Solar System

                                Solar system
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."
Massive energy
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.
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
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 fact
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 try it.


The 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.
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

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.
Historical 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 facts
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.
Final fact
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.

                                  The Moon

Though 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
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.
Final fact
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 background.
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.

Historical notes
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.
Inside 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.
Around Jupiter
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.
Historical notes
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

Final fact
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
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  much

Historical notes
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 rocky core.
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).
Historical notes
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.
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.
Historical notes
Neptune was discovered in 1846 after mathematical  calculations  of  Uranus'
movements predicted the existence of another large body.


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 orbit
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 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 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 object.
Historical notes
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  is
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.

Meteor showers
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 seldom round).

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  craters.

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
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
of impact.
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


1. Astronomy , B. A. Vorontsov-Veliaminov, Moscow 1991.
2. English for success, Margareta Dushciac, Teora 2000.

                                Comet Halley

"Solar System"