The Atmosphere

The Atmosphere: Definition

Atmosphere is defined as the gaseous layer that surrounds the planet. It is made up of a mixture of different gases that are retained close to the Earth by the planet's gravity force.

The atmosphere is very important for life on our planet, because it provides gases to living things. It also absorbs ultraviolet radiation, that it is extremely deleterious. The atmosphere, besides, warms the Earth's surface through heat retention and reduces extreme changes of temperature between day and night: it keeps the heat accumulated during the day, so the global temperature doesn't descend dramatically during the night.

Finally, the atmosphere is essential to complete the water cycle. It has not only water in gaseous state (water vapour), but also liquid or even solid water water stored in the clouds.

Chemical Components

The atmosphere is made up of a mixture of gases. The composition of the gaseous layer has changed throughout the Earth's history. The primitive atmosphere was rich in carbon dioxide (CO₂), ammonia (NH₃), methane (CH₄) and water vapour. Oxygen was, however, rare.

The atmospheric water vapour condensed when the Earth's temperature descended, causing intense precipitations and forming the current oceans.

The reduction of the volcanic activity and the photosynthetic activity of living beings that appeared three thousand million years ago changed the gas proportions. Living beings use carbon dioxide to produce organic matter and release oxygen. Due to this, the relative amount of carbon dioxide dropped and the oxygen raised for million years.

The most abundant gas in the current atmosphere is the Nitrogen (N₂). It is 78% of the total mass. This gas is colourless and odourless. Although nitrogen is an essential element for living beings, nearly none of them are able to use the atmospheric nitrogen (only some microorganisms).

The second most abundant gas is the oxygen (O₂). It is 21%of the total mass. As we have just studied, most of the atmospheric oxygen comes from the photosynthetic activity of autotrophic living beings (living beings that can produce their own food). It is transcendental to the respiration of many living beings.

The rest of the gases are only 1% of total mass. Some of them are extremely important gases. Carbon dioxide (CO₂), for instance, is necessary for photosynthesis, but it is also one of the main responsible for the greenhouse effect. 

The water vapour (H₂O) is generated during the water cycle. The amount of water vapour in the atmosphere is called humidity and it is related to the apparent temperature (the higher the amount of water vapour, the higher the transmission of heat by the air).

The ozone (O₃) is rare, but very important because it absorbs the ultraviolet rays, that are dangerous for living beings.

Hydrogen is very rare, because it reacts with the oxygen to form water.

Other gases are inert. Helium (He), Argon (Ar), Neon (Ne) or Krypton (Kr) are noble gases that don't react with any other substance. Due to this, the amount of these inert gases has been nearly the same throughout the whole Earth's history.

Atmosphere: Layers 

The atmosphere can be divided into five consecutive layers:

• Troposphere.
• Stratosphere.
• Mesosphere.
• Thermosphere.
• Exosphere.

Troposphere

The troposphere is the lowest and thinnest layer of the atmosphere. It is 17 kilometres in height on average. It is thicker in the equator and thinner in the poles.

The troposphere contains 75% of the total atmosphere's mass. It is quite thin, but gases are very condensed. Its temperature declines with the altitude, around 6.5°C per kilometre.

It is the layer where the meteorological phenomena occurs, so it is responsible for the Earth's weather.

It is bound above by the tropopause, that separates this layer to the stratosphere.

Stratosphere

The stratosphere is the second sayer of the atmosphere. It is 40 kilometres thick on average (from 12 kilometres to 55 kilometres, more or less). It has very low atmospheric pressure and its temperature is lower at the tropopause, around -60°C, and rises with the height, reaching 0°C in the higher limit.

The atmospheric conditions in this layer are very stable, there are only  some peculiar clouds and there are not relevant atmospheric phenomena. But it contains the ozone layer, that protects the Earth from ultraviolet radiations.

It is bound above by the stratopause, that separates this layer to the mesosphere.

Mesosphere

This layer lies directly against the stratosphere. It is 50 kilometres thick on average, it extends from 50 to 100 kilometres of altitude. It is thinner in summer. During this season, its upper limit descends to 85 kilometres.

It has a very low amount of gases, so its atmospheric pressure is very low. Its average temperature descends with increasing height, from 0°C in the tropopause to -143°C in the upper boundary, called mesopause.

Although the atmospheric phenomena are rare in this layer, there are typical mesospheric structures called noctilucen clouds (or polar mesospheric clouds), that are made of ice crystals. 

The upper part of the mesosphere belongs to a atmospheric layer called ionosphere (a part of the ionosphere is in the mesosphere, another part is in the thermosphere).

The mesosphere is bound above by the mesopause, that separates this layer to the thermosphere.

Noctilucten Clouds

Thermosphere

This layer, that lays just above the mesopause, is very thick: more than 400 kilometres. Its low boundary is between 80 and 100 kilometres high and the upper limit is more than 500 kilometres high.

It has few gases, only vestigial particles. As a result, the atmospheric pressure is extremely low. Its temperature increases with increasing height due to the absorption of solar electromagnetic radiation. The temperature near the upper boundary can reach 2500°C during the day although there is not warm sensation. This fact results from the extremely low amount of matter: there are not particles capable of transmitting heat.

Many artificial satellites or devices such as the International Space Station orbit in the thermosphere.

Exosphere

The exosphere is the atmospheric volume surrounding the Earth, made up of residual particles orbiting the planet by gravitational attraction.

It doesn't have a definite upper boundary. For this reason, the exosphere is frequently considered a part of the outer space.

Atmospheric Conditions

The general tropospheric conditions are not constant, but they change. These changes in the atmospheric conditions are very important, because they are related to weather.

The most relevant atmospheric conditions are:

• Atmospheric pressure.
• Temperature.
• Wind.
• Humidity.
• Clouds.
• Precipitations.

Atmospheric Pressure

The atmospheric pressure is defined as the force of the air on a surface by the weight of the atmospheric air column above that surface. Summing up, it is the weight of the air on a concrete point.

The unit for pressure in the International System (IS) is the Pascal (Pa). Meteorologists,  however, more frequently use other units, mainly bars (bar) or the millesimal unit derived from it, the millibars (mbar). It is measured by an instrument called a barometer.

The atmospheric pressure is variable. It depends on the air density, so it changes when the air temperature changes. It also depends on the amount of air above the surface. For this reason, air pressure at sea level tends to be higher, because the air column is also higher.

Variations in the air pressure are related to changes in weather. Weather maps have lines that connect points with the same atmospheric pressure. In these maps we can find high pressure centres, usually marked with a H, and low pressure centres, usually marked with a L.

The air pressure is related to the movement of air masses or, in other words, it is related to the wind. The air always move from high pressure centres to low pressure centres. As a result, clouds move from high pressure centres to low pressure centres (carried by the wind currents). Thus, low pressure centres are zones where clouds tend to accumulate. So high pressure centres are usually associated to fair weather, whereas low pressure centres are associated to bad weather.

This is how isobaric maps and barometers help us to predict the weather. But there are other essential parameters to complete the prediction, such as the temperature.

There are three types of maps to forecast the weather: isobaric maps, that we have just studied, graphic maps, with symbols that show how the weather is going to be, and real maps, taken by satellites, showing the distribution of clouds above the Earth.

Temperature

The atmospheric temperature is the amount of heat stored in the gases. The International unit for temperature is the Kelvin (K) although in our regular life we usually use other units,  above all degrees Celsius (°C). The instrument used to measure temperature is called  thermometer.

Changes in the air temperature lead to changes in air properties. Hotter air has less density, so hot reduces the atmospheric pressure. Colder air has more density, so cold increases the atmospheric pressure. Hot air, besides, tends to ascend to upper atmospheric layers, because of its low density.

The air temperature is also related to the amount of water vapour of the atmosphere. Colder air can support less water vapour. It the air temperature is extremely cold, the water vapour freezes, so the humidity descends. The driest air of the planet can be found in the Antarctic.

Difference of temperature between different points or zones of the planet are related to the movement of large masses of air, due to changes in the atmospheric pressure. The air in the equator tends to become hotter, then in ascends and moves towards hotter places. The air in the poles tends to become colder, then it descends and move towards hotter places.

All these movements cause atmospheric currents, that are related to the movement of clouds and the local climate in different parts of the planet.

Wind

The wind is the air in movement. As we have just studied, it comes from differences of atmospheric pressure. Wind is responsible for the movement of clouds. The movement of air masses with different temperatures causes air currents. These currents are responsible not only for the transportation of clouds, but also for the transmission of heat from one place to another.

There are two important characteristics of the wind in one particular region: the direction and the speed. The wind direction is measured by wind vanes. The wind speed is measured by the anemometer. The wind speed is related to the strength of the wind. 

Wind can be classified according to its strength. From weaker to stronger, we can define:

• Breeze.
• Gale.
• Storm.
• Hurricane.
• Typhoon.

Humidity

Humidity is defined as the amount of water vapour in the atmosphere. The water vapour is invisible, it can only be seen when it condenses. The most usual system to measure the atmospheric humidity is using a percentage. When this percentage reaches 100% the water vapour condenses, changing from gaseous to liquid state. 

The instrument used to measure the atmospheric humidity is called hygrometer.

The atmospheric humidity is related to the thermal sensation. The higher amount of water vapour in the air, the better heat transmission. Due to this, environments with high humidity increase the thermal sensation. Thus, cold temperatures are perceived as colder and high temperatures are perceived as hotter.

Clouds

Clouds are visible masses of tiny drops of water suspended in the atmosphere. They are formed, in general, by the condensation of atmospheric water vapour.

The condensation takes place when the temperature decreases. This process is specially frequent in high parts of the troposphere, but it can occur at any altitude, even very close to the Earth's surface, forming fog.

In fact, condensation can take place under different conditions of humidity, pressure or temperature. This is the reason why there are different types of clouds.

According to the height, there are low, medium and high level clouds.

Low level clouds.

• Stratus: they are grey, flat and uniform clouds. They show horizontal layering with uniform base. Thick stratus can produce precipitations. When these clouds are very low, they form fog.

• Cumulus: they are low clouds with vertical development. They are cotton-like clouds and usually indicate fair weather. They can also be precursors of other clouds, such as cumulonimbus.

• Stratocumulus: they are hybrid of stratus and cumulus, characterised by dark large masses. They don't usually to produce precipitations, but they are typically visible after rains or before storms.

• Nimbostratus: dense clouds, similar to stratus but thicker. They are made of multiple layers or strata. They produce precipitations, mainly rain and snow.

• Cumulonimbus: thick cotton like clouds, made of multiple layers or strata. They are cumulus with high vertical development. They are related to atmospheric instability, heavy rains and storms.

Middle level clouds.

• Altostratus: flat, grey clouds forming layers. They are similar to stratus, but higher. Due to these characteristics, they are usually translucent. They can produce light precipitations.

• Altocumulus: high white or grey globular clouds, similar to cumulus, but at high altitude. They are related to variable weather.

High level clouds.

• Cirrus: thin and long clouds, similar to white strands. They are always white or light grey. They are made of tiny ice crystals. Although they usually indicate that weather may soon to deteriorate, they can also precede warm fonts. Cirrus are also formed during tropical cyclones.

• Cirrostratus: widespread cirrus or, in other words, high stratus made of ice crystals. They indicate high humidity in high tropospheric layers. They are a sign of precipitations in the following hours, although they can also be related to warm fronts.

• Cirrocumulus: they are made up of liquid water and water crystals mixed. They are high white cotton like clouds. When they form dense groups, they precede rains. When they are isolated, they are related to fair weather.

Precipitations

When the tiny drops of water that made the clouds condense, forming bigger water drops or flakes and little balls of ice, they fall from the clouds due to the gravity force. This process is called precipitation.

The device used to measure the amount of precipitation (mainly of liquid precipitation) is called pluviometer or rain gauge.

There are three types of precipitations: rain, snow and hail.

Rain

Rain is the precipitation formed by liquid water droplets. It occurs when the tiny drops of water that made the clouds condense, forming bigger drops that fall as a result of the gravity force. This condensation can take place due to changes of pressure or temperature.

Rain is the most responsible for the deposition of fresh water. 

Snow

Snow is a precipitation in form of flakes of crystalline water ice. The tiny drops of water that made the clouds condense and freeze slowly. These condensed frozen drops form the crystalline flakes.

The flakes fall from the clouds. Obviously, snow is related to cold weather, because water solidifies under cero degrees Celsius. And it is always formed from clouds with high vertical component.

Hail

Hail is a precipitation in form of ice balls. It is related clouds with high vertical component and also high amount of water.

Hail is formed when the little drops of water condense and freeze abruptly due to low temperatures, but air currents make them ascend instead of falling. These little frozen drops can adsorb more water and freeze again, producing bigger ice balls that fall from the cloud.   

Atmosphere and climate

Different parts of the planet have different climatological characteristics, mainly depending on the latitude, but also on other factors, such as altitude or distance from the sea or air currents.

Latitude

Latitude is defined as the distance of one point or region from the equator.

The latitude is related to the solar radiation intensity. The solar rays reach the Earth in more perpendicular way in equatorial regions. Due to this, radiation is more intense in equatorial zones and less intense in the poles.

So that, the furthest the distance from the equator, the lower the average temperature. 

Altitude

Altitude is defined as the vertical distance from the sea level. The average temperature descends with the altitude. In other words, the higher the altitude, the lower the average temperature.

This is the reason why the higher mountains of the planet have perpetual snow. The line for perpetual snow is not clearly defined, it depends on the latitude or location of the mountain. 

Distance from the sea

The sea moderates changes of temperature, because liquid water heats and cools slowly. Summing up, sea water preserves temperature.

Near the sea, winter tends to be not so cold and summers not so hot. Inland regions, however, tend to have colder winters and hotter summers.

Air and ocean currents

On the one hand, the ocean in equatorial zones heats the air. On the other hand, the oceans in polar zones cools the air. These differences of temperature cause air movements. Hot air tends to ascend, whereas cold air tends to stand close to the earth. 

The difference of the water temperature also causes ocean currents. Sea currents change the air temperature, modifying the main air currents or causing other secondary air movements. 

The Earth in the Universe

The Universe and Human Beings

Human beings have been watching the Universe since ancient times. At first, people looked at the stars and constellations to guide themselves or to predict the seasons. 

First theories about the origin and structure of the Universe were simply speculations and myths, without any scientific support. The Ptolemaic Model, also called Geocentric Model, was the most accepted theory for centuries. According to this theory, the Earth was in the centre of the Universe and the Sun, the Moon and the rest of celestial bodies orbited around it. It is logical because, after all, we currently know that the Earth is moving, but we don't perceive it. And this model was useful for more than 1500 years.

But the model is not correct and new observations revealed some mistakes, many astronomical phenomena could not be explained by this theory, so it was replaced by the Heliocentric Model. In this model the Sun is in the centre of the Universe and the Earth and planets revolve around it.

Geocentric Model.

Nowadays we know that the Earth and the planets of the Solar System revolve around the Sun, although neither the Sun nor the Earth are in the centre of the Universe. The Sun is our closest star, and we live in a planet located in a Galaxy called the Milky Way.

Heliocentric Model.

Currently, the most accepted theory that explains the origin of the Universe is called “The Big Bang Theory". According to this theory, the whole Universe was concentred at a single point (singularity). 13.8 billion years ago, this point started standing, leading to the formation of the stars, planets and other celestial bodies. Galaxies are still moving, the Universe is expanding and the distance between galaxies is getting bigger and bigger throughout the time.

Measuring the Universe

The Universe is the space and time where we live. And all the things that we see, touch, feel or perceive are, in fact, a component of the Universe. This means that it groups all the things that exist, have existed or will exist in the future. It is a vast place, probably infinite. Only the part that we can see or study using telescopes, called the observable universe, is so large that light would take 91 billion years to cross it.

In such a  huge place, the units used to measure distances on Earth are not enough. The unit for distance in the International System is the metre, that is a good unit if we want to measure how tall we are or how far is the school from our house. For bigger distances on Earth, we usually use another unit, the kilometre. We can measure in kilometres the distance between two towns, or even the radius (6371km) or the circumference (around 40000km) of our planets.

But when we try to study the Universe, kilometres lose their meaning. The main issue is that the distances in the Universe are enormous and many people tend to think that the diagrams of the solar system that they can see in our books have something to do with reality. But they are not real, the schemes and drawings are a simply approach. If the Sun was the size of a basketball, the Earth would be a small 2mm diameter ball (more or less, the size of one of these letters). And if we put the basketball in the middle of a football pitch, the 2mm sized Earth would be orbiting beyond the goal. Neptune, the furthest planet in the Solar System would be orbiting more than 2.5km from the Sun.

The Moon is the Earth satellite and its closest celestial body It is 384400km from the Earth. It is a big number. The Sun, our closest star, is 270000000000000000km from us. That is, in fact, a huge number. But, as we have seen, it is really small if we compare it with the distance between the Sun and other celestial bodies, such as far extrasolar planets or other stars. We need other units to measure these kind of distances. 

Solar System

Astronomical Units (AU).

The Solar System is made up of the Sun and the celestial bodies orbiting around it. To measure distances between the Solar System elements the most common unit used by astronomers is the Astronomical Unit (AU).

We define Astronomical Unit as the average distance between the Earth and the Sun. So the Earth is 1AU from the Sun. Mars is 1.5AU from the Sun. Jupiter is 5.2AU from the Sun.

Galaxy

Light-Years.

Astronomical Units are still too small to measure distances between stars. So astronomers use another more appropriate unit, called the light-year (ly).

We define a light-year as the distance that light can travel in a year. Light travels at 300000km per second in a vacuum, so the distance it travels in a year is really enormous, around 9460700000000km.

Proxima Centauri, the closest star to the Sun, is about 4.25 light-years from us.

Parsec.

Currently astronomers tend to use another unit, called parsec (pc), to measure stellar distances. Parsecs are even bigger than light-years. 1 parsec is equal to 3.26 light years.

Components of the Universe

What is the Universe made up of? Basically, there are two main components, matter and energy.

We can perceive energy and its effects: light, radiations, etc.

Matter can form different structures called Astronomical Objects. They can be classified according to their size and physical properties. The following are the most important astronomical objects:

• Stars.
• Planets.
• Dwarf Planets.
• Satellites.
• Asteroids.
• Comets.
• Others.

Stars

Stars are massive luminous spherical bodies. In general, they are big and bright celestial corpus. Their main chemical components are hydrogen and helium. They are so enormous, and  have such an enormous mass, that the hydrogen and helium in their core r

The Sun.

eact (in nuclear fusion reactions) releasing huge amounts of energy.

The emitted energy is merely electromagnetic radiation. Visible light is a kind of electromagnetic radiation that we can perceive with our eyes. Other important radiations are the infrared rays, that transmit heat. Some radiations emitted by stars are extremely deleterious for living beings, but luckily they do not reach the Earth in significant quantities; X-rays, Gamma Rays and Ultraviolet rays are three typical examples.

Stars are not solid structures, they are frequently a plasma of gases condensed by gravitational forces. These gases are usually at extremely high temperatures, due to the nuclear reactions that take place in them. And, as we have said, the main chemical components are hydrogen and helium, although other heavier elements can also be found in them, above all in the inner core.

Stars are formed when clouds of gases and dust are pulled together by gravitational attraction. As a result, all the stars have a big cloud of non-collapsed materials surrounding them called Nebula. 

Nebula

During the first phases of formation, nebula are bigger, and some outer components of this surrounding materials join, forming other celestial bodies that will orbit the stars, such as planets, satellites or asteroids.

The group of a stars with all the celestial bodies orbiting it is called Solar System. The Sun is the star of our Solar System.

Stars are classified according to their size and temperature. These two characteristics are related to their colour. The Sun is a yellow star.

Stars are not static structures, they evolve and change and some types of stars change into other when the nuclear reactions consume some components. At the end of their life, some stars can transform into other celestial bodies, such as Supernovas, Neutron Stars or Black Holes.

Stars sometimes group to form star clusters, that are groups of hundreds or even thousands of stars. Galaxy clusters sometimes group forming galaxies. Galaxies can have millions of stars.

Our Solar System is in one of the arms of a spiral galaxy called the Milky Way.

The Sun is the nearest star to Planet Earth. It provides us with light and heat, and it is absolutely essential to support life.

The Sun is in the centre of the Solar System. It is much bigger than the rest of celestial bodies that orbit it. In fact, Sun contains 99% of the total mass of the Solar System.

Planets

Planets are spherical bodies orbiting a star across a clean orbit, what it means is that no other celestial body share its orbit. Planets are held to their star by gravitational forces.

Planets are always smaller than stars and never emit electromagnetic radiation (nor light or heat), because they are not large enough to support nuclear reactions. They are, however, bigger than other celestial bodies like comets or asteroids.

Planet Movements

Planets have two movements: rotation and revolution, also called orbit.

Rotation is the spinning movement of the planet around its axis. The period a planet takes to complete a rotation is known as a day.

Revolution is the movement of a planet around a star. The time a planet takes to complete a revolution around its star is known as a year.

All the planets in the Solar System revolve around the Sun in the same direction: anti-clockwise as seen from the Sun northern pole. All the planets in the Solar System but Venus and Uranus rotate in an anti-clockwise direction.

There are two kind of planets, rocky and gaseous planets. Rocky planets are smaller and denser, whereas gaseous planets are bigger and less dense. Inner planets in the Solar System (closes to the Sun) are rocky planets, outer planets are gaseous planets.

Planets in the Solar System

There are eight planets in the Solar System. The four closest planets, Mercury, Venus, Earth and Mars are rocky planets. The next four planets, Jupiter, Saturn, Uranus and Neptune are gaseous planets.

Mercury

Mercury is the closest planet to the Sun. It is also the smallest planet in the Solar System.

It doesn't have atmosphere and its rotation period is only a bit longer than its revolution, in other words a day is nearly as long as a year. Due to this, one of the sides faces the Sun for a long time. The side of the planet that faces the Sun is very hot and the other side, however, is very cold because the lacking of atmosphere provokes that it loses a lot of heat.

Mercury.

Venus

Venus is the second planet of the Solar System. It is bigger than Mercury, but a bit smaller than the Earth. It rotates in clockwise direction. Its rotation period is longer than its revolution, in other words its day is longer than its year.

Venus has a dense atmosphere, rich in carbon dioxide. Although it is further from the Sun than Mercury, it is the hottest Planet of the Solar System due to the greenhouse effect provoked by its atmosphere. This is the closest planet to the Earth and it is the brightest astronomic object in the night sky after the Moon.

Venus.

Earth

Planet Earth is the third planet in the Solar System. Its distance from the Sun and the density of its atmosphere, rich in nitrogen and oxygen, provide an average temperature which ranges from -90°C to 60°C (depending on the place). This temperature allows that about 71% of its surface is covered by liquid water. Probably due to these two factors it is the only known planet that support life. Planet Earth has one satellite, the Moon.

Earth

Mars

It is also called the red planet, due to the colour of the iron oxide, that is very abundant in its surface. This planet has a thin atmosphere and an average temperature slightly lower than in the Earth, from -135°C to 35°C. Mars has two small satellites.

Mars

Jupiter

Jupiter is the largest planet in the Solar System, it has 2.5 times the mass of all the planets combined.

It is a giant gaseous planet with at least 67 satellites and a faint ring.

It has an extremely fast rotation, but it takes more than 11 earth years to complete a revolution. Its average temperature is -110°C in spite of its dense atmosphere.

Jupiter

Saturn

It is the second largest planet in the Solar System. It most well known characteristic is its prominent ring system, made of dust, rocks and small asteroids.

It also has 62 satellites with formal designation, although some asteroids of the ring system could be considered as small satellites.

Just like Jupiter, it rotates quite fast (its day is less than ten hours long). It has a long orbit, so it takes nearly 30 Earth year to complete one revolution around the Sun. It is a bit colder than Jupiter, its average temperature is -139°C.

Saturn

Uranus

Uranus is the only planet which rotation axis is almost horizontal. Its rotation is also inverse, it has clockwise rotation. It has 27 satellites and a faint ring in vertical position, perpendicular to its rotation axis.

It is the second furthest planet. This long distance, added to its atmospheric characteristics make this planet the coldest in the Solar System, its average temperature is -224°C.

Uranus takes more than 80 Earth years to complete a revolution around the Sun. 

Uranus

Neptune

Neptune is the farthest planet in the Solar System. It is really far, more than 30AU from the Sun.

Due to this, its average temperature is really low, around -210°C, although it is a bit higher than in Uranus.

It has 14 known satellites and also has a faint ring.

Neptune.

Dwarfs Planets

Dwarfs Planets are spherical bodies, smaller than planets, orbiting a star. They usually have strange, eccentric orbits. All of the known dwarfs planets are orbiting the Sun beyond Neptune.

Currently there are five recognised dwarfs Planets: Pluto, Ceres, Haumea, Makemake and Eris, although its estimated that there may be hundreds. The most famous one is Pluto (that was relegated to this category in 2006).

Pluto

Satellites

Satellites are spherical celestial bodies orbiting a planet.

All the planets of the Solar System but Mercury and Venus have satellites. The large gaseous planets have extensive satellite systems. Jupiter and Saturn have more than 60 satellites, for instance.

The two satellites that orbit Mars are very small. The Moon, however, is a big satellite, the bigger one in relation to its planet. In fact, only half of the satellites in the Solar System are comparable in size to the Earth’s Moon.

Calisto (Jupiter's moon)

Asteroids

Asteroids are celestial rocky bodies with irregular morphology. They are smaller than planets and dwarfs planets, although vary greatly in size.

The majority of known asteroids orbit between Mars and Jupiter. This huge group of asteroids orbiting together form an asteroid belt.

Another asteroid belt is known as the Kuiper Belt, that is made of asteroids orbiting beyond Neptune.

Asteroid (recreation).

Comets

Comets are celestial bodies that orbit the Sun in extremely elliptical orbits. Their nucleus is made up of a mass of gas, dust and ice. When they move close to the Sun, some of the ice evaporates, creating the tail of the comet (so when they are far away from the Sun, they do not have a tail).

Due to their eccentric elliptical orbits, comets have wide range of orbital periods.

Most of them were originated in the Kuiper Belt or in the Oort Cloud.

Comet.