a cloudy surface, rapid winds, and a small
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 seasons.
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.
The eighth 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.
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 body.
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.
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.
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
a similar thing happens with Jupiter's moons: Many orbit on the ecliptic, but
some are inclined from that plane.
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?
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.
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.
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
·Pluto is smaller than any
other planet by more than a factor of 2.
·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
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 object.
HADES IN ANCIENT MYTH, ROMAN GOD OF THE UNDERWORLD
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.
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
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.
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 is produced.
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.
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.
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 seldom round).
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
Size and makeup
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 craters.
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.
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.
an asteroid, or a part of it, crashes into Earth, it's called a meteorite.
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
The threat of impact
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.
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 say.
asteroids that are believed to strike Earth every 1,000 to 10,000 years could
destroy a city or cause devastating tsunamis.
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.
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
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 of impact.
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.
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.