we look into the night sky, the impression we first get may be of an
unchanging scene. Certainly, the Moon and planets change position
against the backdrop of stars from night to night, and over a period of
hours the satellites of Jupiter can be seen to shift around their
orbits - but everything appears to happen slowly. Apart, that is, from the brief streak of light from a meteor.
Capable of appearing in any part of the sky without warning, these
objects can catch the observer completely off-guard. Many people have
never seen one, but in fact meteors are visible every cloudless night -
if only one has the patience to watch out for them. But these are more
than just a means to a free firework display; for two centuries,
meteors (or more correctly, the objects which caused them) provided
science with its only source of material from beyond the Earth. Meteors
are caused by pieces of debris floating through the solar system,
entering the Earth's atmosphere at very high speed and exciting the
atoms in the air. The piece of material itself is usually destroyed by
the immense heat caused by friction with the air. Although the vast
majority of such objects are destroyed long before they can reach the
surface of the Earth, some of the larger pieces do survive the journey, and fall to the ground. These pieces of rock from beyond the reaches of our planet are known as meteorites.
Meteoroids, Meteors and Meteorites
nature of meteors remained a mystery for a considerable time, and there
was much debate about whether "shooting stars" were phenomena of the
Earth, or space. However, the fact that meteorites really were visitors
from beyond our atmosphere was proved by Ernest Chladni in 1794, and
Jean-Baptiste Biot in 1803 when a meteorite was observed to fall near a
are three distinct stages to the phenomenon, and in the vast majority
of cases, an object will only survive to the second. These stages are:
When a piece of debris , later to enter the Earth's atmosphere, is
travelling through space towards our planet, the object is called a
Meteor. The brief flash of light we see in the night sky and caused not
by the material "burning" with friction from the atmosphere, but rather
from the atoms which have been excited in the air from the object's
The name given to the relatively small number of objects which are not
completely destroyed in the upper atmosphere, but survive to the ground.
Meteors and Meteor showers
pieces of material which cause meteors enter the upper atmosphere of
Earth at very high velocity - typically around 260,000 km/h. The flash
of light which we see occurs when the object is approximately 100 km
above the surface of the Earth. Perhaps the most surprising feature of
these objects is their size. To create a flash which is visible to the
naked eye, the particle must be at least the size of a grain of sand. A
particle the size of a grape would be a spectacular object, and may
even cast shadows on the ground. Such bright meteors are given the name
of fireballs. The image here (left, courtesy of the Dutch Meteor Society) shows such a fireball.
to the Earth's motion, the best time to see meteors is after midnight,
since before this time, only those objects which are travelling faster
than the Earth can catch up and fall through the atmosphere. After
midnight, the night time sky faces the direction of the Earth's motion,
and in effect "scoops up" more meteors.
Meteor showers and their origins
certain times of the year, many more meteors than usual can be seen at
night. The reason for this is linked to the origins of the vast
majority of meteors: comets. As a comet orbits near the sun, it loses
material, the majority of which is ejected in the tail. The dust and
rock particles ejected from the comet are spread behind it in a trail
which follows the orbit, as shown on the diagram (left, courtesy of
Cambridge University press). At certain times of the year, the Earth
crosses these dirt-laden paths, and the particles of material which
they contain are swept up by our planet's atmosphere to appear in the
sky as a shower of meteors. The dates of these showers are accurately
known, and astronomers can prepare for them in advance. Often, the
comet responsible for particular showers is known: for instance, both
the Eta Aquarid shower (which is at its height around May 4th) and the
Orionid shower (October 20th) are both caused by the trail of debris
left by Halley's comet. The name of each shower comes from the position
of its radiant. Radiants are an effect of perspective - the
effect which makes railway tracks seem to meet far in the distance. If
the meteors from a particular shower are plotted on a map, and a line
drawn through the tail and continuing on behind the meteor, all the
lines appear to converge at a particular point on the sky. This point
is called the radiant, and is shown in the figure below.
shower is named after the constellation in which the radiant lies -
such as Gemini for the Geminid shower (December 13th). In the case of
the Eta Aquarids, the radiant is found to lie close to the star Eta
The movie presented here (courtesy of NASA) shows a fireball of the 2003 Perseid meteor shower.
majority of meteors seen in our skies, including probably all those in
the showers, are from these cometary trails, and most - even the very
bright ones, are caused by particles very small in size (from dust
grains to pieces a few centimetres across). These particles are,
generally, too small to survive the journey to the surface of the
Earth, since they are burned away too rapidly. However, much larger
chunks of material do land, and it is believed that these meteorites
are from a different source. Rather than a by-product of the passage of
a comet, meteorites probably originate from the asteroids. Asteroids
may collide with each other, and with other pieces of space debris.
This may break pieces off the asteroid, and in some very severe cases,
the asteroid may shatter completely. The result is a cloud of "rubble"
- large pieces of rock and metals from the original asteroid which may
wander into the path of the Earth and fall through the atmosphere. Far
more rare than a "normal" comet related sighting, the resulting meteor
is remarkably bright - again, capable of casting strong shadows.
Although a great deal of the object will be burned away on its passage,
some may still survive, and the resulting meteorite provides scientists
with a mine of information about the composition of asteroids, and also
that of the early solar system when these objects formed. Although
travelling at great speed, recordings of injuries from meteorite falls
an object fell to earth on August 14th, 1992 in Mbale, Uganda. The
meteorite was made of stone and broke up on its fall, scattering debris
over an area of around 3 x 7 km. At least 48 separate impacts were
found, and it is thought that the original object must have had a mass
of around 1000 kg, and so far, 150 kg of fragments have been found. The
image here (left, courtesy of the Dutch Meteor Society) shows the
largest fragment recovered.
most meteorites almost certainly came from the asteroids, there are
some exceptions. Some are thought to have come from comets, and others
have a composition which is very close to the rocks which are found on
the Moon, which suggests that some of the impacts from objects which
caused the craters on the Moon's surface, threw pieces of the lunar
surface off, and some of these pieces ended up falling into the Earth's
atmosphere. At least seven other meteorites in collections are of a
type which match closely the materials found on the planet Mars, also
probably as a result of impacts on the surface of that planet. A
martian meteorite is shown in the image on the left (courtesy of LANL).
The composition of meteorites
to their asteroidal origins, meteorites provide us with information
about the composition of their distant parents. The objects which fall
to Earth are very diverse in their composition and appearance,
reflecting the different materials in the original bodies.
Meteorites are generally divided into three classes according to their composition:
This is the most commonly encountered meteorite. The majority of the
material (90%) is iron, with a smaller amount of nickel mixed in. The
meteorites which fell at Barringer, Arizona, and Wolf Creek, Australia,
were both irons. You can click here to see an image of a slice through an iron meteorite.
are the most common type to fall; however, because they are very
similar in composition to native Earth-rocks, they are difficult to
spot, and so this class is not the most common type to find. Some stony meteorites contain small glassy spheres called chondrules, and objects with these spheres are known as chondrites, a sub-class of the stony meteorites. (Stony meteorites without the spheres are called achondrites). This image
shows a fragment of the meteorite which fell in Peekskil, New York,
USA. This is the same meteorite which was captured on video and was
presented above. (Photo courtesy of AstroMall).
Stony Irons are the final major class of meteorite, and contain small pieces of stone embedded in a body of iron. The image
here shows a slice of a meteorite of this class. The meteorite fell at
Esquel Chabut, Argentina, and is pictured courtesy of AstroMall.
Impacts with the Earth
we are fortunate that the vast majority of debris from space which
enters the Earth's atmosphere is small - so small that it rarely
survives the fall to the ground. A very small fraction of the pieces do
make it all the way, and are recovered as meteorites, usually
causing no more damage than a hole in a house roof (as in the image on
the right, courtesy of the Dutch Meteor Society), or a small pit in the
things were not always this way; throughout the history of the Earth
our planet has been hit by very much larger objects which have had a
profound effect on its evolution, and in many cases have left visible
scars on the surface. For example, some scientists believe that the
disappearance of the Dinosaurs about 65 million years ago was caused by
the impact of a large asteroid in an area now covered by the Indian
Ocean - although this is by no means the only theory for their demise.
Theories for the creation of the moon also include one in which the
early Earth was hit by a huge body - possibly planet-like in size,
which caused a large piece to fracture off our planet and form the
satellite which we see today (again, this is only one of many theories
put forward to explain the existence of the Moon).
photographs of the Earth show a surface remarkably free from the
craters which scar the surface of our satellite, this appearance is
deceptive. Whilst the Earth's atmosphere does shield it from the
smaller bodies, it is inadequate to prevent large pieces of material
from hitting the surface; the reason for an apparent lack of craters on
our planet is erosion. Whilst the moon is a relatively quiet and
inactive body, the Earth has always been far more restless. Craters
from long ago can be "wiped off the face of the Earth" by the action of
the oceans, weather, volcanoes, earthquakes and other large scale
events. However, there are several impact sites still identifiable.
These are shown on the chart below.
World crater sites - courtesy of the Canadian Geological Survey
are the major meteorite falls of today so small in comparison with
these apocalyptic events? Early on in the history of the solar system,
the pieces of debris which caused major impacts were far more common.
The surface of the moon, pock-marked with huge craters, is evidence to
the fact that the planets went through a period of very intense
bombardment millions of years ago. Through time, the space around the
planets has been "swept clear" of the majority of these objects, and so
large events are more rare. However, the scars from some of these
events are still visible, as shown in the images below, also from the
Canadian Geological Survey.
Wolfe Creek, Western Australia.
than 0.3 million years old, this crater has a diameter of 0.9 km. You
can also see an image of a fragment from the Wolf Creek meteorite.
Manicouagan, Quebec, Canada.
Around 214 million years old, this crater (pictured here from the Space Shuttle) has a diameter of about 100 km.
Barringer, Arizona, USA.
Possibly the most famous terrestrial impact crater, Barringer is 1.2 km across, and is about 49,000 years old.
has also been suggested that there are regular periods of bombardment.
By looking at the rocks from different depths in the Earth's crust,
scientists find layers of debris which indicate massive planet-wide
events. The depth at which these debris layers are found is an
indication of the date of the event, and some scientists belive that
these impacts occur at regular intervals. They have suggested that this
may be caused by a companion star, called Nemesis, orbiting the
Sun every 26 million years. This star periodically disturbs the Oort
cloud, and sends many comets hurtling in to the centre of the solar
system, so that impacts between planets and large meteorites become far
perhaps the most intriguing possibility is the one put forward by some
scientists who look into the origins of life: some believe that the
seeds of the life found on Earth today could have been carried here on
some unknown meteorite, millions of years ago.
An artists impression of the impact of a large meteorite on the Earth. (Courtesy of Meteor Crater Enterprises)