How does thermal radiometry help astronomers estimate asteroid sizes?

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Illustration showing a massive asteroid above a city map with a dinosaur silhouette for scale, highlighting the asteroid's size compared to major urban areas. — Asteroid This rock would take out more than just a *T. rex*.

Thermal radiometry is the most widely used technique for estimating the size of asteroids. This method relies on the principle that the infrared radiation (heat) emitted by an asteroid must be in balance with the solar radiation it absorbs.1

To derive an asteroid's diameter, astronomers employ a thermal model that balances the measured intensity of the asteroid's emitted infrared radiation with its observed brightness at visual wavelengths. Because an asteroid's apparent brightness is influenced by both its size and its albedo—the ratio of reflected light—this modeling allows researchers to disentangle these factors to calculate the object's physical dimensions.1

While direct methods like stellar occultation or direct imaging provide more precise measurements, they are limited to specific conditions or nearby, bright objects. Consequently, thermal radiometry remains the primary tool for characterizing the vast majority of known asteroids, with measurements for objects larger than 20 km typically maintaining an uncertainty of less than 10 percent.1