Asteroids, Meteoroids, Meteors, Meteorites, Comets Asteroids Upsc
Contents
Summary Table
| Asteroid | A celestial body bigger than 10 m orbiting the Sun, mainly between Mars and Jupiter | Meteoroid | Similar to an asteroid, but significantly smaller. Mostly debris of comets, sometimes debris of asteroids. |
| Meteor | A bright tail of light caused by a meteoroid during its atmospheric flight, also called a shooting star or falling star. | Fireball | A very bright meteor (brighter than the planet Venus). |
| Bolide | A fireball that explodes during its atmospheric flight, often with visible fragmentation. | Meteorite | The part of a meteoroid or asteroid that survives the passage through our atmosphere and reaches the Earth's surface. |
| Comet | A smaller celestial body mainly composed of ice and dust. If a comet approaches the Sun it can generate a tail of gas and/or dust. |
Asteroids
An asteroid is a celestial body - composed of rock, metal or a mixture of both - that is orbiting the Sun. Most of them are in the asteroid belt between Mars and Jupiter. Even though there are millions of asteroids with sizes up to more than 500 km (like Pallas and Vesta) they are of no danger to the planet Earth. The biggest body in the asteroid belt - Ceres - is officially not called an asteroid anymore but a . If you try to envision the asteroid belt don't get fooled by some science fiction films: travelling around in the asteroid belt with your spacecraft doesn't require constant steering in order to avoid crashes with asteroids. The scale of the solar system is so immense that even inside the asteroid belt the average distance between two asteroids is above one million km - or three times the distance between Earth and the Moon.
Asteroid Itokawa, an Apollo asteroid with a length of 500 metres. Credit: JAXA
Some asteroids have very elliptical trajectories, crossing the orbits of the inner planets Mars, Earth or Venus. The cause of these elliptical trajectories could be collisions within the asteroid belt or the gravitational influence of the massive planet Jupiter changing the orbits of some asteroids gradually over time (see .
Meteoroids and meteors
Generally speaking, meteoroids are all the smaller objects in orbit around the Sun. Most of them originate from comets that lose gas and dust when they approach the Sun. Other meteoroids are basically small asteroids. There is no exact diameter that distinguishes an asteroid from a meteoroid. Wikipedia states 10 metres; other trustworthy sites call anything smaller than 1 km a meteoroid. Anyhow, the vast majority of all meteoroids are just a few millimetres and less in size. The smallest and by far the most numerous ones have sizes of small dust particles and are called micrometeoroids; they do not leave any visible trace behind when they enter the Earth's atmosphere.
Perseid meteor shower on August 12, 2012. Image taken by David Kingham in Snowy Range (Wyoming).
The ones about the size of a pebble leave behind a flash of light when they completely vaporise. Most people call this flash a "shooting star" or a "falling star", but more accurately spoken this is a meteor. A meteor is the light that you can see when a small meteoroid enters the Earth’s atmosphere. This normally happens with speeds between 11 and 73 km/s and at altitudes of about 75-120 km. Under a clear sky an observer can see 5 to 10 meteors per hour, especially after midnight when the Earth has rotated so far that the observer's part of the sky is positioned in the direction of the Earth's motion around the Sun. During so called meteor showers the rate of observable meteors per hour can increase significantly. Meteor showers are caused when the Earth crosses higher than usual concentrations of particles that are themselves in an eccentric orbit around the Sun. Since the orbit of these particles is fixed, we encounter this stream every year at the same time - just its density cannot be foreseen. This sometimes leads to sparse meteor showers and sometimes very intense meteor showers with more than 1000 meteors per hour, also called meteor outbursts or meteor storms. The meteors we see can be debris from a comet (> 90% of all meteors we see) or an asteroid. The most famous meteor showers are the (mid-November). The meteors during these meteor showers almost all emerge from the same section of the sky; indeed the meteor showers are named for the constellations from which the meteors appear to originate.
Leonid meteor, image taken during the peak of the 2009 Leonid Meteor Shower (17th November). Credit: Wikipedia.org, user: Navicore.
But what causes the light path of the meteor that we can see in the sky? Smaller meteoroids will be heated by until the point when they completely disintegrate. However, the light emission we observe is mainly caused by interactions between evaporated and detached components of the fast moving meteoroid and air molecules. Both the meteoroid atoms and the air molecules ionize during this encounter. When the free electrons recombine with the ionized atoms in the tail of the meteoroid they emit the light that we can observe. The light track can have a length of up to several tens of kilometres and an initial diameter of a few metres. The colour of the meteor is an indicator of the material of the meteoroid; e.g., a yellow colour is caused by iron, a blue-green colour by copper and a red colour by silicate material.
A meteor that is larger and brighter than normal is called a fireball; brighter than the brightest planet in our night sky (Venus). If these fireballs also break apart or explode during their atmospheric flight - sometimes accompanied by considerable audible sounds - they are called a bolide.
Meteorites
Finally, every asteroid or meteoroid that survives its passage through Earth's atmosphere (and this is the rare exception) can be advanced to be called a meteorite. Meteorites are made of rock (.
The three main types of meteorites. Credit: Sun.org - www.sun.org, released under CC-BY-SA 3.0
Comets
Comet Hartley 2 has a length of about 2 km and a short orbital period of just 6.46 years. CreditNASA/JPL-Caltech/UMD.
Comets are asteroid-like objects which are composed of ice, dust and rocky particles; that's why they are also called 'dirty snowballs'. The sizes of their nuclei vary between a few hundred metres to tens of kilometres in diameter; their visible tails can extend to above 150 million km in length. They originate from outside Neptune's orbit and - like many asteroids and meteoroids - are unmodified remnants of the formation of our solar system about 4.568 billion years ago. When comets approach the Sun the solar radiation and solar winds cause particles to sublimate and detach from the comet, forming a tail of particles which often makes them visible in the night sky even to the naked eye. We say 'sublimate' (a direct phase transition from the solid to the gas phase) since with zero pressure in space, water will not exist in the liquid phase. Anyhow, below its surface there can also be reservoirs of liquid water which can vaporise and feed jets of water vapour.
Comets orbit as around the Sun on elliptical orbits until all of their volatile material has evaporated away. The orbital periods vary between a few years (like , our guest in 2013, the next time.
The Kuiper belt is the region from where most of the short-period comets derive. Credit:NASA.
Short-period comets mainly originate from the Kuiper belt, a region in the solar system with many millions of icy bodies extending from about 30 can cause these comets to change their trajectory around the Sun and approach the inner parts of the solar system. The star Gliese 710 will approach within a distance of just 1 light year from the Sun in about 1.4 million years, scratching the Oort cloud and causing many objects to change their trajectories around the Sun.
Comet Lovejoy. This image is taken from the International Space Station (ISS) on December 22, 2011. At the bottom of the image you can see Earth's atmosphere. Credit: Dan Burbank (ISS Expedition 30, NASA)
Comets from the Kuiper belt tend to orbit the Sun within the plane of the solar system because the Kuiper belt itself is aligned with the plane of the solar system. Comets from the Oort cloud can arrive from all different directions since the Oort cloud has a spherical shape. A comet's tail is caused by gas and dust particles that are sublimated and/or vaporised by sunlight and then blown away by the solar wind. The tail always streams out in the direction opposite to the Sun, but it doesn’t arise until the comet enters the inner parts of the solar system (somewhere between Mars and Jupiter), so that the sunlight can sufficiently heat up the comet.
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