Planetary Defence System

A near-Earth object (NEO) impact would cause massive tsunamis or multiple firestorms, and an impact winter caused by the sunlight-blocking effect of large quantities of debris ejected into the stratosphere, creating an extinction level event.

A collision 66 million years ago between the Earth and an object approximately 10 km wide is thought to have produced the Chicxulub crater and triggered the Cretaceous–Paleogene extinction event that caused the extinction of most dinosaurs.

Many scientists including Stephen Hawking considered an asteroid collision to be the biggest threat to the planet. In 2019, scientists reported that asteroids may be much more difficult to destroy than thought earlier, and an asteroid may reassemble itself due to gravity after being disrupted. In 2021, NASA astronomers reported in a Planetary Defence Conference that 5 to 10 years of preparation may be needed to avoid a virtual impactor.

Collision avoidance and a Planetary Defence System
A working Planetary Defence System is a Type I project. These are possible methods of avoidance:


 * A nuclear explosive device around the surface of a NEO is a deflection option, changing the velocity, or "nudging", the object off course by the reaction, following Newton's third law, with ejecta going one way and the object being propelled in the other. This would produce enough of a change in the object's orbit to make it miss the Earth.
 * A massive object, such as a spacecraft or even another NEO, could be sent out into a collision course with the NEO, creating a kinetic impact and knocking it off course.
 * A gravity tractor could move the asteroid slowly over time using small but constant amount of thrust to deviate an object sufficiently from its course. This could use an ion thruster and would likely have to spend several years beside the asteroid to be effective. An alternative involves the use of the ion thruster pointed at the NEO from a nearby spacecraft, producing a slow-but-continuous force that can "shepherd" the asteroid in a similar way as the gravity tractor.
 * Solar energy could be focused onto the NEO's surface to create thrust from the resulting vaporization of material. This could be from a space station or spacecraft with a system of large collecting mirrors such as a ring-array concentrator.
 * A mass driver is a system on the NEO to eject material into space thus giving the object a slow steady push and decreasing its mass. It will use local material as propellant.
 * Attaching a spacecraft propulsion device would have a similar effect of giving a push, possibly forcing the asteroid onto a trajectory that takes it away from Earth. A rocket engine such as an ion thruster capable of imparting an impulse of 106 N/s will have a small effect on a an asteroid that has a mass of roughly a million times more.
 * Focusing sufficient laser energy on the surface of a NEO to cause flash vaporization / ablation can disintegrate or ablate away the asteroid mass. Directed Energy System for Targeting of Asteroids and ExplorRation (DE-STAR) is a proposed orbital planetary defense system capable of heating the surface of potentially hazardous objects to the point of vaporization, using a modular phased array of kilowatt class lasers powered by photovoltaics.

News
In September 2022, the binary asteroid system of Didymos and Dimorphos will be within 11 million kilometers of our planet. NASA's DART mission (Double Asteroid Redirection Test) will lift off on November 23 2021 aboard a SpaceX Falcon 9 rocket and will deliberately crash into Dimorphos to change the asteroid's motion in space.