Planetary defense

Planetary defense concerns the natural impact hazard posed by near-Earth asteroids and comets. It is a chain of related activities: discovering objects, observing them to determine their orbits and physical properties, estimating possible consequences, communicating credible risks, and developing ways to reduce a threat if needed. Most near-Earth objects are not on a collision course with Earth; the purpose of monitoring is to identify and assess the uncommon cases that might require action, not to imply that any particular object is dangerous.

Astronomers use ground-based observatories, radar and spacecraft observations to discover and follow near-Earth objects. Repeated measurements help refine an object's orbit and estimate whether it could approach or strike Earth. NASA's Near-Earth Object Observations Program funds surveys and follow-up work to find undiscovered objects, refine orbits, determine physical properties and study mitigation technologies. Infrared observations can help reveal dark asteroids and comets that reflect little visible light. NASA's NEO Surveyor mission is being built specifically to find large numbers of potentially hazardous asteroids and comets, including some that can be difficult for ground-based observatories to see.

Because a serious impact hazard would have consequences beyond one country, planetary defense depends on international data-sharing and planning. The International Asteroid Warning Network (IAWN) brings together asteroid observers and modelers. Its functions include coordinating observations, maintaining an international focal point for validated information, assessing hazards and assisting governments with impact-consequence analysis and response planning. The Space Mission Planning Advisory Group (SMPAG) is an inter-space-agency forum that considers the technologies and mission planning needed for possible near-Earth-object deflection.

A mitigation response would depend on the object's orbit, size, composition, warning time and other characteristics. One investigated method is a kinetic impactor: a spacecraft deliberately collides with an object, imparting a small change in its motion. In 2022, NASA's Double Asteroid Redirection Test (DART) struck Dimorphos, the small moonlet of asteroid Didymos. Follow-up observations found that Dimorphos's orbital period around Didymos became shorter by about 32 minutes. This was a demonstration of a deflection technique, not a test on an Earth-threatening asteroid; neither member of the Didymos system posed an impact hazard to Earth. The result informs future research, but the effectiveness of any response would depend on the properties and trajectory of a real target.