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.
Jupiter
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.
JUPITER: RULER OF THE ROMAN GODS, ALSO JOVE
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
impact.
Final fact
You could stuff 1,300 Earths into Jupiter
Saturn
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.
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.
Uranus
The seventh 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
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).
Historical notes
Uranus was thought to be a star until William Herschel discovered in 1781
that it orbited the Sun.
Neptune
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.
NEPTUNE: ROMAN GOD OF WATER
Historical notes
Neptune was discovered in 1846 after mathematical calculations of Uranus'
movements predicted the existence of another large body.
Pluto
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.
PLUTO: HADES IN ANCIENT MYTH, ROMAN GOD OF THE UNDERWORLD
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.
Comets
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
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.
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.
Asteroids
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).
Orbits
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
meteorite.
Origin
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
say.
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
decade.
Literature
1. “Astronomy” , B. A. Vorontsov-Veliaminov, Moscow 1991.
2. “English for success”, Margareta Dushciac, “Teora” 2000.
3. www.space.com
4. www.NASA.gov
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[pic]
Comet Halley
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