Small Bodies of the Solar System Transcript
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Small Bodies of the Solar System Transcript Date: Wednesday, 4 March 2015 - 1:00PM Location: Museum of London 04 March 2015 Small Bodies of the Solar System Professor Carolin Crawford Introduction The Solar System is a surprisingly crowded place. Swimming through space alongside the major planets are a whole host of moons, moonlets, comets, dust particles, dwarf planets and asteroids, all also in orbit around the Sun. In today’s talk I shall concentrate on some of these smaller bodies: the asteroids and the dwarf planets and how they contribute to the overall architecture of the Solar System. An asteroid is simply a space rock (containing ice and metal too). The characteristics are known of over 100,000 asteroids, and there are most likely several millions in total. With sizes ranging from a few metres to 550 km in diameter, they are shaped more like potatoes than planets, and display battered and cratered surfaces. A dwarf planet is the stepping stone between these space rocks and the major planets. Larger and more massive than the asteroids, they have sufficient self-gravity to be pulled into a spherical shape, but not enough to have cleared their own solitary path round the Sun. However, both the first asteroids and the first dwarf planets to be discovered were originally classified as planets and their role re-evaluated with time, as astronomers learned more about the contents and distribution of the Solar System. Now recognised as the remnant debris from the formation of the planets, they give us different perspective on how the Solar System came to be. Discovery of Asteroids Discovery of Uranus The six planets out to, and including, Saturn have always been known to humanity, and their movements against the background of fixed stars have been monitored with the unaided eye since antiquity. A full understanding of these motions had to wait until the Renaissance, where intuitive science from Copernicus, Brahe and Kepler established the Sun at the heart of the Solar System, and the laws that dictate the orbital mechanics of the planets. The growing understanding of the shape of the Heavens was then consolidated by the first new data that emerged with the development of the use of telescopes for astronomical observations at the start of the 17th Century. But the vision of a Solar System encompassing only six planets remained unchanged until 1781, when Sir William Herschel stumbled across Uranus as he was using his new state-of-the-art telescopes to chart the positions of faint stars. He observed an object that was extended in shape – not point-like, but showing a greeny-coloured disc – and too bright to be one of Messier’s diffuse nebulae. The clincher was that it moved relative to the stars from one week to the next. His first, and entirely logical, interpretation was that he had found a new comet. Only as he calculated its orbit from the observations made over the next few months did he realise that it followed a circular path well out beyond that of Saturn. At such a great distance, the fact that the object appeared as a disc in his telescope immediately meant that it was much larger than a comet – indeed, it was bigger than any the terrestrial planets, although not quite on the scale of the giant planets Jupiter and Saturn. Quite accidentally, Herschel had discovered the first new planet. Bode’s law One of the fascinating things about the discovery of Uranus was that the size of its orbit complied with a pattern known as the Titius-Bode ‘law’. This observes that the relative distances of each of the planets from the Sun (in practice, the semi-major axis of their elliptical orbits) obey the mathematical formula D = 0.4 + 0.3 x 2n for n = 0, 1, 2… and a D=1 corresponds to one astronomical unit (1 AU; about 150 million km), the separation of the Earth from the Sun. The only glaring inconsistency is that the law predicted that a planet should lie between Mars and Jupiter at around 2.8 AU, where none had been observed. Had something been missed? Discovery of the brightest asteroids At this point astronomers had a new hobby – who would waste time searching for comets when there might be new planets waiting to be discovered! The ‘missing’ planet between Mars and Jupiter seemed to have been found on the 1st January 1801, when the Italian astronomer Guiseppe Piazzi observed something moving within this gap, and named his discovery Ceres, after the Roman goddess of agriculture. But almost straightaway things began to get a little more complicated when the German astronomer Heinrich Olbers stumbled on another new object while studying Ceres in 1802. His discovery, Pallas, became the ninth planet. The strange thing, however, was that Pallas moved around the Sun in a similar orbit to Ceres. And the situation was further compounded by the subsequent discovery of two more planets occupying this gap: Juno, the tenth planet discovered in 1804, and Vesta, the eleventh found in 1807. But after that, and despite intense searching through the next decade, no more objects were found. The Solar System had ten planets. Like Uranus, all four objects had required the development of powerful telescopes for their discovery as they are too faint to be seen with the unaided eye. But unlike Uranus, this wasn’t because they were so far away, but because they were so small. Even with a telescope they did not appear extended, but star-like, and it was only their motion against the stationary stars that gave them away. On their discovery they were immediately accepted as planets, but there was debate about the role of these new bodies in the hierarchy of the Solar System. The main dissenter was Herschel, who proposed they be described by the term ‘asteroid’ meaning ‘star-like’ … perhaps a not entirely impartial move from someone who might have a vested interest to be the only person to have discovered a new planet! Discovery of Neptune By 1846 Herschel was no longer alone in having been the first to find a major planet, when the discovery of Neptune was announced. For the first time, a new major planet had been discovered not by serendipity, but by detailed mathematical calculation comparing the observed and predicted motions of Uranus. Any discrepancies were assumed to be due to the gravitational effect of a hitherto undiscovered planet orbiting the Sun beyond Uranus. Neptune was found in the correct region of the Sky and found to be comparable to Uranus in size, and fully recognised as a planet, even if its orbital distance now broke with the pattern suggested by the Titius-Bode law. and yet more asteroids The arrival of Neptune into our knowledge suddenly confused the situation regarding Ceres, Pallas, Juno and Vesta. The ambiguity was further compounded by a deluge of new discoveries which started just a year before Neptune’s discovery. A new small body was found in the gap between Mars and Jupiter a full 38 years after Vesta had been found; and then at least one further object added to the list each year after that. By the end of the 19th Century the growing accumulation of ‘asteroid planets’ all swarming in the same region in the Solar System had blurred the understanding of what and what was not a planet. A subtle consensus decision gradually moved from describing all these small bodies as planets to using Herschel’s term asteroid, including even the largest and first four discovered; this reclassification acknowledging that there were differences between the planets and asteroids. how we discover new asteroids now The ability to detect asteroids in great numbers was enhanced by the devlopment of photography as an astronomical technique at the end of the 19th Century. Either a camera was fixed to a telescope that tracks the motion of the stars across the sky so that a faster-moving asteroid is revealed as a streak on the image; or comparison of two images of the same part of the sky taken at slightly different times reveals an objects which has changed its position in the meanwhile. Today’s asteroid discoveries may employ computers and ccd imaging on telescopes that survey the sky continuously and methodically, but the basic principle remains the same. An asteroid is revealed by its motion relative to the stars. Today there are over 200,000 named asteroids charted, following well-determined orbits, and many many more which have been discovered even if their orbits are not yet fully mapped out. A new asteroid is not just given a name (which the discoverer is allowed to choose, as long as there is no duplication), but it is also assigned a number. The numbers increase according to the chronological order of ‘discovery’, although the order of discovery point rests not necessarily on the first viewing, but also on the reliable determination of the asteroid’s orbit. So each small space rock out there is uniquely identified with both a number and a name: 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta…and even 134340 Pluto! Properties of the Asteroids The asteroid belt The majority of asteroids are collected into a torus-shaped swarm that fills the space between the orbits of Mars and Jupiter and which is known as the main ‘asteroid belt’. It contains millions of objects, and spans the region about 2.2 to 3.3 AU out from the Sun, where objects have orbital periods lasting between three and six Earth- years. Physical properties of the asteroids Nearly all but the largest of asteroids in the asteroid belt appear star-like in even today’s large telescopes because they are physically small.