Impact event

An impact event is a collision between astronomical objects in space. These collisions can involve asteroids, comets, or meteoroids. While many impacts are small and have minimal effects, larger ones can cause massive changes. When large bodies strike terrestrial planets like Earth, they carry immense energy. This energy comes from their high speed. Most impacts occur at several kilometers per second (km/s). The minimum speed for any object striking Earth is the planet's escape velocity.

The mechanism of an impact depends on the object's size and speed. As an object enters the atmosphere, the air can mitigate some of the energy. However, many large bodies retain enough energy to reach the surface. When they hit solid ground, they create impact craters. These craters are large structures that shape the landforms of the Solar System. Sometimes, an object does not reach the ground. Instead, it undergoes an airburst, which is an explosion in the atmosphere. The energy released depends on the object's diameter, density, velocity, and impact angle.

Impacts can be categorized by their size and how they release energy. Small objects, called meteoroids, frequently collide with Earth. Stony asteroids with a diameter of 10 to 20 meters enter our atmosphere about once a year. These often result in airbursts. For example, the 2013 Chelyabinsk meteor was about 20 meters wide. It produced an airburst 30 times more powerful than the atomic bomb dropped on Hiroshima. Larger objects, such as those with a diameter of 1 kilometer or more, are much rarer. Large collisions involving objects 10 kilometers wide happen only once every twenty million years on average.

Humans have documented many of these events throughout history. In 1908, the Tunguska event occurred in a remote part of Siberia, Russia. This massive explosion shattered windows hundreds of kilometers away and damaged many trees.

In 1994, the Comet Shoemaker–Levy 9 impact provided the first direct observation of a collision. We watched as the comet broke apart and hit Jupiter. Scientists like Eugene Shoemaker were vital to our understanding. He helped prove that meteorite impacts have significantly affected Earth's history.

Major impacts have shaped the very existence of our world. Scientists believe a massive impact early in Earth's history helped form the Earth–Moon system.

Some theories suggest that impacting comets delivered water to Earth. Others suggest impacts brought the building blocks of life through a process called exogenesis. However, impacts can also be destructive. The Chicxulub impact occurred 66 million years ago. This event caused the Cretaceous–Paleogene extinction event. It also accelerated the evolution of mammals, eventually allowing humans to rise.

We can also observe these events happening in other star systems. In 2013, NASA's Spitzer Space Telescope detected a massive planet impact around the star ID8. This was an extrasolar impact, meaning it happened outside our Solar System.

In our own system, the frequency of impacts is tied to the size of the object. There is an inverse relationship where larger objects hit much less often. For instance, asteroids 1 kilometer wide strike Earth every 500,000 years on average.

Today, humanity is working to prepare for future risks. The B612 Foundation and physicist Stephen Hawking have noted the threat of devastating asteroids. In 2022, the Double Asteroid Redirection Test was a major success. This was the first human experiment to deflect an asteroid. The mission changed the target body's orbital period by 32 minutes. This exceeded the success requirement of 73 seconds.

Such technology helps us defend our planet from future cosmic collisions.