Near-Earth Objects (NEOs) are comets and asteroids whose orbits bring them within 50 million kilometers (about 30 million miles) of Earth’s path around the Sun. The question of how many asteroids pose a potential collision risk is central to the field of planetary defense. The term “potential collision course” is a designation based on orbital proximity and size, which helps prioritize which space rocks require the most attention. International scientific bodies constantly monitor and catalog these objects to understand the probability of impact and provide the necessary lead time for mitigation efforts. While the number of known close-approaching objects is large, the number of actual threats remains extremely low.
Defining the Threat
Near-Earth Objects (NEOs) are tracked and categorized based on two main factors: proximity to Earth’s orbit and estimated size. The population of known NEOs has grown significantly due to dedicated survey programs, with over 37,500 objects currently cataloged.
A specific subset of NEOs is designated as Potentially Hazardous Asteroids (PHAs), which are monitored most closely. To earn the PHA label, an asteroid must meet two strict criteria. First, its orbit must intersect Earth’s orbit at 0.05 astronomical units (AU) or less (approximately 7.5 million kilometers). Second, the object must have an estimated minimum diameter of about 140 meters (460 feet) or more, large enough to cause significant regional damage.
The size criterion is often determined by the asteroid’s absolute magnitude. Scientists estimate the total population of PHAs larger than 140 meters to be around 4,700, though many remain undiscovered. As of early 2024, the number of known PHAs is approximately 2,350 to 2,500. The vast majority of these known objects have been determined to have no chance of striking Earth for at least the next century.
The System for Tracking and Cataloging
The process of finding and cataloging NEOs and PHAs is managed by a network of globally distributed observatories and centralized data centers. NASA’s Planetary Defense Coordination Office (PDCO) oversees this effort in the United States, utilizing data gathered from various ground-based astronomical surveys. These surveys are the primary infrastructure for discovery, continuously scanning the night sky for moving objects that betray their close proximity to Earth.
Major contributors to NEO discovery include the Catalina Sky Survey (CSS) and Pan-STARRS, which together account for a large portion of new finds. The ATLAS network also plays a significant role, designed to survey the entire visible sky multiple times each night. Once an object is spotted, initial observations are reported to the Minor Planet Center (MPC), the international clearinghouse for all minor body observations.
After initial discovery, follow-up observations refine the object’s orbital parameters. This continuous monitoring is paramount because initial measurements often contain high uncertainties. Repeated observations over days or weeks significantly reduce this uncertainty, allowing scientists to accurately model the asteroid’s trajectory and definitively rule out a collision risk for the foreseeable future.
Collision Probability and Scale
Assessing the actual risk of impact requires quantifying the likelihood and potential consequences of a collision. Scientists use the Torino Impact Hazard Scale, a color-coded format ranging from 0 to 10. Level 0 indicates a negligible chance of collision, and Level 10 signifies a certain, global-scale catastrophe.
When a newly discovered asteroid’s orbit is calculated, it initially contains a range of possible paths, referred to as a “probability corridor.” Limited early data might cause this corridor to briefly intersect with Earth, resulting in a temporary, non-zero rating on the Torino Scale, typically Level 1 or 2.
As more observations are gathered, the probability corridor narrows dramatically. This refinement of the orbital data usually eliminates the possibility of impact, causing the object’s rating to be downgraded to Level 0. The scale is designed to assess short-term threats over the next century. Long-term risks are tracked separately and involve complex gravitational modeling to account for subtle planetary perturbations.
Planetary Defense Strategies
When an asteroid is confirmed to pose a future threat, deflection methods require significant lead time, ideally decades, to be effective.
Kinetic Impactor
The kinetic impactor technique is the most developed strategy, successfully demonstrated by NASA’s Double Asteroid Redirection Test (DART) mission. This method involves deliberately smashing a spacecraft into the asteroid to alter its velocity and subtly change its orbital path.
Gravitational Tractor
The gravitational tractor is a “gentle push” approach that relies on gravity rather than impact force. A large, heavy spacecraft is positioned near the hazardous asteroid, and the mutual gravitational pull slowly tugs the asteroid off its collision course. This technique offers a more controlled and gradual deflection, avoiding the possibility of fracturing the asteroid, but it requires a longer operational period to achieve the necessary trajectory change.