IN AND around our planet, there are thousands of comets and asteroids known as near-Earth objects (NEOs). Multiple space agencies and government affiliates are responsible for tracking them, especially those known as potentially hazardous asteroids (PHA). These objects are so-designated because they will cross Earth’s orbit and may even collide with it someday. Considering how impacts in the past have caused mass extinctions (like the Chicxulub impact event that killed the dinosaurs), future impacts are something we would like to avoid!

Monitoring PHAs is a huge responsibility that requires a worldwide effort, including tracking, alerts, and disaster preparedness. Last year, over 100 participants from 18 countries (including NASA scientists and the NEOWISE mission) conducted an international exercise that simulated an encounter with an asteroid that made a close flyby to Earth. As NASA revealed in a recently-released study, the exercise was a complete success. The lessons learned could help avert real impacts in the near future or significantly limit the devastation one could cause.

The study, which appeared in the May 31 issue of The Planetary Science Journal (titled “Apophis Planetary Defense Campaign“), was conducted by the Planetary Defense Exercise Working Group and led by Vishnu Reddy — an Associate Professor at the University of Arizona’s Lunar and Planetary Laboratory (LPI). The working group is made up of more than 100 participants from 18 countries and includes facilities like NASA’s Planetary Defense Coordination Office (PDCO), the ESA NEO Coordination Centre, the Russian Academy of Sciences, the Korea Astronomy and Space Science Institute (KASI), and many universities and research institutes worldwide.

As Reddy and his colleagues describe in the paper, the planetary defense exercise was the culmination of work that began in 2017, which was designed to test the operational readiness of our global planetary defense capabilities. The exercise was carried out with the support of NASA’s PDCO, the Minor Planet Center (MPC) — the internationally-recognized authority for monitoring the position and motion of small celestial bodies — and the International Asteroid Warning Network (IAWN). The exercise was named the “Apophis Campaign” since it coincided with the close approach of the NEO (99942) Apophis, which flew past Earth from December 2020 to March 2021.

Apophis is one of many PHAs regularly monitored by the planetary defense-monitoring database. Shortly after it was discovered in 2004, Apophis was determined to have a significant chance of impacting Earth in 2029 or later. But after years of tracking and several close approaches, astronomers have refined Apophis’ orbit and concluded that it poses no risk of impacting Earth for a century or more. Apophis was specifically selected for this campaign because planetary defense experts knew it would closely approach Earth in early December 4, 2020.

To make the exercise more realistic, the MPC removed Apophis from the planetary defense-monitoring database to see whether it could be properly detected anew, tracked, and characterized by the planetary defense system. With no prior record of it in the database, astronomers had nothing to reference it to, making it seem like astronomers were seeing it for the first time. Other goals included the ability of the system to conduct observations, hypothetical risk assessment, risk prediction, and hazard communication.