My Science School

COMETS

More than a trillion comets surround the planetary region of the Solar System. They follow long orbits around the Sun and together make up a vast sphere called the Oort cloud. Each comet is a lump of dirt and snow, called a nucleus, or “dirty snowball”. Comets are so small that they are only visible when they travel close to the Sun and grow large and bright enough to be seen.

• CHANGING COMET: As a comet approaches the Sun it warms up. The snow turns to gas, which, along with loose dust, flows from the nucleus. When the comet passes closer to the Sun than the orbit of Mars, this material forms a head (called a coma) and two tails, one of gas and one of dust.
• COMET DISPLAY: More than 2,300 comets have been identified as they passed through the Sun’s neighbourhood. About 200 make return visits, but most pass by just once. Three or four times a century, a spectacular one, such as Comet McNaught in January 2007, makes a stunning display.
• PASSING THE SUN: These images from the SOHO spacecraft, track the progress of Comet McNaught as it rounds the Sun. Like most comets, it is named after its discoverer, Robert McNaught, who saw it first on 7 August 2006. It was at its biggest and brightest in January 2007, when closest to the Sun.
• COMET STRUCTURE The nucleus of a comet is a city-sized lump, two-thirds snow and one-third rock dust. Halley’s Comet orbits the Sun every 76 years. When it drew close to it in 1986, the Giotto spacecraft flew into the comet’s coma and captured images of its nucleus.
• BREAKING UP As a comet passes a massive body, such as the Sun or Jupiter, it may be pulled apart by its gravity. Comet Schwassmann Wachmann 3 orbits the Sun every 5.4 years and astronomers have observed that it is disintegrating.

Picture Credit : Google

PLANETS

Hurtling around the Sun are eight planets. Those closest to the Sun - Mercury, Venus, our home planet Earth, and Mars - are made of rock. The vast outer planets - Jupiter, Saturn, Uranus, and Neptune - are called “gas planets” because all we see of them is their gas. All eight travel in the same direction around the Sun. The time taken to make one circuit, or orbit, increases with distance. Mercury takes just 88 Earth days to orbit, while Neptune’s longer journey takes 164.8 Earth years.

1. JUPITER: The largest and most massive planet, Jupiter is also the fastest spinner, rotating once on its own axis in less than 10 hours. This giant world is made mainly of hydrogen and helium, with a central rocky core. A thin faint ring encircles Jupiter, which also has a large family of moons.
2. SATURN: Sixth from the Sun, and second largest, is pale yellow Saturn. Its distinctive feature is its ring system, which is made of billions of pieces of dirty water ice. Saturn is mainly hydrogen and helium with a rocky core. It has a large family of moons.
3. URANUS: Nineteen times the distance of Earth from the Sun, Uranus is a cold, turquoise world bounded by a layer of haze. A sparse ring system encircles the planet’s equator. Uranus is tilted on its side, so that its rings and moons seem to orbit it from top to bottom.
4. MERCURY: Mercury is a dry ball of rock, covered by millions of impact craters. It is the smallest planet, the closest to the Sun, and has the widest temperature range of any planet. During the day it is baking hot, but at night it is freezing cold.
5. VENUS: Second from the Sun, Venus is the hottest planet. This rock world is permanently covered by thick cloud that traps heat and makes it a gloomy planet.
6. NEPTUNE: Neptune is the most distant, coldest, and windiest of all eight planets. Like Uranus, it is made mainly of water-, methane-, and ammonia ices with an atmosphere of hydrogen- rich gas. It is encircled by a thin ring system and has a family of moons.
7. MARS: Sometimes called the “red planet”, Mars is the outermost of the rocky planets and a cold, dry world. It has polar ice caps, giant volcanoes, frozen desert, and deep canyons, formed in the distant past. Mars has also two small moons.
8. EARTH: The only place known to have life is Earth, the largest of the rocky planets and third from the Sun. It is also the only planet with liquid water. Movements in Earth’s crust are constantly changing its surface. Earth has one moon.
9. DWARF PLANETS: The Solar System has five known dwarf planets - small, roundish objects that orbit the Sun amongst other objects. Ceres orbits between Mars and Jupiter within a belt of rocky asteroids, while Pluto, Haumea, Makemake, and Eris are icy worlds that orbit beyond Neptune in a region called the Kuiper Belt.

PLANET SCALES Jupiter, fifth planet from the Sun, is much larger than all the other planets. It measures 142,984 km (88,846 miles) across and is made of about two and a half times as much material as all the other planets put, together. The seven other planets and the dwarf planets are shown here roughly to scale.

Picture Credit : Google

MOONS

The Solar System has more than 190 moons orbiting six of the planets — only Mercury and Venus are moonless. They range in size from Ganymede, a satellite of Jupiter, which is larger than Mercury, to S/2009 S1, a 300-metre moonlet orbiting within Saturn’s rings. All are made of rock, or rock and ice, and many have surfaces littered with impact craters, formed when the moons were bombarded by asteroids in the past. Nineteen Solar-System moons are more than 400 km (250 miles) wide. These large moons are round but the more numerous smaller moons are irregular in shape.

1. THE MOON: The Moon is Earth’s only natural satellite. It is about a quarter the size of Earth and the fifth largest of all moons. The surface of this dry ball of rock is covered in impact craters.
2. IO: Colourful lo is the most volcanic moon in the Solar System. Its surface is constantly being renewed as molten rock erupts through its thin silicate-rock crust, and fast-moving columns of cold gas and frost grains shoot up from surface cracks.
3. EUROPA: The smallest of Jupiter’s four major moons, Europa has an icy surface criss-crossed with networks of brownish grooves. The crust slowly drifts around on top of a deep ocean of liquid water that might be home to alien life.
4. GANYMEDE: At 5,262 km (3,267 miles) across, Ganymede is the largest moon in the Solar System and belongs to the largest family of moons - the moons of Jupiter. Astronomers know of 79, but the number is likely to rise as smaller moons are detected. Ganymede is made of rock and ice with an icy crust.
5. TITAN: Titan is the largest of Saturn’s 62 moons. On its surface are bright highlands, dark plains, and methane lakes and seas. It is the only moon with a substantial atmosphere, which is rich in nitrogen and extends out for hundreds of kilometres.
6. TITANIA: Titania is the largest of the 27 moons orbiting Uranus. Titania and the planet’s other major moons, Oberon, Umbriel, Ariel, and Miranda, are named after characters in English literature. Impact craters and large cracks are seen on its grey, icy surface.
7. IAPETUS: Iapetus is a moon of contrasts. Most of its crater-covered terrain is bright and icy, but the rest appears to be coated by a dark material. It is one of Saturn’s seven major moons, along with Titan, Rhea, Dione, Tethys, Enceladus, and Mimas.
8. TRITON: Triton is by far the largest of Neptune’s 14 moons, a rock-and-ice ball with a young, icy surface. It is nicknamed the cantaloupe as its linear grooves, ridges, and depressions resemble a melon’s skin.
9. SMALL MOONS Most moons are less than 400 km (250 miles) across and irregular in shape, like Saturn’s Epimetheus and Hyperion. Many of these smaller moons, like Mars’s two moons Phobos and Deimos, may have started off as asteroids or comets.

Picture Credit : Google