An asteroid that could be twice as large as the height of the Empire State Building is set to pass Earth on Friday, January 7, 2022.
According to NASA's Center for Near-Earth Object Studies (CNEOS), the asteroid—496860 (1999 XL136)—will pass our planet at a distance of around 8.3 million miles.
Earth's nearest planetary neighbor, Venus, is three times as far away from our planet as the distance at which this asteroid will pass. Mars is located at an average of about 17 times this distance, around 140 million miles from Earth.
It will be traveling at a speed of around 37,500 miles per hour. That's around 30 times as fast as a jet fighter and around 18 times as fast as a bullet fired from a rifle.
The asteroid last made a close approach to Earth in April 2017 and it won't return to our cosmic neighborhood until July 2034.
The asteroid has a diameter of between 330 meters and 750 meters. This means that it could be as small as the Eiffel Tower, or could be double the size of the Empire State building.
The reason for this large difference in size estimates is a result of how astronomers calculate the sizes of asteroids.
How Do Astronomers Measure Asteroids?
To calculate the diameters of asteroids researchers use something called albedo, a measure of the visible light from the sun which reflects off the surface of the asteroid.
There can be a large uncertainty in size that arises from this technique because size isn't the only determining factor in how much light an asteroid reflects and the albedo depends highly on how reflective the surface of an asteroid is.
The other factors that majorly determine the albedo of an asteroid depends are the material from which it is composed, how that material exists at the object's surface, and also how densely this matter is packed.
Albedo also hinges on the color of the asteroid. For example, a lighter "chalky" asteroid that has looser or dusty material at its surface could appear deceptively large to astronomers.
On the flip side of this, an asteroid with a charcoal-like surface will reflect less light. Thus, it could appear smaller to astronomers than it actually is.
This means that using this technique, from millions of miles away, a large dark asteroid can appear the same size as a small, light one.
Researchers also have another, more robust method of determining asteroid sizes with much smaller uncertainties. This involves measuring the heat that an asteroid gives off as it travels through space. This can be done by observing the object using infrared light.
Infrared light, and by extension heat, maybe a better indicator of size than reflected visible light as a larger object would appear brighter in infrared light, and this isn't affected by the amount of visible light it reflects.
According to NASA's Jet Propulsion Laboratory at the California Institute of Technology, which operates CNEOS, the best way to determine asteroid size is a combination of observing the visual light it reflects and measuring its infrared signature.
