Planetary defense is a global effort focused on the detection, tracking, and characterization of Near-Earth Objects (NEOs) that pose a potential impact hazard to Earth. An NEO is an asteroid or comet whose orbit brings it within approximately 30 million miles of Earth’s orbit. The goal of planetary defense is to prevent impacts entirely by developing the means to alter the trajectory of potential threats so they safely pass by Earth. Threats range from smaller objects that could cause regional devastation to larger ones capable of global catastrophe.
Locating and Characterizing Near-Earth Objects
The initial step in defense is the systematic search for objects whose orbits cross Earth’s path, carried out by a network of ground-based and space-based observatories. Ground-based surveys, such as the Catalina Sky Survey and Pan-STARRS, use wide-field telescopes to scan the sky nightly, detecting moving points of light against the stationary background. Once an object is detected, follow-up observations gather data to compute a precise orbit, known as the ephemeris. This calculation determines if the object is a Potentially Hazardous Object (PHO), which are NEOs typically larger than 460 feet (140 meters) and coming within 4.6 million miles of Earth’s orbit.
Characterizing a PHO involves determining its physical properties. Techniques like planetary radar and infrared measurements help scientists estimate the object’s size, shape, rotation rate, and composition. Infrared data reveals the albedo, or surface reflectivity, which provides clues about the asteroid’s density. Understanding if an asteroid is a solid monolithic rock or a loosely bound “rubble pile” is necessary for selecting the most effective deflection method, since a rubble pile might fragment if struck too hard.
Deflection Through Kinetic Impactors
The kinetic impactor technique is the most developed method for altering an asteroid’s path, relying on the direct transfer of momentum from a high-speed spacecraft. The concept involves intentionally crashing a heavy, fast-moving spacecraft into an asteroid to change its velocity by a tiny amount. Applied far enough in advance, this small change accumulates over time to shift the asteroid’s orbit sufficiently so that it misses Earth. The goal is a subtle nudge, not an attempt to destroy the object and create hazardous fragments.
NASA’s Double Asteroid Redirection Test (DART) mission provided the first validation of this technology. In September 2022, the DART spacecraft successfully impacted Dimorphos, the moonlet orbiting the larger asteroid Didymos. The collision changed Dimorphos’s orbital period around Didymos by 32 minutes, far exceeding the initial expectation of a ten-minute change. This amplified effect was largely due to the recoil from the ejected material kicked up by the impact, which provided an additional push. The success of DART demonstrated that the kinetic impactor is a viable option for planetary defense.
Gradual and Non-Contact Deflection Techniques
For threats identified decades in advance, or for objects whose composition makes a kinetic impact risky, gradual, non-contact deflection methods are available. One such method is the Gravity Tractor, which uses the subtle gravitational pull between a spacecraft and the asteroid to slowly tow it off course. A spacecraft hovers near the asteroid, using its mass to create a small gravitational force that gently accelerates the asteroid. The spacecraft’s thrusters fire away from the asteroid to maintain position, ensuring the exhaust plume does not interfere with the gravitational interaction. This method is highly controllable and does not depend on the asteroid’s structural properties, making it a reliable option if there is a decade or more of lead time.
Laser Ablation is another non-contact technique that transforms the asteroid itself into a propulsion system. This method involves focusing a powerful laser beam onto a small spot on the asteroid’s surface, heating the material to extreme temperatures. The heated surface material vaporizes, forming a plume of gas, or ejecta, that shoots away from the asteroid. According to Newton’s third law, this plume acts like a tiny, continuous thruster, imparting a small but steady force that nudges the asteroid off its original path over years.
A third gradual technique involves Albedo Modification, which harnesses solar radiation pressure. This concept proposes altering the asteroid’s surface reflectivity by “painting” a section of it with a bright, reflective material to increase its albedo. The change in albedo affects how much sunlight the asteroid absorbs and re-radiates, creating a slight, non-gravitational force known as the Yarkovsky effect. By controlling the solar radiation pressure exerted on the object, this constant force can slowly change the asteroid’s orbit over a very long duration.
The Role of Advance Warning and Mission Readiness
The time between discovery and potential impact determines the success of any deflection mission. A smaller asteroid requires less energy to deflect but provides less time for detection due to its faintness. For a deflection attempt to be effective, even a small change in velocity must be applied far from Earth. This allows the deviation in the asteroid’s path time to accumulate into a large miss distance. Deflection missions like the Gravity Tractor, which rely on slow, gentle forces, require many years, often decades, of advance warning to be viable.
Mission readiness also depends on established frameworks for international cooperation and decision-making. Organizations like the United Nations have established recommendations for an international response to an NEO threat, recognizing the global consequences of an impact. Government agencies routinely conduct tabletop exercises to test national readiness and coordination for hypothetical impact scenarios. These exercises help determine the complex political and logistical steps required before a mission is launched, such as when a threat is deemed serious enough to warrant a deflection effort.