The largest asteroids in the asteroid belt between Mars and Jupiter are Ceres, Vesta, and Pallas. If all non-satellite objects in the solar system are included, the largest are Eris, Pluto, and Ceres. An asteroid larger than 12 miles in diameter could kill all complex life on Earth. The Moon is thought to have been created by a Mars-sized object hitting the Earth.
The answer to this question depends on the definition of an asteroid. If only objects within the asteroid belt between the orbits of Mars and Jupiter count, then the largest are Ceres, which is about 580 miles (930 km) wide and considered a dwarf planet; Vesta, which is about 326 miles (525 km) in diameter; and Pallas, which is about 338 miles (544 km) across, but less massive than Vesta. These comprise 32%, 9% and 7% of the total mass of the asteroid belt, respectively. If the definition is expanded to include all sub-planets, non-satellite objects in the solar system, including bodies beyond the orbit of Neptune, the largest are Eris, another dwarf plant with a diameter of about 1,445 miles (2,326 km); Pluto, which is approximately 1,433 miles (2,306 km) wide; and Ceres, in that order. By comparison, the Earth has an average diameter of 7,917.5 miles (12,742 km).
To get an idea of the size of the largest asteroids, a person can consider the largest one to hit Earth: It was only about 6 miles (10 km) in diameter. This impact, thought to have occurred about 2 billion years ago, is represented by the Vredefort crater in South Africa. A similar impact represented by the Sudbury Structure in Canada is dated to 1.8 billion years ago. At that time, only microbes existed as life. Wilkes Land Crater under the East Antarctic Ice Sheet could mean an even bigger impact.
There is a consensus among scientists: the impact of an object larger than 12 miles (20 km) in diameter could kill all complex life on Earth by blocking out the Sun, disrupting photosynthesis for years at a time and causing global flight cooling down. Tens of thousands of cubic kilometers of crust would be instantly vaporized, spewing incandescent ejecta into an area the size of a continent. The only survivors would be microbes, especially (extremophile) bacteria and fungi.
Even the third largest of the asteroids is more than 20 times larger than would be needed to kill off all multicellular life on Earth, had it impacted. Fortunately, these objects are in stable orbits and are extremely unlikely to collide with the planet.
Of the largest, only one has so far been visited by astronauts or space probes. The Dawn mission (launched 2007) entered orbit around Vesta in July 2011 and departed in September 2012, bound for Ceres. New Horizons (launched in 2006) will reach Pluto in 2015. Currently, the best telescopes provide only blurry images of these bodies, so scientists’ knowledge about them is limited. Pluto is known to have a reddish tint, similar to Mars, while Ceres has mysterious dark and white spots (presumably craters) that appear and disappear over the years.
In the early history of the solar system, much larger asteroids were common. The Moon is thought to have been created when a Mars-sized object called Rhea formed in the same orbit as the Earth, eventually hitting it. The crust ejected by this impact formed the Moon. This theory is well supported because the composition of the Moon is similar to that of the Earth’s crust.
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