9.1.5.2 The Asteroid Belt
Beyond Mars is the Asteroid Belt, probably the debris of a planet that never formed. Te mass of all the asteroids is less than that of Earth’s moon. Te belt is made up of more than 200 objects over 100 kilometres in diameter, more than 750,000 asteroids larger than 1 km in diameter, and millions of smaller ones. A few, like Ida, even have their own moons. Ida’s egg-shaped moon, named Dactyl, 1.5 km in diameter, is orbiting at a distance of about 90 km. Some asteroids, known as near-Earth asteroids,
sometimes cross the path of Mars and Earth. In February 2013 the asteroid 2012 DA14 was approaching Earth but luckily passed about 28,000 km above our surface. NASA has mapped the orbits of all known Potentially Hazardous Asteroids (PHAs), numbering over 1,400 as of early 2013. Being fairly large (more than 140m in size) and following orbits that pass close to the Earth’s orbit, they may pose an impact threat to Earth. Eros is a peanut-shaped asteroid 34×11×11 km3
Figure 9.13: The Chelyabinsk meteor on 15 February 2015 – the largest meteor since the 1908 Tunguska event, an explosion in Siberia that flattened nearly 2,000 km2
of forest. , named after
the Greek god of love, which may evolve into an Earth-crosser within as short an interval as two million years. It would be much larger in size than the Chicxulub asteroid that created the Chicxulub crater and led to the extinction of the dinosaurs. On 1 January 1801, Giuseppe Piazzi discovered the first
asteroid, a tiny moving object which he dubbed Ceres. He initially believed it to be a comet, but its lack of a coma suggested it was a planet. Fifteen months later, Heinrich Wilhelm Matthäus Olbers found a second object in the same region, Pallas. Apart from their rapid movement, they appeared indistinguishable from stars. Accordingly, in 1802, Frederick William Herschel, court astronomer to King George III after his 1781 discovery of Uranus, suggested they be placed into a separate category, named ‘asteroids’, after the Greek asteroeides, meaning ‘star-like’. Upon completing a series of observations of Ceres and Pallas, he concluded: “Neither the appellation of planets, nor that of comets, can with any propriety of language be given to these two stars… Tey resemble small stars so much as hardly to be distinguished from them. From this, their asteroidal appearance, if I take my name, and call them Asteroids; reserving for myself however the liberty of changing that name, if another, more expressive of their nature, should occur.” Today, we know that the moniker asteroid was proposed to Herschel by Charles Burney Sr., a poet with whom he was collaborating on an educational poem about the cosmos, by suggesting the words ‘asteriskos’ or ‘stellula’ to describe the new celestial objects. A meteor is an asteroid or other object that upon entry
into the Earth’s atmosphere burns and vaporises; meteors are therefore commonly known as ‘shooting stars’. If a meteor survives the fall through the atmosphere and lands on the surface, it’s known as a meteorite. On 15 February 2013, a 20m-wide meteor took everyone by surprise when it slammed into the atmosphere over the Russian city of Chelyabinsk. It exploded at a height of 20 km, releasing energy equivalent to 500 kilotons of TNT, 30 times the yield of the nuclear bomb over Hiroshima. Te shock wave broke windows and flying glass injured 1,500 people. Around five tons of meteoritic
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material reached the ground, including a 654 kg meteorite that was recovered by divers from the bottom of Lake Chebarkul.
9.1.5.3 The Gaseous Planets
Outside the Asteroid Belt come the four gas giants: Jupiter, Saturn, Uranus and Neptune. Around their small, solid cores, they have retained their original atmospheres of light gases. Tey all have rings and complex moon systems. Beyond Neptune is the dwarf planet Pluto, which is made mainly of ice
Figure 9.14: The inner solar system, the asteroid belt, and Jupiter. Ceres (named after the Roman goddess of agriculture and patron of Sicily) and Vesta (named after the virgin goddess of home and hearth) are the two most massive residents of the asteroid belt. Vesta is a rocky body, while the dwarf planet Ceres is believed to contain large quantities of ice. The profound differences in geology between these two protoplanets form a bridge from the rocky bodies of the inner solar system to the icy bodies, all of which lie beyond in the outer solar system. Observations by the Herschel space observatory between 2011 and 2013 found that Ceres has a thin water vapour atmosphere. This is the first unambiguous detection of water vapour around an object in the asteroid belt.
Konstantin Kudinov/Wikipedia
NASA
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