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LP4 R2: Asteroid facts

ASTEROIDS are  rocky bodies left over from the formation of the Solar System. They are often called “minor planets”. The word “asteroid” means “star-like” in greek, because asteroids look like tiny shiny stars when we look at them through telescopes.

They are irregularly shaped and vary in sizes from 1km to 1000 km.

The three largest asteroids are Ceres (975 km), 4 Vesta (500 km) and 2 Pallas (500 km).


Asteroids are concentrated in a few groups and orbit the Sun as part of their group:

  1. Most asteroids concentrate in the area between orbits of Mars and Jupiter called Asteroid Belt. The Belt contains 1-2 million asteroids larger than 1 km and millions of smaller ones. Ceres, 4 Vesta and 2 Pallas are part of the Belt. Though there are so many asteroids in the Belt, the total mass of the asteroids is 4% of the mass of the Moon (with Ceres, Vesta and Pallas together making up a half of this mass). Asteroids in the part of the belt closest to the Sun take about 3 years to orbit the Sun, in the furthest from the Sun part they take about 6 years to orbit.
  2. Trojans are asteroids that share the orbit with a planet or moon but don’t collide with it because they orbit either ahead or behind their planet or moon. Jupiter shares the orbit with Trojans. Mars has a few Trojans too.
  3. Near-Earth Asteroids (NEAs) are asteroids whose orbit is close to the orbit of the Earth. Scientists think there is about a 1000 NEAs larger than 1 km and 13.000 smaller ones.


There are 3 known types of asteroids (depending on the composition):

  1. C-type (carbon reach)
  2. M-type (metallic)
  3. S-type (silicate)

Over 75% asteroids we find are C-type asteroids.

More recent classification suggests not 3 but 24 types of asteroids!


Only one asteroid called 4 Vesta (the second biggest asteroid in the Asteroid Belt) is bright enough to be seen with the naked eye.

If we cannot see them, than how do scientists “hunt” for new asteroids? They use telescopes with photo cameras! The trick is to take images of the same area of the sky during a course of an hour and then compare those images to see if there are any objects on them that changed their position. Everything that moved can be an asteroid!   

A few asteroids were visited by spacecrafts, for example a probe called NEAR Shoemaker orbited asteroid Eros and then touched down on that asteroid at the end of its mission. Dawn probe has been orbiting Vesta for a year and is now orbiting Ceres (since 2015).


Fun Facts:

  • June 30 is an “International Asteroid day”. The goal of this day is to educate people about asteroids and what can be done to protect our planet.
  • Some asteroids have moons.
  • People plan to mine asteroids in the future.


LP4 R3: Comet facts

COMETS are icy bodies that orbit the Sun.

The word comet means “long haired” in greek. This is because comets grow tails when they get close to the Sun.


Comets orbit the Sun just like planets, but their orbits are much more elongated (so for part of its orbit the comet stays relatively close to the Sun, and for part of the orbit it is very far away) and it can take comets from a few years to few million years to orbit the Sun.

Short-period comets are comets that take less than 200 years to orbit the Sun.

Long-period comets are comets that take over 200 years to orbit the Sun.

  1. Short-period comets come from an area called Kuiper belt. Kuiper belt is a region that starts beyond the orbit of Neptune. Astronomers sometimes measure space distances in Astronomical Unit (AU). 1 Astronomical Unit, or 1 AU, is a distance equal to the distance from the Earth to the Sun. Kuiper belt is an area that starts at about 30 AU from the Sun and ends at 55 AU.
  2. Long-period comets come from an area called Oort cloud, a cloud of icy bodies surrounding the Sun between 50.000 and 200.000 AU.


The solid part of the comet is called a nucleus. The nucleus is a dirty snowball made of rock, water ice, dust and frozen gases. When the comet’s orbit takes it closer to the Sun (closer than the orbit of Jupiter) the comet warms up and ice and gas that were trapped inside turn into a cloud around the nucleus – coma. Coma is usually made of water vapour and dust. After that solar wind and solar radiation push the coma and the comet develops two tails  – two long trails of gas and dust. The gas tail of the comet always points away from the Sun, no matter where the comet is on its orbit. The dust tail is usually curved.


How big is each of this parts?

Nuclei (that’s plural for nucleus) come in a variety of sizes from one hundred meters to tens of kilometers. Their comas can be thousands of kilometers across, the largest comas are as big as 15 Earths! The tails can stretch to one astronomical unit!


When the comet is far away from the Sun it is impossible to see: it is too small, dark and far away. When the comet gets closer to the Sun and develops coma and tail, then it can finally be spotted, because coma and tail reflect the sunlight and glow! The tails are easy to tell apart too: the dust tail is white and very bright and the gas tail (also called plasma tail) is blue, it is smaller and less bright. Most comets can only be seen through the telescope, although there are comets that can be seen with a naked eye even in the daytime (in the past 332 years people saw only 9 such comets).

A few comets were visited by spacecrafts. Rosetta probe studied the comet called “67P Churyumov – Gerasimenko” for over 2 years before landing on the comet and finishing the mission.


When comets get closer to the Sun, the ice on the surface evaporates and comets develop a ”tail” of gas and dust. When the tail crosses the orbit of the Earth, the comet’s debris enters our atmosphere and heats up. We see shooting stars – the glowing hot air trails rather than the rocks themselves.

One of the most beautiful meteor showers we can see every year in October is Orionid meteor shower (showers are called by the constellation they seem to be coming from, in case of Orionid shower the point from which meteors seem to be coming from lies in Orion constellation)

Orionid meteor shower is associated with the famous Halley comet.

This comet is locked into its orbit by Jupiter’s gravity and takes around 76 years to go around the Sun. Halley’s oval-shaped orbit brings it from beyond Neptune’s orbit (when farthest from the Sun) to within the orbit of Venus (when closest to the Sun). It is the only comet that we can see with the naked eye! Halley comet’s debris moving through the Earth’s atmosphere produces the Orionid meteor shower.


  • Comets contain the same materials as the newborn Solar System, this is why scientists really want to study them
  • In the past, people were scared of comets and thought them to be bearers of bad news


LP4 R4: Quiz for lesson 4



  1. Asteroids are made out of: a) ice b) rock
  2. The word “asteroid” means: a) “star-like” b) ”long hair” c) “little planet”
  3. The biggest asteroid is called: a) Ceres b) Vesta c) Pallas
  4. Asteroid belt is located between the orbits of: a) Jupiter and Saturn b) Mars and Jupiter  
  5. How many Near-Earth Asteroids have been found? a) less than 20.000 b) less than 200.000 c) more than 200.000
  6. The nickname for the comet is “dirty ice ball”. a) True b) False
  7. Comet is made of four parts: nucleus, coma, gas tail and dust tail. Find them on the picture


8. Which way does the gas tail of the comet point? a) along the comet’s orbit b) towards the Sun c) away from the Sun

9. When do we see meteor showers? a) When the comet’s nucleus goes through the Earth’s atmosphere b) When the Earth passes through a cloud of asteroids on its orbit c) When the debris from the comet’s tail goes through the Earth’s atmosphere  

10. How big are meteoroids? a) Smaller than 10 meters b) 10-100 meters c) larger than 100 meters

11. How do we call a small piece of rock that enters the Earth’s atmosphere? a) meteoroid b) meteor c) meteorite




  1. b
  2. a
  3. a
  4. b
  5. a
  6. True
  7. A – gas tail, B – coma, C – dust tail, D – nucleus
  8. c
  9. c
  10. a
  11. b


Useful websites / further reading -The offical NASA website – European Space Agency – BBC science of space Physics space topic – BBC KS2 Space education resources – BBC KS1 Earth and Space resources – UK Space Agency – Tim Peaks Journey to the ISS – Videos and games about the solar system – Up to date news on astronomy – Great site on space exploration – ROSCOSMOS is a Russian Space State Corporation – Full information guide to astronomy


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