The solar system is filled with countless objects orbiting the sun, many of which cross Earth’s path. These Near-Earth Objects (NEOs) range significantly in size, from fine dust grains to objects kilometers across. Public attention often focuses on rare, catastrophic impacts, but the most frequent interactions involve much smaller space rocks. This natural cosmic bombardment occurs constantly, with a particular size class of small asteroids frequently encountering our planet. Exploring the rate and outcome of these events provides a clearer picture of Earth’s relationship with its immediate space environment.
Defining the “Car-Sized” Object
The term “car-sized” is a non-scientific description that astronomers use to refer to objects approximately one to five meters in diameter. An object in this size range orbiting the sun is classified as an asteroid. When this rocky body enters Earth’s atmosphere, it transitions into a meteoroid. The bright flash of light it produces is called a meteor, commonly known as a shooting star. If any remaining fragments survive the fiery descent and land on the ground, they are classified as meteorites.
Annual Impact Rate
Objects in the one to five-meter size range enter Earth’s atmosphere relatively often, demonstrating the abundance of small debris in the solar system. An automobile-sized asteroid, estimated to be around four meters across, strikes the atmosphere approximately once every year. Space rocks measuring about one meter across impact our planet a few times annually. The frequency increases dramatically for smaller objects, following a size-frequency distribution. The overall mass of meteoritic material hitting Earth daily is estimated to be over 100 tons, though most of this is microscopic dust that vaporizes unnoticed.
Atmospheric Disintegration
The frequent atmospheric entry of car-sized objects does not pose a threat because Earth’s atmosphere functions as an effective natural shield. When a space rock enters the atmosphere at high speed, it violently compresses the air in front of it, generating immense pressure and heat. This leads to the object’s swift destruction through an airburst, where the meteoroid shatters and vaporizes high above the surface. Objects smaller than about 25 meters across are highly likely to disintegrate completely before reaching the ground. The 2013 Chelyabinsk event, caused by a 19-meter object, demonstrated this process; it disintegrated over Russia, creating a shockwave that injured people from shattered glass, but the mass did not strike the surface.
Tracking Challenges
Detecting these smaller space rocks before they enter the atmosphere presents a significant challenge for astronomical surveys. Car-sized asteroids are faint, move quickly, and do not reflect enough sunlight to be easily spotted by telescopes until they are very close to Earth. Due to their small size and speed, they are often only discovered hours before impact, or sometimes not until the resulting airburst is observed.
Programs like NASA’s Planetary Defense Coordination Office (PDCO) are in place to monitor the skies for threatening objects. The focus of these efforts is primarily on tracking larger, potentially hazardous objects greater than 30 to 50 meters in diameter, which could cause regional devastation if they impacted the surface. While systems exist to identify imminent impactors, the sheer number and faintness of the smallest objects mean that routine, complete tracking of every car-sized NEO is not currently feasible or a priority.