This week’s guess the planet image is a type of solar system object we’ve not covered before. This is an image of the Asteroid 234 Ida. It comes from a close approach by the NASA Galileo spacecraft in 1993, and credit goes to the NASA Jet Propulsion laboratory. More information about how this image was acquired can be found at this link. Interestingly the Galileo team didn’t know precisely where the asteroid was located prior to the close approach, so this image is made up of five frames out of 30 which were taken in this region of space, to make sure that the asteroid was imaged.

Asteroids are found throughout the solar system, but one of the largest groupings is the asteroid belt between Mars and Jupiter, which is where 234 Ida is located. This is quite an interesting region of space, as it contains a very large number of bodies, including the dwarf planet Ceres.

The asteroid belt was discovered as a result of a peculiar phenomenon called the Titius-Bode Law. During the eighteenth century astronomy was taking off. One issue that intrigued the early astronomers was whether there were other planets beyond the six which had been known since antiquity. They were looking for patterns in the skies, and a German astronomer named Johann Daniel Titius seemed to have found one.

He observed a pattern in the distribution of the known planets, namely that each planet was approximately twice as far from the sun as the previous one. He was able to derive a mathematical relationship which seemed to describe the precise positions of the planets, but there was a gap in the sequence.

Titius’ model predicted that there should be a planet in what is now known to be the orbit of the asteroid belt. This gave the astronomers of the day somewhere to start looking, and it wasn’t long before they discovered Ceres, and later the other large asteroids. It seemed as though this law could be used to derive the locations or further planets, and astronomers began searching for further objects in the outer solar system, ultimately discovering Uranus in pretty much the location predicted by the law. Unfortunately there was no physical reason why the distribution of the planets should fit this pattern.

It wasn’t until the discovery of Neptune, in an entirely different location to that predicted by Titius, that faith in the law began to fade. The discovery of further dwarf planets, which also don’t fit the model, has provided further proof that the supposed relationship was coincidental. The Titius-Bode law has now been thoroughly discredited, but it played an important role in the discovery of some of the most interesting objects in the solar system.

It is possible that this pattern evolved from the orbital resonances caused by the interactions of the planets. Large planets influence the orbits of their neighbours, and can create regions where planets do not easily form, and stable orbits cannot occur, since the motion of objects there is gradually perturbed by contact with its neighbours.

This brings us back to the asteroid belt. It was initially thought that the asteroids might be the remnants of a planet which had been destroyed by a catastrophic impact. However this doesn’t seem to be the case. Almost all of the mass of the asteroid belt is found in the four largest bodies, and there isn’t sufficient mass to produce a large planet.

It seems more likely that the gravitational effect of Jupiter prevented the formation of a planet in this region of space, stopping the various planetesimals from coalescing into a larger body. This is supported by the variety of compositions found across the asteroid belt. Different objects are made up of different materials. Silicaceous or stony asteroids like Ida are composed primarily of rock, while others have a more metallic composition.