In 2022 humans nudged an asteroid, and beat the target by a mile
On 26 September 2022 NASA deliberately crashed a spacecraft into a small asteroid to see whether it could shift its path. Mission planners would have counted a change of 73 seconds in its orbital period as success. The actual change was 32 minutes. For the first time, people had demonstrably moved a natural body in space.
The stakes are real. The B612 Foundation said in 2018 that a devastating hit is certain, only the timing is not, and Stephen Hawking's final book named an asteroid strike as the planet's biggest threat. Anything hitting Earth arrives at 11.2 kilometres per second at least, Earth's escape speed, and asteroids average about 17. The most likely angle of arrival is 45 degrees.
Size and frequency trade off steeply. Rocks about 4 metres across enter the atmosphere roughly once a year and usually vaporise high up. Something 20 metres wide arrives about twice a century; the 2013 Chelyabinsk meteor over Russia was that size, exploded with around 500 kilotons of energy and injured more than 1,000 people. Kilometre-wide asteroids strike on average every 500,000 years, and the last known object of 10 kilometres or more hit 66 million years ago at Chicxulub, ending the age of dinosaurs and clearing the way for mammals.
Oddly, scientists long doubted that impacts shaped Earth at all. Meteor Crater in Arizona was blamed on a volcanic blast, a fair guess given the San Francisco volcanic peaks only 48 kilometres away, and lunar craters were also credited to volcanoes. The Arizona crater was recognised as an impact scar in 1903 to 1905, and Eugene Shoemaker nailed the proof in 1963. Space probes then showed that every solid body surveyed carries craters.
Collisions have also been creative. A giant impact is widely thought to have formed the Moon, and impacts may explain why Venus and Uranus spin backwards, while comets may have delivered some of Earth's water. In July 1994 astronomers watched their first direct collision as fragments of comet Shoemaker–Levy 9 slammed into Jupiter. Only a handful of small asteroids, 2008 TC3 among them, have been spotted before hitting Earth.
Source: Impact event
