How NASA's Planetary Defense Coordination Office Protects Earth from Asteroids and Comets

Overview

In 2016, NASA consolidated several programs into the Planetary Defense Coordination Office (PDCO) to coordinate federal efforts to detect, track and respond to hazardous asteroids and comets. The PDCO works with other agencies—such as the Federal Emergency Management Agency (FEMA), international partners, and the research community—to monitor the skies, assess impact risk, and communicate with emergency managers and the public when necessary.

What PDCO monitors

Near-Earth objects (NEOs) are comets or asteroids that come within roughly 121 million miles of the Sun, which means they can pass near Earth's orbit. A subgroup, potentially hazardous objects (PHOs), are those that come within about 4.7 million miles of Earth's orbit and are larger than about 500 feet in diameter — large enough to cause significant regional damage.

Scale of the challenge

As of September 2025, NASA has cataloged more than 39,000 near-Earth asteroids. Nearly 900 are larger than about 3,000 feet (with an estimated ~50 very-large asteroids still undiscovered), and more than 11,000 known asteroids exceed roughly 500 feet in diameter (with estimates indicating ~14,000 of that size still unlocated). These numbers show why continuous detection and tracking are essential.

Why size matters

Impacts from even modest-size objects can be devastating. The Tunguska event in 1908 was likely caused by an object under 300 feet across and flattened nearly 800 square miles of Siberian forest. An impactor larger than 500 feet could destroy a city and create a crater over a mile wide; an object on the order of 3,000 feet or larger could inject enough dust into the atmosphere to cause global cooling and major agricultural disruption.

Response and technology

NASA and international partners are actively testing planetary-defense techniques. The DART mission (Double Asteroid Redirection Test) successfully demonstrated a kinetic impactor technique in 2022, showing it's possible to alter an asteroid's trajectory. However, exercises and simulations highlight limits: a 2021 Planetary Defense Conference simulation found that, for some scenarios, available options might be insufficient to avert an impact if detection occurs too late.

The role of PDCO going forward

PDCO's ongoing priorities include improving early detection, refining impact probability forecasts, coordinating contingency planning with emergency-management agencies, advancing deflection and disruption techniques, and leading public communication during potential threats. Continued investment in surveys, international collaboration, and technology tests will strengthen our ability to protect Earth from hazardous space objects.

Bottom line: PDCO coordinates detection, preparedness and response for asteroid and comet threats, but early discovery and robust technology development remain critical to reducing impact risk.