the early 1600's, the astronomer Johannes Kepler formulated laws which
described the shape and size of the planetary orbits. However, these
laws still did not describe why the planets in our solar
system were found at precisely the distances from the Sun that we
observe. However, in the latter half of the eighteenth century, the
German astronomer Titius discovered a curious mathematical pattern:
take a sequence of numbers 0, 3, 6, 12, 24, and so on, each number
being twice the previous. Then add four to every number, and finally
divide each by ten. The result is another series of numbers: 0.4, 0.7,
1.0, 1.6, 2.8, 5.2, and so on. This is a remarkable series, because
these numbers correspond very closely with the distances of
the planets from the Sun in astronomical units. Apart, that is, from
the number 2.8. There was apparently a gap in between Mars and Jupiter
where the series predicted there should be a planet, but where astronomers found none.
a search was begun for this "missing" planet, but to no avail. Then, on
January 1st 1801 the Sicilian astronomer Guiseppe Piazzi discovered a
"star" which moved from night to night - a clue that the object was not
in fact a star, but belonged to the solar system. After studying its
motion, it was found that this new object, named Ceres lay at
precisely the distance suggested by the Titius-Bode law. Soon
afterwards three other similar objects were discovered at the same
distance - they were named Pallas, Juno and Vesta. However, studies showed that in comparison to the planets, these were small objects. The asteroids had been discovered. Today many thousands of asteroids (also called minor planets)
are known and catalogued, with more being discovered each year - but
the total mass of all the asteroids is still less than the mass of the
Moon. The diagram below shows the position of the asteroids in the
solar system. These minor planets occupy the asteroid belt.
Courtesy of the Los Alamos National Laboratory
was once believed that the asteroids were the debris of a large planet,
orbiting between Mars and Jupiter, which had suffered a major
catastrophe and fragmented into many smaller parts. However, it is now
thought that no planet could ever have formed in this zone, because the
strong gravitational influence of the newly formed Jupiter would have
prevented the smaller planet from forming. Instead, the "building
blocks" of rock, built up through collisions with the smaller particles
present at the formation of the solar system, were left, and are what
we call the asteroids today, with sizes ranging from the largest
(Ceres, 1,000 km in diameter) to objects with diameters of 100 km and
objects may have collided with others through history, fragmenting yet
further. Impacts with very small objects would have broken up the
surfaces of the asteroids into very fine particles called regolith,
and because the asteroids have a very weak gravitational pull, this
layer of material may be very thick, and easily cratered from further
collisions. The photograph of the asteroid Gaspra at the top of this
page clearly shows the pock-marked surface.
The influence of Jupiter
the asteroids orbit in the gap between Mars and Jupiter, it is not
surprising that the massive planet affects them. Astronomers in the
1800s noticed that the asteroid belt has gaps in it, particularly at
distances of 2.5 and 3.28 astronomical units from the Sun. The
astronomer Daniel Kirkwood explained these gaps by considering the
orbit which a body at this distance would have. He discovered that any
asteroids in these gaps would be lined up with Jupiter very often, and
so it would be pulled by the gravitational influence of the planet, out
of the gap. For this reason, these are now called the Kirkwood gaps,
and now there are several known. This, however, is not the only effect
which the largest planet in our solar system has on these small
are placed into groups according to the type of orbit which they have;
and some of these groups are directly linked to Jupiter. These classes
are given below.
These asteroids "share" the orbit of Jupiter, and are found ahead, and
behind, the planet in its orbit. Several hundreds of such asteroids are
known, and it is also believed that there may be similar groups
accompanying Mars, Venus, and even the Earth.
Apollos. Such asteroids are within the Earth's orbit when they pass closest to the Sun (a point called perihelion, and may pass across the Earth's orbit.
Atens and Armors are also classed according to their orbits, which also cross the Earth's.
are also named groups within the main asteroid belt. In each case, the
group is named after the largest asteroid of that particular class.
recently, all observations of asteroids had to be done from Earth,
using optical telescopes to plot the way their brightness changed as
the objects tumbled in space. Today, using radio telescopes and, more
recently, space probes, more is being revealed about their nature.
Ida and Dactyl
Ida with moon Dactyl (courtesy of LANL).
asteroid Ida was imaged by the Galileo spacecraft on August 28th 1993,
approaching within 2400km of the surface. The object measures around 56
x 24 x 21 kilometres, and makes one rotation in just 4.5 hours. The
heavily cratered surface is clearly visible. The most interesting
feature of this asteroid is that it has a moon, called Dactyl, the first satellite of an asteroid ever discovered.
The satellite Dactyl (courtesy of LANL).
satellite measures around 1.2 x 1.4 x 1.6 km across, and is made of the
same material as the main asteroid. Perhaps it was created when Ida was
hit by another object, with pieces being broken off - or perhaps both
Ida and Dactyl are just fragments of a larger body shattered by a
collision long ago.
Radar map of Toutatis, courtesy of LANL.
was discovered in 1989, and its orbit takes the minor planet from the
main asteroid belt, to within the Earth's orbit - so it is called an
Earth-orbit-crossing asteroid. In December of 1992, it passed within 4
million km of the Earth, and at this time radar images like the one
above, were made using powerful transmitters in the Californian Mojave
desert. These images show craters in the surface; two in particular are
4 and 2.5 km wide. The asteroid has a very peculiar shape, and is also
"tumbling" in space. This indicates that at some time, the asteroid was
subject to very severe collisions. (Most asteroids rotate on a single
axis, like the major planets). This peculiar rotation can be seen in
the animation presented here (Courtesy Scott Hudson, Washington State University).
11 views of the approach to Gaspra, courtesy of LANL.
was discovered in 1916, and it became particularly interesting to
scientists when it was found that the Galileo spacecraft, en route to
Jupiter, would pass close by. This event occured on October 29th, 1991
when the spacecraft approached within 1600 km of the object. Craters
are clearly visible, but all are small. The asteroid is believed to be
made of metal-rich material.
image presented here shows the view of Gaspra from Galileo as the
spacecraft drew closer in to the object, and span 5 hours 15 minutes
from first to last image. Since the rotation period of Gaspra is about
7 hours, these images show the asteroid through almost a complete
revolution. Note that a second image of Gaspra is presented at the top
of this page. The colours in the image have been enhanced to show
variations in the brightness of the surface, which is reflecting
sunlight (the Sun is shining from the right). It is believed that the
brighter areas are fresh rock exposed by comparatively recent
collisions, with darker areas being a much older surface.
Vesta, as seen by the Hubble Space Telescope (courtesy of LANL).
is an interesting asteroid, because of the light and dark patches
clearly visible on the surface. The movie presented here (courtesy of
LANL) shows the asteroid viewed through the Hubble Space Telescope over
one rotation period. It has a diameter of about 525 km. The surface is
thought to consist of a deep impact crater, and old lava flows. This
suggests that asteroids may at one time have had molten cores. It is
believed that when Vesta was created, by smaller pieces of material
colliding and "sticking" together, some of the material was
radioactive, probably the result of a supernova explosion in a nearby
region. This hot, radioactive material may have caused the core of the
asteroid to melt. Today, the activity has ceased and Vesta is now
the most interesting point to note in connection with Vesta is the
possibility that we already have a piece of it on Earth to study. In
October 1960, a fireball (a very bright meteor) was observed in
Australia. Ten years later, fragments of the meteorite were found. The
meteorite fragments were chemically analysed, and were found to be
remarkably similar to the material found in Vesta. (Astronomers can
find out about the material an object is made of by breaking down the
light into its individual components - the same effect that causes
rainbows in the sky and multiple colours on the surface of a compact
disc). So the Australian meteorite (pictured above, courtesy of LANL)
was probably one of a shower of pieces "chipped" off the surface of
Vesta by collisions with other objects, and in this instance, was
travelling in just the right direction to reach the Earth.
Collisions with Earth
the history of the solar system, the Earth and other planets have been
subjected to impacts from smaller bodies such as comets and asteroids -
sometimes with catastrophic consequences (for instance, it is believed
that many species have been wiped out as a result of the effects of
such impacts; the dinosaurs may well have disappeared following such an
event). Whilst this subject is dealt with in more detail in the meteors
section, it is worth mentioning that the majority of meteors which are
large enough to survive and hit the surface of the Earth (when they are
referred to as meteorites), probably originate from asteroids which have been broken up or chipped by collisions with other objects.