No book about planets would be complete without a discussion of asteroids, because these are the most common objects to hit planets in the inner Solar System (where asteroid impacts are about ten times more common than comet impacts). In addition, the largest asteroid, Ceres, is offi cially classifi ed as a dwarf planet.

Shapes, sizes, and compositions

Ceres is the goal for NASA's　Dawn spacecraft, which will spend fi ve months orbiting it in 2015, having already spent the year beginning in July 2011 at Vesta, the second most massive asteroid.

A few smaller asteroids have already been visited by spacecraft, providing images ( Figure　25 ) that confi rm their irregular shapes. Visualize a pock-marked potato scaled up to any size between tens of metres and a few hundred kilometres, and you should have a serviceable mental image of a typical asteroid. Telescopically observed periodic variations in asteroids'brightness show that mostly they take only a few hours to rotate. Generally, rotation is at right angles to their length, so they rotate like sausages twirled on a cocktail stick.

About 1 asteroid in 50 probably has its own satellite, and it was lucky that Ida, the second asteroid to be visited by a spacecraft when Galileo fl ew past in 1993, turned out to be one of these. That was the fi rst confi rmed discovery of an asteroid satellite, but subsequently many more have been found using advanced telescopic techniques, such as adaptive optics to compensate for the shimmering of the Earth’s atmosphere. Asteroid satellites range from the comparatively tiny up to sizes similar to the main asteroid. In fact, the asteroid named Antiope appears to consist of two mutually orbiting bodies of indistinguishable 110-kilometre size, whose centres are only about 170 kilometres apart. So far,there are two asteroids known to possess two small satellites each.Some asteroid satellites may be fragments from a collision, and others may be captured objects. Neither case is readily explicable,because it is hard to end up with objects orbiting rather than fl ying apart.

25.Images of asteroids at different scales. Top: Ida, a 54-kilometrelongmain belt asteroid, with its tiny satellite Dactyl to its right.Lower left: Eros, a 33-kilometre-long near-Earth asteroid. Lowerright: Itokawa, a 0.5-kilometre-long Earth-crossing asteroid. Thereare many impact craters visible on Ida and Eros, but the much smallerItokawa is boulder-strewn

Asteroid densities have been measured between 1.2 and 3.0 g/cm 3 .However, stony meteorites, which are clearly bits of asteroid, have densities of about 3.5 g/cm 3 and stony-iron meteorites have densities close to 5.0 g/cm 3 , so none of the measured asteroids can be an intact solid body. Rather, they must be porous rubble piles.Some, such as Itokawa, visited by the Japanese probe Hayabusa in 2005 ( Figure 25 ), and others whose shape has been determined by radar, appear to be ‘contact binaries’ consisting of two main masses joined at a narrow waist. However, the numerous boulders on much of Itokawa’s surface show that the two main masses are themselves composed of many pieces.

Asteroids are not strongly coloured, but can be grouped into several classes according to their refl ectance spectrum. There are three main types. S-types have the characteristics of silicate rock,and are evidently the same material as stony meteorites. They make up the majority of asteroids with orbits between about 2.0and 2.6 AU from the Sun, whereas from 2.6 to 3.4 AU, C-types,having the characteristics of carbonaceous chondrite meteorites,are the most common. Beyond 3.4 AU, asteroids tend to be dark and somewhat red in colour. These are dubbed D-type, and may be coloured by a tarry surface residue formed from carbonaceous material during prolonged exposure to solar radiation (space weathering). These tarry substances are usually referred to as‘tholins’, a term coined from the ancient Greek word for ‘mud’ by the American astronomer Carl Sagan (1934–96).

Scattered here and there are asteroids that appear largely metallic(M-type), clearly related to iron meteorites, and a few that appear to have basalt on their surface, notably Vesta, from which they take their designation V-type. These, or their now fragmented parent body, may have once been hot enough for internal melting and volcanic eruptions.

Asteroid orbits

Most known asteroids (equivalent to about 4% of the Moon’s mass)have orbits lying between the orbits of Mars and Jupiter, in the so-called ‘asteroid belt’. Over 3,000 main belt asteroids have been documented. More than half the total mass of these resides in the four largest examples, Ceres, Vesta, Pallas, and Hygeia, with diameters of 950, 530, 540, and 430 kilometres respectively (Vesta is denser than Pallas, so is more massive despite being slightly smaller).Undiscovered objects range down in size through individual lumps of rock to dust particles. Nevertheless, the asteroid belt is virtually empty space, and you should not think of it as replete with jostling rocks. All space probes that have been sent through the asteroid belt have survived without mishap, and even have to be steered carefully to come close enough to any asteroid to study it in passing.

Jupiter’s gravity has considerable infl uence on main belt asteroid orbits. Notably, it prevents asteroids settling into orbits whose periods would be in resonance with its own. There are virtually no asteroids whose orbital periods are simple 4:1, 3:1, 5:2, or 2:1 ratios of Jupiter’s. These correspond to average distances from the Sun(orbital semi-major axes) of 2.06, 2.50, 2.82, 3.28 AU,respectively,which are known as the Kirkwood gaps, after Daniel Kirkwood, an American astronomer who discovered and explained them in 1886.Not all orbital resonances are unstable with respect to asteroid orbits, and in fact there is a small family of asteroids whose orbital periods are two-thirds that of Jupiter (a 3:2 orbital resonance).

There are a great many more asteroids with the same orbital period as Jupiter. There may be more than a million of these greater than 1 kilometre in size, with a combined mass about one-fi fth that of the main belt. These occur only close to locations60° ahead of, and 60° behind, Jupiter in its orbit. Those are special places where the combined gravitational force from the Sun and Jupiter allows small objects to orbit stably, and are known as the leading and trailing Lagrangian points. Asteroids in these orbits are by convention given names of heroes from the Trojan War (Greek names 60° ahead of Jupiter and Trojan names60° behind), but are collectively referred to as ‘Trojan asteroids’.

We're all doomed!

A few asteroids are known in similar ‘trojan’ relationship to Mars,but Earth has no trojan companions. However, there are asteroids whose orbits cross ours, known as Earth-crossing asteroids. If you are worried about collisions, this may sound alarming, but asteroid orbits tend to be inclined to the ecliptic, so they almost always pass either ‘above’ or ‘below’ us when they cross our orbit.Only a subset of Earth-crossers are regarded as Potentially Hazardous Asteroids (PHAs), being those that can pass within0.05 AU of the Earth (a range suffi ciently close that perturbations caused by various third bodies could bring about a collision) and that are larger than about 150 metres in diameter (big enough to survive passage through the atmosphere with undiminished speed). By the end of 2009, about 1,100 PHAs had been documented, plus fewer than 100 Potentially Hazardous Comets.

The closest calculated approach by a PHA is by Apophis (350metres long) that will come very close on Friday 13 April 2029.Soon after its discovery, in 2004, its orbit was suffi ciently poorly known that there was a chance (estimated at 2.7%) of a collision,but subsequently a longer series of observations showed that it will pass safely about 30,000 kilometres above the surface. It will be back again on 13 April 2036, and because we do not know exactly how close it will pass in 2029, we do not know exactly how much its trajectory will be affected by the Earth’s gravity during that encounter. However, the chances of collision in 2036 are vanishingly small.

An asteroid that penetrates Earth’s atmosphere with undiminished speed is very dangerous. On hitting the ocean, it could cause a tsunami, and if it hits land, it excavates a crater much larger than itself and devastates the surrounding area.A 2.2-million-year-old, 130-kilometre crater named Eltanin has been discovered under the fl oor of the Bellinghausen Sea, in the southernmost Pacifi c ocean, apparently caused by an asteroid several kilometres in diameter. This would have barely been slowed by the ocean, let alone the atmosphere, before striking the sea bed. According to computer models, the resulting tsunami would have devastated the coast 300 metres above sea-level in southern Chile and 60 metres above sea-level in New Zealand.The quantity of water and dust thrown into the atmosphere might even have been the trigger for climate change leading to the migration of our ancestors, Home erectus , out of Africa, at about this date. The most recent collision between the Earth and a10-kilometre ‘dinosaur-killer’ asteroid happened 65 million years ago, creating the 200-kilometre-diameter Chicxulub crater, now buried beneath sediment in the Yucatan peninsula of Mexico. This caused a global environmental upheaval that is widely credited as the cause of a ‘mass extinction event’ when about 75% of species of life on Earth were wiped out.

Catastrophes of that magnitude are mercifully rare, but statistics show that asteroid impacts rank alongside volcanic eruptions,earthquakes, and extreme weather events as potential causes of death. A 1-kilometre asteroid able to devastate coasts 3,000kilometres from the point of impact strikes the ocean on average about every 200,000 years, whereas a 200-metre asteroid with a signifi cantly smaller tsunami danger radius might be expected about every 10,000 years.

To categorize the hazard posed by each PHA, astronomers use a numerical system called the Torino Scale (agreed at a meeting in Turin, hence the name). This combines the energy that would be delivered and the probability of collision into a single number from 0 to 10, where 0 means negligible chance of collision and/or too small to penetrate the atmosphere, and 10 is certain impact by a ‘dinosaur-killer’ causing global catastrophe. Most PHAs exceeding 150 metres in diameter are ranked either 0 or 1 when they are discovered, and the 1s are usually downgraded to 0 when their orbit has been more adequately determined. Apophis holds the record for having temporarily held a Torino rating as high as 4(‘Close encounter, meriting attention by astronomers; 〉1% or greater chance of collision capable of regional devastation’), but was downgraded to 0 in 2006.

A semi-formal linking of observatories known as Spaceguard has assumed the task of locating and categorizing all PHAs. This is important because, unlike most sorts of natural disaster when all we can do is mitigate the effects, it would be possible to prevent a collision by a PHA. To achieve this, it is necessary to change either the PHA’s speed or its direction of travel. The longer in advance this is done, the smaller the required change. There are various ways to do it, ranging from the brute-force method of equipping the PHA with a rocket motor, to the more subtle ploy of coating of one side in a refl ective substance so that solar radiation-pressure does the job for you. Using a nuclear bomb to blast apart an incoming PHA is not a smart idea, because unless you could guarantee that all the fragments would be too small to penetrate the atmosphere, you might make the problem worse by causing multiple impacts.

Asteroid mining

There is a silver lining, in that asteroids could be valuable sources of raw materials. A 1-kilometre M-type asteroid contains more nickel and iron than the world’s annual consumption, and the most massive example, Psyche, contains enough to last for millions of years. Asteroids, especially M-types, also contain precious metals like platinum.

The investment to begin mining the fi rst asteroid would be very great, but the potential returns are immense too. It remains to be seen whether the main value of asteroids turns out to be supply of raw materials to Earth or to space-based industries. Some near-Earth objects are probably defunct comets with remnant water-ice surviving beneath their dusty surfaces, which could be valuable as propellant and radiation shielding, as well as for drinking.

Names and provisional designations

By 1891,332 asteroids had been discovered visually, but photography had boosted the tally to 464 within 10 years. There are now over 100,000 known objects of all types, each of which needs to be identifi ed in some way. The IAU oversees a system of provisional designations given to each new discovery. This consists of the year of discovery plus a two-letter code coupled with numerical subscripts, corresponding to the date and sequence of discovery. The fi rst letter (A–Y, omitting I) specifi es which half-month the discovery was made in (A for January 1–15, B for January 16–31, and so on, up to Y for 16–31 December), the second letter (A–Z, omitting I, which gives 25 options) is awarded to each discovery in sequence, and a numbered subscript allows the cycle of 25 to be repeated as many times as necessary. Thus,2011 BA would be the fi rst body discovered in the period January16–31 2011; 2011 BB would be the 2nd; 2011 BA 1 would be the26th, and so on. When an object’s orbit has been well determined(which may take several years), it can be awarded a permanent name, which replaces the provisional designation. For example,Apophis originally had the provisional designation 2004 MN 4(signifying the 113th discovery during 16–30 June 2004).

The privilege of suggesting a permanent name is given to the discovery team, though some automated surveys reveal so many new objects that managers are glad of suggestions. Permanent names are a name preceded by a number, added in sequence as each new name is added. So formally we have (1) Ceres, (4) Vesta,(99942) Apophis, and so on. Available mythological names are too few for all these objects, and pretty much anything is allowed,except that names must be inoffensive and unconnected with recent political or military activity. I know several astronomers who have had asteroids named after them (by colleagues; you can’t name one after yourself ), and there is one called (5460)Tsenaat’a’i, which means ‘fl ying rock’ in the Navaho language. The only asteroid that I have had a hand in naming is (57424)Caelumnoctu, named in 2007 to commemorate the 50th anniversary of BBC television’s long-running programme The Sky at Night , which in Latin is Caelum Noctu . We picked it from a list because its number refl ects the date of the fi rst broadcast, which was 1957 April 24 (57/4/24).