Reasons for studying solar system debris

Apart from the planets and moons that are well known and which can be seen from Earth with greater or lesser ease, there is a huge amount of material that also journeys around the Sun. These items range in size from objects many miles in diameter or length (they usually have irregular shapes) to small pieces of rock or even particles of dust. Apart from idle curiosity, why should astronomers bother studying these? And are there aspects of any of these objects that should concern the average non-astronomer?

It is probably best to begin by defining what these objects are in more detail.

Asteroids

These are sometimes referred to as “minor planets”. They range in size from Ceres, at 1000km in diameter, to lumps of rocks no bigger than 1km across. There are doubtless many that are considerably smaller, but their size makes them very difficult to detect. Many thousands of these have been discovered, although their total mass is probably only around 1/2000th that of our Moon. Large asteroids are rare, but smaller ones are very common, regularly showing up as streaks across photographic plates when more distant objects are being surveyed.

The bulk of the asteroids are in a belt between the orbits of Mars and Jupiter, but many have also been discovered outside this region, and with orbits that cross those of other planets, including Earth. The object known as Pluto, regarded as the Sun’s 9th planet for much of the 20th century, is now considered to be an asteroid in an orbit beyond that of Neptune, where many other similar objects have been discovered.

Comets

These are similar to asteroids in that they are relatively small objects that orbit the Sun, but they have highly eccentric orbits that take them well beyond the outer limits of the solar system but which also come close to the Sun and are often visible from Earth. They are composed of rock and ice, as “dirty snowballs” and, as a comet approaches the Sun, some of the ice melts to produce a “tail” of particles that gives the comet its distinctive appearance.

Meteoroids

This term can be used for all the smaller particles that are undetectable until they come into contact with Earth’s atmosphere. They then either burn up and appear as meteors or “shooting stars”, or, if sufficiently large to survive the passage through the atmosphere, can land on the surface as meteorites. Much of this material originates from the debris of collisions or break-ups of larger objects. For example, many regular “meteor showers” occur when Earth passes through the path of former comets that have disintegrated after getting too close to much larger objects.

So why study these objects?

One reason for our interest in the larger objects, such as asteroids and comets, is that we do not want to experience a collision with one of them. There is plenty of evidence that, at times in Earth’s history, such collisions have occurred, often with disastrous consequences. An object probably about 10km in diameter was the likely cause of the mass extinction, 65 million years ago, that caused the sudden end of the age of the dinosaurs. An object only 50 metres across was the likely cause of the Barringer Crater in Arizona, which is 1.2km across and 200m deep.

The massively destructive power of such objects is caused by the huge speed at which they travel through space and the energy they release when crashing into a solid object such as a planet. The “dinosaur” object would have released energy equivalent to the explosion of 100 million megatons of TNT.

Astronomers are therefore keeping a very watchful eye on any object that could pose a threat to life. These are defined as “potentially hazardous asteroids” (PHAs) or “potentially hazardous comets” (PHCs), and to date over 1,100 of the former and 65 of the latter have been identified. Although the chances of a collision are low, the possibility is always there; it has happened before and will undoubtedly happen again. The latest scare is over the asteroid Apophis, which is predicted to pass very close to Earth in April 2036, with an outside chance of a direct hit.

There are other reasons for studying objects that come close to us, apart from fears for our safety. This is mainly due to the fact that this material represents the primitive building blocks of the solar system, given that the objects have been whirling around the Sun ever since they coalesced from the dust and gas that eventually formed the Sun and the planets. By analysing this material we can learn a huge amount about how stars and planets form.

By studying comets it might also be possible to study material from beyond the solar system. Many comets are believed to originate in the “Oort cloud” of objects that lie just within the gravitational pull of the Sun but at a distance of up to 50,000 times the distance from Earth to the Sun (which puts them at about one fifth the distance to the next nearest star). The material that comprises these comets may well have originated at a time when the Sun was being formed as one of a cluster of stars which have since dispersed.

There is, however, plenty of “debris” material that is much easier to study because it is already with us. By this is meant the meteorites that sometimes fall to the ground and can be collected. Of particular interest are those meteorites that may have started out on other solar system bodies, such as Mars. An impact collision by a comet or asteroid, millions of years ago, could have blasted so much material into space that some of it was bound to end up on Earth. Material found in the Australian desert has been reckoned to be of Martian origin, and it is thought that up to half a ton of Martian debris lands on Earth every year. It is therefore possible to analyse rocks from Mars without the expense of actually going there.

Even without the Martian connection, meteorites can tell us a lot about the composition of the early solar system. Many of these consist largely of nickel-iron in an unoxidised form that is not found naturally on Earth. Indeed, many iron objects made by people in prehistoric times probably made use of meteorites as their raw material. Other meteorites comprise stony material, the age of which can be determined by assessing the decay of its radioactive elements. This has put their age at between 4.5 and 4.6 billion years, thus confirming their importance as markers of the age of the entire solar system.

Another fascinating area of research is into whether life on Earth could have had an extra-terrestrial origin. The inspiration for this research has come in part from studies of meteorites, and partly from studies of comets, in particular the Giotto probe to Halley’s Comet in 1986. These have shown the presence of organic molecules in the form of amino acids, which are the building blocks of the proteins that are essential to life. It has also been suggested that the water on planet Earth could have originated from countless bombardment of the young planet by comets.

It is therefore possible that further studies of this material could answer some of the most fundamental questions of science. We could eventually know where life came from and, more importantly, how it might be extinguished, by learning as much we can from the debris that is out there between the planets.