An asteroid like Apophis is a faint dot in even the largest telescopes, yet we say confidently that it is about 340 meters wide. How is that possible? Astronomers combine several methods, each with its own strengths. This article explains how size and speed are measured and why the numbers come with error bars.
The easiest thing to measure is how bright an asteroid appears. Astronomers convert that into an absolute magnitude, written H, which is the brightness the object would have at a standard distance from both the Sun and Earth. A smaller H means a brighter, and generally larger, object.
The problem is that brightness depends on two things at once: size and reflectivity (albedo). A small, shiny asteroid can look the same as a large, dark one. A dark asteroid with an albedo of 0.05 is about three times larger than a bright one with an albedo of 0.4 at the same H. Without knowing the albedo, a size estimate from H alone can be off by a factor of two or three.
Asteroids absorb sunlight and radiate it as heat. A big asteroid radiates more heat than a small one, no matter how dark or bright its surface is. By observing the infrared glow, astronomers can separate size from reflectivity. NASA's NEOWISE mission, which used an infrared space telescope, measured thousands of near-Earth objects this way. Combining visible and infrared data gives much better estimates of both diameter and albedo.
When an asteroid comes close enough, radar can image it. A large dish, such as NASA's Goldstone antenna in California, sends out a radio signal, and a telescope listens for the echo. The timing of the echo gives the distance to within meters, and the way the echo is smeared in frequency, called Doppler shift, reveals the asteroid's shape, spin and size. Radar can even show moons and craters. The Arecibo Observatory in Puerto Rico was long a leader in this work, until it collapsed in December 2020. Radar observations of Apophis in 2012 and 2013 were an important part of removing its 2029 impact risk, as covered in Is Apophis Dangerous?
Sometimes an asteroid passes in front of a star. Observers at different places along a path see the star blink out for a few seconds. Timing the disappearance from several locations draws a profile of the asteroid, like a shadow, and gives its size and shape very precisely. Amateur astronomers contribute many of these measurements.
The most accurate sizes come from visiting an asteroid. NASA's OSIRIS-REx mapped Bennu in detail, and NASA's DART mission and ESA's Hera mission measure the Didymos pair. These results are also used to calibrate the other methods. See OSIRIS-REx and Bennu and Hera and Ramses.
Speed is not read from a single picture. Astronomers measure the asteroid's position on the sky many times over days, weeks or years. Each measurement is called an astrometric observation. A computer then fits an orbit to those positions, and from the orbit it computes the velocity at any moment. Radar adds a direct measurement of speed along the line of sight through the Doppler shift, which can improve an orbit dramatically with just a few observations.
Orbit fitting is the same process used by NASA's impact monitoring systems; details are in How NASA Tracks Asteroids. The accuracy of a future position depends on how long the object has been observed, and forces such as the Yarkovsky effect must be included.
The figure comes from a combination of infrared and radar measurements, and it is an average, as Apophis is not a perfect sphere. Estimates carry an uncertainty of tens of meters. That is why scientists say "about", and why the 2029 flyby, and the missions studying it, will give a much more accurate answer.
Facts in this article are drawn from public scientific sources. For primary information, see: