Near-Earth Asteroids
Near-Earth asteroids, or perhaps 'rocks in space', originate from the main belt between Mars and Jupiter. Within the asteroid belt are a range of asteroid compositional types ranging from silica- rich (or stony) S-types to carbon rich C-types. Silica-based materials with high melting temperatures formed closer to the Sun while the lower melting temperature (volatile), carbon based materials, accumulated further out from the Sun.
Asteroids come in many different size and shapes as shown in the figure below. They are basically fragments of larger bodies.
We can't see inside asteroids but surface features such as grooves and strange shapes suggest their interiors are not completely solid but may be 'rubble piles' or highly fragmented. This is a result of collisions of other asteroids in the main-belt.
As there is no weather in space the aftermath of collisions remains for millions of years as craters.
These collisions between asteroids have produced some of the meteorites and near-Earth asteroids that make their way to Earth.
Every so often the Earth is hit by an asteroid. The result of which is shown in the figure below. This type of impact is thought to occur every six thousand years.
Actually the Earth is protected from more frequent small impacts by the atmosphere. Small bodies less than 50 m in diameter may burn up or fall to the ground as meteorites. A meteorite will strike the ground at a modest speed of 90 to 180 metres per second (a large hailstone may reach 40 metres per second) whereas large asteroids greater than about 50 metres in diameter will impact at over 9400 metres per second or about 15 times the speed of a jet fighter. A meteorite producing fireball is shown in the figure below.
On Earth there are craters from asteroid strikes like the one a Meteor Crater, Arizona thought to have been formed by a 30 m (the size of a hot air balloon) iron asteroid impacting at a speed of about 12 kilometres per second. This speed is about 36 times the speed of sound or 120 times the speed of the signal from your eyes to your brain.
Craters do not last long on the Earth as they are eroded by the weather and water and are difficult to recognise as shown in the figure below.
When was the last asteroid strike? The last large asteroid strike was in 1908 in Siberia when a 60 metre stony body exploded in the atmosphere causing a blast that knocked over trees up to 10 miles from the blast centre. Small asteroid fragments were found embedded in trees around the blast site. An example of the devastation is shown in the figure below.
One of these rogue asteroids shown by the yellow orbits in the figure below is shown crossing the Earth's orbit.
Although Jupiter has a large gravitational pull it needs to make many passes of an asteroid to cause any significant change in their orbit. If the orbital period of Jupiter and an asteroid are some simple ratio then Jupiter can make frequent repeated passes.
For example Jupiter takes approximately 12 years to orbit the Sun. It is possible that an asteroid could be in an orbit of 4 years. Therefore every third asteroid orbit around the Sun Jupiter will make a close approach (relatively speaking of course). This so called 3:1 resonance with Jupiter can change the orbit over periods of several million years.
There is evidence for this mechanism at work in the asteroid belt. There is a depletion of asteroids the orbital periods that are a simple ratio to Jupiter. These Kirkwood gaps can clearly be seen in the figure below.
It is possible that an asteroid fragment may reach a resonance orbit with Jupiter after an asteroid-asteroid collision. Alternatively an asteroid may drift into this resonance region due to a tiny but continuous force due to the asymmetric emission of thermal photons (the Yarkovsky Effect).