Monday, 13 February 2012

Asteroids – Vermin Of The Skies

Asteroid Belt Data
Innermost edge: 2.0 AU
Outermost edge: 3.3 AU
Orbital period at innermost edge: 3.2 years
Orbital period at outermost edge: 6.0 years
Known population: -22 000
Estimated population: 500 000 larger than 1.6 km
Estimated total mass: 4.8 x 1021 kg or 0.0008 Earth’s
Composition of asteroids: rock and metal
Largest asteroid: Ceres, diameter 900 km

Between the orbits of Mars and Jupiter is a cosmic dustbin. This is the realm of the asteroids – potatoshaped chunks of rock and metal, heavily cratered, most of them just a few metres across. For much of the time since the first asteroids were discovered, 200 years ago, astronomers have regarded these objects as the ‘vermin of the skies’. Often, one of them would pass in front of a more interesting object being photographed, the asteroid’s trail as it moved across the field of view marring the result. But lately, astronomers have learned much more about the asteroids – also known as minor planets – than ever before. They are now seen not so much as vermin but as keys to the Solar System’s past.

The Asteroid Belt

Since the first asteroid was found in 1801, astronomers have discovered and mapped the orbits of well over 10 000 more. As more and more were found, it became evident that the great majority of them – about 90 to 95 per cent – lie between the orbits of Mars and Jupiter. This region has become known as the asteroid belt. In total, the asteroid belt may harbour a million stony and metal fragments.

When an asteroid orbit is calculated it is numbered sequentially, and the number becomes part of the asteroid’s official designation. Thus, the first asteroid to have its orbit mapped is called 1 Ceres. Ceres is easily the largest asteroid at about 900 kilometres across or one-quarter the diameter of the Moon. In fact it is the only one that truly deserves the term ‘minor planet’. Ceres is more than twice the diameter of the next largest asteroid, and contributes about one-third of the total mass of all the asteroids combined. Fewer than 20 are larger than 250 kilometres, and most (of those so far found at least) are only about the size of a house or a car. Only the largest asteroids, bigger than about 150 kilometres, are expected to be round. The rest are too puny for their gravities to pull them into spherical shapes. Close up pictures of some asteroids, taken during recent spacecraft missions, confirm this. They also reveal heavily cratered and fractured surfaces, and it seems likely that all of today’s asteroids are the product of collisions. With so many asteroids in the belt, close encounters are no doubt relatively frequent. But the romantic image of a vast field densely strewn with madly spinning boulders, popularised by science-fiction films, is very inaccurate. On average, great distances of tens of millions of miles separate each minor planet, and their spin periods can stretch to weeks – though 10–20 hours is more usual.

Asteroids generally orbit the Sun in more elongated or eccentric orbits than those of the planets. Moreover, asteroid orbits are fairly inclined to the ecliptic, the general plane in which the planets encircle the Sun. On average their orbits form an angle of about 10 degrees relative to the ecliptic, but some of them have inclinations as high as 30 degrees. With their high inclinations, the asteroids actually populate a three-dimensional region of space – more a donut than a belt.

The belt extends from roughly 2 to 3.3 AU. Those closer to Mars take about 3 years to orbit, and the most distant belt asteroids take about twice that. Aside from the differences in orbital timescale across the belt, there is also a marked trend in composition. The innermost asteroids, less than about 2.4 AU from the Sun, are mostly stony. They are designated as S-class. They have generally bright surfaces that reflect up to 15 per cent of incident light. The outermost regions of the belt are generally made up of much darker asteroids. These C-class minor planets have a high carbon content and are exceedingly black – darker than soot or blackboards. The moons of Mars, Phobos and Deimos, are captured C-class asteroids. C-class asteroids are also found in the middle of the belt, from 2.5 to 3 AU, along with a third major type, the metal M-class objects. This trend in composition with distance no doubt reflects the different materials that condensed in the Solar Nebula at different distances.

Other Asteroids

Not all asteroids populate the main belt. Some range much nearer the Earth. The so-called Apollo asteroids, for example, which are mostly all smaller than about 5 kilometres, actually cross the Earth’s orbit. The Aten asteroids are another group that also cross paths with the Earth. Together, the Apollo and Aten asteroid groups – each named after a single prototype asteroid – are known as near-Earth objects, or NEOs. They, more than any other asteroids, pose a constant threat to life on our planet. Other asteroids have even smaller orbits that hug the Sun, coming well inside the orbit of Mercury. The famous prototype is 1566 Icarus. At its closest approach to the Sun, Icarus’ surface temperature soars to 500 Celsius or more.

Other asteroids are found much further out in the Solar System. The Trojans are a group that share Jupiter’s orbit. They remain in the same position relative to Jupiter, 60 degrees ahead of and 60 degrees behind the planet, trapped there as a result of the combined gravity of Jupiter and the Sun. And even further afield, some asteroids roam as far as Uranus.

History of the Asteroids

Irrespective of where they are found, however, all asteroids share a common origin. Astronomers once considered the asteroids to be the remains of a planet that got pulverised in some giant collision. But the combined mass of all minor planets is estimated to be only about 0.08 per cent of the Earth’s mass, or 5 per cent of the Moon’s. This is far too little material to make a world even as puny as Pluto. Evidently the asteroids are not the fragments of an exploded planet. It seems more likely that they are the remains – and evidence – of the process that built our Solar System.

Billions of years ago, the first fragments to emerge from the rubble pile that was the Solar Nebula were the planetesimals and then the planetoids. As we have seen, these then gradually lumped together, through gravity, to form the planets as we know them. But the asteroids, it seems – as well as the comets, which we shall meet later – are testament to the fact that the planet-building process did not entirely finish. One of the reasons for this was the gravitational influence of massive Jupiter. Astronomers are fairly happy with this explanation because, even today, there are apparent zones in the asteroid belt where many members have been thrown out by Jupiter’s gravity. These so-called Kirkwood gaps coincide with special orbits that are said to be in resonance with Jupiter’s. The ratio of the orbital period of an asteroid in a Kirkwood gap to that of Jupiter is an exact fraction such as 1 : 2 or 2 : 3. This means that an asteroid in a resonance orbit of 2 : 3, say, orbits the Sun three times in exactly the same time it takes Jupiter to go around twice. So, every three orbits of the asteroid – and every two orbits of Jupiter – the asteroid and the planet end up in the same relative positions, and the asteroid gets a regular gravitational kick from Jupiter, each time at the same point in the asteroid’s orbit. It is exactly like pushing a child on a swing. If you push at the right time, the swing gains energy and can be made to go very high. In the same way, Jupiter’s gravity ‘pushes’ the asteroids in the Kirkwood gaps at just the right time, and the energy gain makes their orbits wildly unstable. Ultimately they escape from the belt altogether or are flung towards the Sun. This very process enabled Jupiter to depopulate the Solar Nebula where it was forming. In addition, the gravitational perturbations made the planetesimals collide too quickly to stick together, and they broke apart instead. Even today, the asteroids that remain cannot coalesce to form a single planet because of these continuing perturbations.

The largest two asteroids, 1 Ceres and 4 Vesta, are complete planetoids, with ancient surfaces. Other asteroids have obviously been in collision, planetesimals that fragmented. Some even have little moons – most likely bits that were broken off by impacts.

In general, most asteroids have changed little in billions of years. They are among the oldest pieces of the Solar System, and valuable evidence of its traumatic past. They owe their continued existence almost entirely to the presence of Jupiter, our next stop as we move away from the Sun.

Source :
Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.



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