What would happen if an asteroid hit the Earth?

Impact energy scales with the cube of diameter, so the difference between sizes is not gradual. Objects up to a few metres across burn up in the atmosphere many times a year and are seen as fireballs. The Chelyabinsk object in 2013 was around 20 metres and released roughly 500 kilotonnes of energy as an airburst, breaking windows and injuring about 1,500 people, mostly from flying glass. A 50-metre object comparable to the one that flattened 2,000 square kilometres of forest at Tunguska in 1908 arrives perhaps once every few centuries. The Chicxulub impactor, at 10 to 15 kilometres, released on the order of 10²³ joules — comparable to a hundred million megatonnes — and killed roughly three quarters of species, primarily through the global effects of ejecta, wildfire and years of darkened skies rather than the impact itself. Objects of that size are now catalogued, and none currently known is on a collision course.

What happens, in order

How confident this is

This one is settled. The mechanism is understood and the consequences follow from measurements or arithmetic rather than from guesswork.

Physics

• What would happen if gravity disappeared? The Earth itself would come apart — gravity is the only thing holding a planet together.
• What would happen if the Sun became a black hole? Earth's orbit would not change at all — the mass would be the same. It would simply go dark and cold.
• What would happen if you fell into a black hole? Into a small one, tidal forces would stretch you apart before you arrived; into a supermassive one, you would cross the horizon without noticing.
• What would happen if you fell into a hole through the Earth? You would oscillate from one side to the other and back, with a one-way trip of about 38 minutes.
• What would happen if you travelled at the speed of light? You cannot — accelerating any mass to light speed requires infinite energy. Approaching it, time dilates dramatically.
• What would happen if gravity doubled? You would weigh twice as much, most tall structures would need rebuilding, and no bird larger than a sparrow could fly.
• What would happen if you were shrunk to the size of an ant? The square-cube law would break you: you would freeze, suffocate, and be unable to drink from a surface of water.
• What would happen if the speed of light were slower? Chemistry, nuclear fusion and the mass-energy relationship would all change — stars would not work as they do.
• What would happen if a gamma-ray burst hit the Earth? A nearby one aimed at us would strip a large fraction of the ozone layer within seconds, with the damage arriving as ultraviolet over the following years.