Dimorphos is on the small side, spanning 525 feet—which is about the size of the Great Pyramid.
In 2005, Congress created a mandate for NASA to find asteroids larger than 460 feet in diameter, and so far the agency has detected and tracked almost all of the really huge near-Earth objects.
The DART probe, which is short for the Double Asteroid Redirection Test, has been in the works since 2015.
The mission also depends on observatories in Arizona, New Mexico, Chile, and elsewhere; astronomers are keeping their telescopes focused on Dimorphos and Didymos to measure the post-impact deflection as precisely as possible.
The smaller asteroid is so tiny it can’t be seen from Earth telescopes—but astronomers can track it by measuring how often it dims the already-faint light from its bigger sibling as it orbits around it.
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