The answer to this question depends on your definition of asteroid. If only objects within the asteroid belt (between the orbit of Mars and Jupiter) count, then the largest asteroids are Ceres (975 km, 606 mi), Vesta (530 km, 329 mi), and Pallas (570 km, 354 mi). These comprise 32%, 9%, and 7% of the asteroid belt's total mass, respectively. If you expand your definition of asteroids to include all sub-planet, non-satellite objects in the solar system, including bodies beyond the orbit of Neptune, then the largest are Eris (~2,500 km, 1,553 mi), Pluto (1,195 km, 743 mi), and Ceres (975 km, 606 mi), in that order. By comparison, the Earth has a diameter of 12,756 km (7,926 mi).

To get an idea of the size of the largest asteroids, consider the largest asteroid known to have impacted the Earth: about 10 km (6 mi) 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 the time, only microbes existed as life. The Wilkes Land crater under the East Antarctic ice sheet may signify an even larger impact.

There is one consensus among scientists: impact of an object larger than 20 km (12 mi) in diameter would be likely to kill all complex (multicellular) life on Earth, by blocking out the Sun, halting photosynthesis for years at a time, and causing runaway global cooling. Tens of thousands of cubic kilometers of crust would be instantaneously vaporized, spewing red-hot ejecta across a continent-sized area. The only survivors would be microbes, especially bacteria (extremophiles) and fungi.

So we see that even the third-largest of the asteroids is more than 20 times larger than necessary to kill all multicellular life on the planet, if it made impact. Luckily, these asteroids are in stable orbits and extremely unlikely to cross paths with us.

Of the largest asteroids and dwarf planets, none have thus far been visited by astronauts or space probes, although the Dawn mission (launched in 2007) is scheduled to visit Vesta in 2010 and New Horizons (launched in 2006) will reach Pluto in 2015. Currently, our best telescopes only provide fuzzy pictures of these bodies, so our knowledge about them is limited. Pluto is known to have a reddish tint, similar to Mars, while Ceres possesses mysterious white and dark spots (presumably craters) which appear and disappear over the years.

In the early history of the solar system, much larger asteroids were common. It is thought that the Moon was created when a Mars-sized object named Rhea formed in the same orbit as the Earth, eventually impacting it. The crust ejected from this impact formed the the Moon. This theory is well-supported because the composition of the Moon is similar to that of the Earth's crust.