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:
There are 3 known types of asteroids (depending on the composition):
Over 75% asteroids we find are C-type asteroids.
More recent classification suggests not 3 but 24 types of asteroids!
CAN WE SEE THEM?
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).
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.
PARTS OF THE COMET
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!
CAN WE SEE THEM?
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.
COMETS AND METEOR SHOWERS
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.
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
www.nasa.gov -The offical NASA website
www.esa.int/ESA – European Space Agency
www.bbc.co.uk – BBC science of space
www.bbc.co.uk/education – BBC KS2 Space education resources
www.bbc.co.uk/education – BBC KS1 Earth and Space resources
www.gov.uk – UK Space Agency
https://principia.org.uk – Tim Peaks Journey to the ISS
www.kidsastronomy.com – Videos and games about the solar system