Planets
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Planets

BLASTING OFF TO OTHER WORLDS

For over fifty years, a number of nations have been involved in the exploration of outer space. This research is very costly. Has this money been well-spent or wasted?

Some people believe that most space research should be eliminated because of its expense. These people point out the fact that it costs billions of dollars to send astronauts to the moon, but all they bring are some worthless rocks. These people say that the money wasted in outer space could be spent on more important projects on earth, such as providing housing for homeless people, improving the education system, saving the environment, finding cures for diseases. However, other people believe that space research has provided many benefits to mankind. They point out that hundreds of useful products, from personal computers to foods, are the direct or indirect results of space research. They say that weather are communication satellites have benefited people. Supporters of the space program point to the scientific knowledge, acquired about the sun, the moon, the planets.

I agree with those people who support space research and want it to continue. Space research will bring even more benefits in the future. Moreover, just as individual people need challenges to make their lives more interesting, I believe the human race itself needs a challenge. I think that the peaceful exploration of outer space provides just such a challenge.

Mars or ,,The Red Planet“ is looked upon as the next frontier to space researches. Mars has several enormous canyons. Mars has one mountain which is twice as tall as Everest. It could cost as much as 700 billion dollars to put an astronaut on Mars. That makes it the most expensive journey in history. Even so, it’s a journey which scientists are planning, and they hope that it will happen in the next 25 years in five stages.

There are long-term prospects for space travel, too. After we have explored our own solar system we predict to travel to other galaxies in huge ,,star ships“. These will wander through space for thousands of years. Each will contain a large human population. When the ship discovers a suitable new planet, some of these people will colonise it. This way, the human race will gradually colonise the whole universe.

In fact, one day, life on Earth could be just distant memory. The TV series ,,Star Trek“ could hold the answer.



PLANETS

The planets divide neatly into two broad categories: terrestrial and jovian. The terrestrial are basically small, rocky worlds and include Mercury, Venus, Earth, Mars; the jovian planets are gas giants and consist of Jupiter, Saturn, Uranus, Pluto, the outermost and smallest planet (although some scientists argue that it shouldn’t be considered in this privileged class), is an oddball that doesn’t fit easily into either category.

The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto. Jupiter is so big that all the other planets could fit inside it.

In general, planetary scientists have scrutinized the terrestrial planets in far greater detail than the jovian planets. Earth, of course, has been an object of scientific inquiry ever since people first started to ponder their place in the universe. Sophisticated spacecraft have examined both Venus and Mars from orbit and from the surface. (NASA’s most recent success placed two rovers on the martian surface during 2004.)

Mercury remains the most enigmatic of the terrestrial worlds. It lies so close to the Sun that observations from Earth reveal preciuos little. In the mid-1970s, NASA sent the Mariner 10 spacecraft on three separate flybys of the innermost planet. The spacecraft revealed a startling fact: Mercury has such I high density that more than half of it must be made out of iron and nickel. The planet’s surface shows lots of craters, most dating from the age of heavy bombardment that characterized the solar system about 4 billion years ago. During this period, errant comets and debris left over the solar system’s formation pummeled most planets and moons.

Of all the planets, Venus most resembles Earth. The two have nearly the same size and density, yet the moniker ,,Earth’s twin“ fails miserably for Venus. Earth has a relatively benign climate conducive to the presence of liquid water (and thus life). The surface of Venus, however, bakes at a temperature of 8000 Fahrenheit. Its massive atmosphere of carbon dioxide traps solar radiation and creates a runaway greenhouse effect. Atmospheric pressure at the planet’s surface is nearly 100 times greater than that on Earth’s. Craters on the surface of Venus show that volcanic activity resurfaced the entire planet about 600 million years ago. This contrasts with Earth, where steady volcanism and erosion gradually covered up signs of ancient impacts.

Mars has long fascinated humans, in no small part because its surface is the only one that can be seen clearly from Earth. Changes in its appearance led some imaginative scientists to believe that a dying civilization was tapping into dwindling water supplies. Those hopes were dashed when the first spacecraft images revealed a crater and apparently barren surface. Yet subsequent missions revealed a more nuanced world, where craters share space with massive (albeit extinct) volcanoes, giant canyons, and dry channels. The most recent rovers have left little doubt that liquid water once existed on the
martian surface. So the question remains: Could life

ever have started on so Red Planet? A famous meteorite from Mars known as ALH84001 contains tantalizing evidence of possible microfossils. And, if water once flowed on the surface, life might have followed.

The jovian planets seem to have less diversity than terrestrial counterparts because all we see are the tops of their cloud layers. Jupiter, Saturn, Uranus and Neptune all have thick atmospheres consisting largely of hydrogen and helium. Various minor constituents create the subtle colors that cause them to look different through a telescope. The term ,,gas giant“ fits these planets perfectly – even the smallest, Uranus, weights in at 15 times the mass of Earth. All the jovian planets have ring systems as well, although only Saturn’s shines bright enough to be seen easily from Earth.

Prior to Charon’s discovery, astronomers believed that Pluto was much larger. Because Pluto is so distant, the images of Charon and Pluto were blurred together making the planet appear much larger. Pluto stands apart from the other planets because it is much smaller and less massive than others and has the most elongated orbit. It also consists of a mixture of ice and rock, which puts it more in line with some of the moons of the outer planets. Most scientists now consider it to be the largest Kuiper Belt object, a group of objects now numbering more than 700 that orbit beyond Neptune. Even so, it also remains officially a planet.

Moons in the solar system run the gamut from small objects that likely were captured by their parent planets – think of the martian satellites Phobos and Deimos, as well as most of the dozens of small, irregular satellites orbiting the gas-giant planets – to big objects that rival the planets themselves in size. Jupiter tows its own miniature solar system with it as it orbits is the Sun. Its four large moons – Io, Europa, Ganymede, and Callisto (in order from Jupiter)- were discovered by Galileo when he first pointed this telescope at Jupiter in 1610. The largest, Ganymede, has a diameter of 3.270 miles, making it larger than Mercury. Tidal forces from Jupiter heat Io’s interior so intensely that this moon is the most volcanically active body in the solar system. The same tidal forces heat Europa’s interior, melting the moon’s subsurface ice and creating perhaps the largest ocean of liquid water in the solar system.

Saturn also hosts several moons, including the mysterious Titan. This moon, second in size to Ganymede, possesses a hazy, nitrogen-rich atmosphere thicker than Earth’s atmosphere that hides its surface from view. The equally enigmastic Iapetus features one hemisphere that appears ten times brighter than the opposite one. Both will be prime targets for NASA’s Cassini spacecraft, which went into orbit around Saturn in early July 2004.

Of all the moons in the solar system, none has been studied more thoroughly than Earth’s. Even from Earth, the Moon appears big enough to show detail through a telescope. Its highly crater highlands stand in stark contrast to the darker, lightly crater Maria, crater by giant impacts that took place during the era of heavy bombardment and subsequently filled with lava. The Moon ranks as the fifth largest satellite in the solar system and was born in what seems to be a unique process. Most of the large moons in the solar system were created in protoplanetary disks, dusty disks that surrounded the planets during their formation. The moons condensed out of these disks in much the same way as the planets condensed out of the solar nebula. But our Moon appears to have formed when an object the size of Mars gave a glancing blow to the protoEarth, ejecting debris into orbit that eventually coalesced into the Moon.

The Inner Planets vs. the Outer Planets

The inner planets (those planets that orbit close to the Sun) are quite different from the outer planets (those planets that orbit far from the Sun).

The inner planets are: Mercury, Venus, Earth, Mars. They are relatively small, composed mostly of rock, and have few or no moons.

The outer planets include: Jupiter, Saturn, Uranus, Neptune and Pluto. They are mostly huge, mostly gaseous, ringed and have many moons (again, the exception is Pluto, which is small, rocky and has only one moon).

Temperatures on the Planets

Generally, the farther from the Sun, the cooler planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere.

Density of the Planets

The outer, gaseous planets are much less dense than the inner, rocky planets.

The Earth is the densest planet. Saturn is the least dense planet; it would float on water.

The Mass of the Planets

Jupiter is by far the most massive planet; Saturn trails it. Uranus, Neptune, Earth, Venus, Mars, Pluto are orders of magnitude less massive.

Gravitational Forces on the Planets

The planet with the strongest gravitational attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet’s surface is proportional to its mass and to the inverse of the planet’s radius squared.

A Day on Earth of the Planets

A day is the length of times that it takes a planet to rotate on its axis (3600 ). A day on Earth takes almost 24 hours.

The planet with the longest
is Venus; a day on Venus takes 243 Earth days. (A day on Venus is longer than its year; a year on Venus takes only 224.7 Earth days).

The planet with the shortest day is Jupiter; a day on Jupiter only takes 9.8 Earth hours. When you observe Jupiter from Earth, you can see some of its features change.

The Average Orbital Speed of the Planets

As the planets orbit the Sun, they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun.

A Tenth Planet?

No tenth planet beyond Pluto has been directly observed. A few astronomers think that there might be a tenth planet (or companion star) orbiting the Sun far beyond the orbit of Pluto. This distant planet/companion star may or may not exist. The hypothesized origin of this hypothetical object is that a celestial object, perhaps a hard-to-detect cool, brown dwarf star (called Nemesis), was captured by the Sun’s gravitational field. This tenth planet is hypothesized to exist because of the unexplained clumping of some long-period comet’s orbits. The orbits of these far-reaching comets seem to be affected by the gravitational pull of a distant.

GENERAL INFORMATION ON MERCURY

Mercury is the closest planet to the Sun and the eighth largest. Mercury is slightly smaller in diameter than the moons Ganymede and Titan but more than twice as massive:

orbit: 57.910.000 km (0.38 AU) from the Sun

diameter: 4.880 km

mass: 3.30e23kg

An account of the non-discovery of a planet inside Mercury’s orbit. A much more interesting tale than you might imagine. In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky.

Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was given two names by the Greeks: Apollo for its apparition as a morning star and Hermes as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit is the Sun, not the Earth.

Mercury has been visited by only one spacecraft, Mariner 10. It flew by three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safety imaged by HST). A few discovery-class mission to Mercury, MESSENGER was launched in 2004 and will orbit Mercury starting in 2011 after several flybys.

Mercury’s orbit is highly eccentric; at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million. The perihelion of its orbit processes around the Sun at a very slow rate. 19th century astronomers made very careful observations of Mercury’s orbital parameters but could not adequately explain them using Newtonian mechanics. The tiny differences between the observed and predicted values were a minor but nagging problem for many decades. It was thought that another planet (sometimes called Vulcan) might exist in an orbit near Mercury’s to account for the discrepancy. But despite much effort, no such planet was found. The

real answer turned out to be much more dramatic: Einstein’s General Theory of Relativity. Its correct prediction of the motions of Mercury was an important factor in the early acceptance of the theory. Until 1962 it was thought that Mercury’s ,,day“ was the same length as its ,,year“ so as to keep that same face to the Sun much as the Moon does to the Earth. But this way shown to be false in 1965 by doppler radar observations. It is now known that Mercury rotates three times in two of its years. Mercury is the only body in the solar system known to have an orbital/rotational resonance with a ratio other than 1:1 (though many have no resonance at all).

This fact and the high eccentricity of Mercury’s orbit would produce very strange effects for an observer on Mercury’s surface. At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving three times faster across the sky. Observers at other points on Mercury’s surface would see different but equally bizarre motions.

Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to 700 K. The temperature on Venus is slightly hotter but very stable.

Mercury is in many ways similar to the Moon: its surface is heavily cratered and very old; it has no plate tectonics. On the other hand, Mercury is much denser than the Moon (5.43 gm/cm3 vs 3.34). Mercury is the second densest major body in the solar system, after Earth. Actually Earth’s density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth. This indicates that Mercury’s dense iron core is relatively larger than Earth’s, probably comprising the majority of the planet. Mercury therefore has only a relatively thin silicate mantle and crust.

Mercury’s interior is dominated by a large iron core whose radius is 1800 to 1900 km. The silicate outer shell (analogous to Earth’s mantle and crust) is only 500 to 600 km thick. At least some of the core is probably molten.

Mercury actually has a very thin atmosphere consisting of atoms
blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury’s atmosphere is constantly being replenished.

The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high. Some cut through the rings of craters and other features in such a way as to indicate that they were formed by compression. It is estimated that the surface area of Mercury shrank by about 0.1% (or a decrease of about 1 km in the planet’s radius).

One of the largest features on Mercury’s surface is the Caloris Basin; it is about 1300 km in diameter. It is thought to be similar it the large basins (maria) on the Moon. Like the lunar basins, it was probably caused by a very large impact early in the history of the solar system. The impact was probably also responsible for the odd terrain on the exact opposite side of the planet.

In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Some may be the result of ancient volcanic activity but some may be the result of the deposition of ejecta from cratering impacts.

A reanalysis of the Mariner data provides some preliminary evidence of recent volcanism on Mercury. But more data will be needed for confirmation.

Amazingly, radar observations of Mercury’s north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters.

Mercury has a small magnetic field whose strength is about 1% of Earth’s.

Mercury has no known satellites.

Mercury is often visible with binoculars or even the unaided eye, but it is always very near the Sun and difficult to see in the twilight sky. There are several Web sites that show the current position of Mercury (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Mercury is so close to the Sun that you can see it near sunrise or sunset.

The gravity on Mercury is 38% of the gravity on Earth. A 100 pound person on Mercury would weight 38 pounds. To calculate your weight on Mercury, just multiply your weight by 0.38 (or go to the planetary weight calculator).

Mercury’s thin atmosphere consist of trace amounts of hydrogen and helium. The atmospheric pressure is only about 1×10-9 millibars; this is a tiny fraction (about 2 trillionths) of the atmospheric pressure on Earth.

Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. Also, there is no ,,greenhouse effect“ on Mercury. When the Sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.

Mercury is just over a third as far from the Sun as the Earth is; it is 0.387 A.U. from the Sun (on average). Mercury’s orbit is very eccentric; at aphelion (the point in the orbit farthest from the Sun) Mercury is 70 million km from the Sun, at perihelion Mercury is 46 million km from the Sun. There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet’s orbit. Since Mercury’s axis is directly perpendicular to its motion (not tilted), it has no seasons.

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