Solar System

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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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