Category Modern Science

ASTEROIDS

Asteroids are small, mostly rocky, irregular-shaped bodies. They are found orbiting the Sun in a band filling the 550-million-kilometre gap between Mars and Jupiter. The largest, Ceres, measures just under 1000 kilometres across, but only a handful have diameters greater than 100 kilometres. About 4000 have been recorded, but there are many thousands more too small to be identified.

Astronomers believe that, during the formation of the Solar System, Jupiter’s strong gravitational pull caused nearby planetesimals to smash into one another rather than build up into another planet. This left the belt of fragments we call the asteroids.

The asteroids have continued to collide with one another since their formation, producing smaller fragments called meteoroids. These have occasionally crashed on to Earth’s surface (when they are known as meteorites). It is feared that one day a large meteorite may devastate Earth, causing climatic change sufficient to wipe out many life-forms.

            Most asteroids are rocky, indicating they come from the outer layers of a former minor planet. But some are metallic – they come from the core of such a planet.

            A close-up view of the irregular shaped objects that make up the asteroid belt between Mars and Jupiter. From study of asteroid fragments that have fallen to Earth, scientists have dated the age of the Solar System to 4.6 million years ago.

COMETS

Comets are potato-shaped lumps of dust measuring only a few kilometres across, but accompanied by (when near the Sun) tails of has or dust that stretch for hundreds of millions of kilometres across space. The lump of dust is fused together by frozen gases and water ice. Like all other objects in the Solar System, comets orbit the Sun, although their orbits are often very elliptical (elongated ovals), looping in towards the Sun from distant reaches of the Solar System. When a comet approaches the Sun, part of its ices melt and the gas and dust escape, forming a surrounding cloud, or coma. As it rounds the Sun, the coma is swept back into two tails, a straight gas tail and a broader, curved dust tail, always pointing away from the Sun.

Sometimes, small pieces of debris break off from comets. Great showers of these fragments, called meteors, sometimes come quite close to Earth. Millions of tiny particles burn up in Earth’s atmosphere. Commonly known as shooting stars, they appear to us as split-second streaks of light in the night sky.

FAMOUS COMETS

The English astronomer Edmund Halley (1656-1742) was the first to realise that comets were orbiting objects. He once made a famous prediction: a comet that he observed in 1682 would return to the skies in 1758. Halley believed that comets recorded in 1531 and 1607 were simply earlier sightings of the one he saw in 1682. Halley did not live to see his prediction come true. Halley’s Comet, as it has been known ever since, was duly sighted on Christmas Day 1758 and has reappeared every 75 to 76 years. When Halley’s Comet appeared in March 1986, the space probe Giotto flew within 600 kilometres of it, sending back pictures and sampling the gases and dust particles given off by it.

A sighting of a comet is always a great event. The 1997 appearance of Comet Hale-Bopp was the most spectacular of recent years. Comets can also be destructive if they pass too close to a planet. In July 1994, drawn in by gravity, fragments of Comet Shoemaker-Levy smashed into Jupiter, creating massive fireballs on impact.

            On 30th June 1908 there was a huge explosion in the Tunguska region of Siberia, Russia. Trees in an area about 100 km across were felled by the blast, but no crater was found. The Tunguska fireball may have been a comet exploding at an altitude of about 6 km.

MOONS

Moons, also known as satellites, are relatively small worlds that orbit the planets of the Solar System. Earth has one moon, known simply as the Moon, but other planets have many more – Saturn, for example, has at least 18 moons. Moons are very varied in size and form. Many have unusual landscape features that intrigue astronomers.

Moons are created in different ways. Some are the result of fragments of rock or ice being pulled together by gravity to form a globe. Others are asteroids that have been “captured” by a planet’s gravitational force.

All seven of the moons illustrated here larger than the smallest planet, Pluto, while the largest moons, Ganymede and Titan, are even bigger than Mercury, the second smallest planet. Jupiter’s four largest moons are all in the top seven. They are called the “Galileans” after the Italian scientist Galileo Galilei who first discovered them with one of the first telescopes in 1610. Ganymede has an icy surface with cratered plains and areas showing strange “grooved” patterns.

Titan, Saturn’s largest moon, is the only moon to have a thick atmosphere, made mainly of nitrogen. Beneath its continuous cloud layer, there may be a sea of methane.

Callisto, Jupiter’s second largest moon, is heavily cratered. Measuring 600 kilometres across, its most prominent crater, called Valhalla, is surrounded by a series of ripples. Io, the third of Jupiter’s Galileans, with its crust a vivid mixture of yellows, oranges, reds and blacks, looks a little like a pizza. In fact it is peppered with active volcanoes and lakes of molten rock.

Our own Moon is the fifth largest moon in the Solar System, although it would take 81 Moons to make up a world the size of Earth. The Moon’s lava plains indicate past volcanic activity, but there are no active volcanoes there today.

Next in order of size comes Europa, the fourth Galilean and an object of great interest amongst astronomers. Looking like a cracked egg, its surface consists of ice sheets that are continually melting and re-solidifying. It is by no means impossible that, beneath those ice sheets, there is a warm ocean of liquid water. Could it be that life has also evolved on Europa and that there are life-forms swimming in its oceans? Future space probe missions may find out.

Triton is Neptune’s largest moon. Its surface is the coldest place known in the Solar System. At -235°C, the temperature is low enough to freeze nitrogen. Triton was photographed in stunning detail by Voyager 2, the last of its close encounters, in 1989.

Picture Credit : Google

PLUTO

Pluto is the smallest, coldest and outermost planet in the Solar System. It was the last to be discovered, identified in 1930 by the American astronomer Clyde Tombaugh. He compared photographs of part of the sky taken six days apart and noticed that a pinprick of light had moved slightly against the background of stars. Pluto was the only outer planet not visited by Voyager 2, so astronomers still know little about it. Some even propose that Pluto is really a comet and not a planet at all.

Pluto has a very elongated orbit, ranging between 7400 and 4400 million kilometres from the Sun, bringing it inside the orbit of Neptune for part of the journey. Pluto’s moon, Charon, is just over half its size and lies only 19,640 kilometres away from it. Both spin in a direction opposite to that of the other planets except Venus.

Pluto is denser than the icy moons of Uranus and Neptune, suggesting that it has relatively large, rocky core.

Pluto’s surface is probably an “icescape” of frozen nitrogen, carbon monoxide and methane. There may be craters made by collisions with rock and ice fragments. Seen from Pluto, the Sun looks no more than a bright, distant star. It still provides just enough heat to evaporate some of the surface frost and create an extremely thin atmosphere. Charon, Pluto’s nearby moon, features prominently in the sky.

Thousands of icy objects may exist in the outer reaches of the Solar System. They may form either a belt or a cloud. This could be the birthplace of comets.

Picture Credit : Google

NEPTUNE

Neptune was discovered by German astronomer Johann Galle in 1846. Its largest moon, Triton, was recorded a few days later. Besides that, very little was known about Neptune until the space probe Voyager 2 visited it in 1989.

A bright blue globe, Neptune almost completely lacks surface features. At the time it was photographed by Voyager, a storm system, called the Great Dark Spot (which later disappeared), could be seen racing in a direction opposite to the planet’s rotation. Winds on Neptune blow at more than 2000 kilometres per hour.

Like the other gas giants, Neptune has a system of rings. There are four extremely faint rings, composed of dark, icy fragments.

VOYAGER 2

The greatest journey by a space probe so far undertaken was made by Voyager 2. Between 1979 and 1989, it flew close by Jupiter, Saturn, Uranus and Neptune, transmitting superbly clear pictures of the planets and their moons. Voyager has since sped away from the Solar System, although it continues to send back signals – 20 billion times weaker than those of a watch battery!

Voyager is playing its part in the search for life in other solar systems. Should aliens ever come across the space probe they will find an audiovisual disc on board. If they play it, they will hear, among other things, the sounds of whales, baby crying and greetings in 55 languages.

Picture Credit : Google

URANUS

Uranus was discovered in 1781 by William Herschel, an amateur German astronomer living in England. More recently, astronomers found that Uranus is tilted 98° from the vertical, meaning that it orbits the Sun almost on its side. So for much of the 84-year-long journey, both poles face long periods of continuous daylight, followed by continuous night.

            Uranus’ relatively small, rocky core is surrounded by a slushy ocean of water with some ammonia. Its thick atmosphere is composed mainly of hydrogen.

            Uranus has a family of 11 faint rings, none more than 10 km wide, each made up of pitch-black blocks, measuring only a few metres deep. They circle Uranus’ equator.

Picture Credit : Google