Asteroids

Introduction

In 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.

Understandably, 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

Origins

It 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 less.

These 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

Because 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 objects...

Asteroid groups

Asteroids 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.

Trojans. 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.

There are also named groups within the main asteroid belt. In each case, the group is named after the largest asteroid of that particular class.

Interesting asteroids

Until 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).

The 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).

The 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.

Toutatis

Radar map of Toutatis, courtesy of LANL.

Toutatis 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).

Gaspra

 

11 views of the approach to Gaspra, courtesy of LANL.

Gaspra 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.

The 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

 

Vesta, as seen by the Hubble Space Telescope (courtesy of LANL).

Vesta 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 relatively unchanging.

Perhaps 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

Throughout 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.