Planetary Defenders

The critical endeavor of planetary defense involves a global effort to identify, track, and potentially mitigate the threat posed by near-Earth objects (NEOs), particularly hazardous asteroids. The discovery of a new potentially hazardous asteroid, such as one recently found measuring approximately 230 meters in diameter with a close approach distance of about 150,000 kilometers, underscores the ongoing need for vigilance.

Historical events, like the 1994 impacts of Comet Shoemaker-Levy 9 on Jupiter, which saw a 2-mile-wide fragment traveling at 40 miles per second among others, serve as a potent reminder of the destructive power of cosmic collisions. The observation of these impacts, including by the Hubble Space Telescope, provided invaluable insights into impact mechanics and the potential for terrestrial devastation, with a similar-sized impactor capable of creating a crater the size of Rhode Island.  

Recognizing this threat, NASA established the Planetary Defense Coordination Office. A 1998 congressional mandate tasked NASA with cataloging 90% of all large NEOs (1 kilometer or more), objects capable of causing "global devastation." Over 95% of these nearly 900 such asteroids have been identified, and none currently pose a threat within the next 100 years. A 2005 directive expanded this to include 90% of asteroids 140 meters and larger, capable of causing significant regional damage; however, only about 40% of these are currently known.  

The process of finding these objects involves ground-based telescopes capturing sequences of images to detect movement. The Minor Planet Center serves as the international data repository, collecting observations and calculating orbits. CNEOS at JPL then uses this data to assess impact hazards for the next 100 years, issuing rapid alerts when necessary. Crucial follow-up observations, employing telescopes like the 0.9-meter SPACEWATCH® and the Goldstone Solar System Radar, refine orbits and characterize the asteroids' physical properties, such as shape and composition, through high-resolution radar imaging and spectral analysis.

The 2013 Chelyabinsk meteor, though only about 18 meters across, demonstrated the destructive power of even smaller, unobserved impactors, injuring 1,500 people with an explosive energy equivalent to 470 kilotons of TNT. The successful international collaboration in observing and ultimately de-risking asteroid 2023 DZ2 exemplifies the effectiveness of these coordinated efforts.

The DART mission in 2022 further demonstrated humanity's ability to actively defend the planet by changing the orbit of the asteroid Dimorphos by 32 minutes using a kinetic impactor. Looking ahead, other mitigation techniques like gravity tractors and ion beam deflectors are being explored. The importance of early detection, providing years or decades of warning, is paramount for any deflection strategy to succeed, a principle regularly practiced in interagency tabletop exercises simulating potential impact scenarios like the hypothetical 2023 TTX in 2038.