Skip to main content

Asteroid Didymos: NASA's galactic target practice for DART

An artist's depiction of Didymos and Dimorphos.
An artist's depiction of Didymos and Dimorphos. (Image credit: NASA/JHUAPL)

Didymos is a binary asteroid system: a pair of harmless, lumpy rocks minding their own business as they spin through space. NASA's DART mission wants to crash a spaceship into them. The smaller asteroid in the system, known as Dimorphos, is the target of a NASA mission to demonstrate its developing planetary defense techniques. While Didymos is not threat to Earth, this technology could protect Earth from other space rocks that may be more dangerous.

What is Didymos?

These 14 radar images show the near-Earth asteroid Didymos (65803) and its moonlet as seen by the Arecibo Observatory radio telescope in Puerto Rico in November 2003.  (Image credit: NASA)

The pair of asteroids together are known as Didymos, but individually the two rocks are Didymos and Dimorphos. Didymos is a large asteroid around half a mile (2,560 feet or 780 meters) in length, while Dimorphos measures a mere 525 feet (160 m), according to the Johns Hopkins University Applied Physics Laboratory (JHUAPL), where the DART mission is based. In some previous coverage from, Dimorphos has been referred to as Didymos' moonlet or as "Didymoon."

As the two asteroids orbit the sun, Dimorphos also orbits Didymos, circling its larger twin every 11 hours and 55 minutes. 

For decades, telescopes on Earth like Arecibo Observatory and Lowell Discovery Telescope have been able to track that movement "just like clockwork," said planetary scientist Nancy Chabot of JHUAPL during a press conference held before DART's launch. As Dimorphos passes between Didymos and Earth, telescopes around the globe can measure the resulting change in brightness from the system. 

Why does NASA want to crash into Didymos?

The clockwork motion of the Didymos system makes it ideal for the DART mission, a demonstration of NASA's ability to redirect dangerous asteroids off collision courses with Earth. Didymos itself poses no threat to our planet, but will serve as a test target. 

Researchers on Earth will be able to observe the effect of the DART spacecraft hurtling head-on into Dimorphos, the smaller asteroid. According to Chabot, after the impact they expect to see Dimorphos to complete a loop of Didymos some 10 minutes faster than its historical schedule. 

DART is also carrying a cubesat that will film the larger spacecraft's impact and beam the footage back to researchers on Earth.

At the time of DART's impact, Didymos will be visible enough to be a good candidate for study and distant enough to be no danger, at approximately 6.8 million miles (11 kilometers) away from Earth.

What else do we know about Didymos?

A simulation of what asteroid Didymos and its moonlet Didymoon may look like. (Image credit: NASA)

Didymos is one of the Amor group of asteroids, near-Earth asteroids named after asteroid 1221 Amor. These asteroids orbit the sun mostly between the orbits of Earth and Mars. Didymos in particular takes a little over two years to complete an elliptical orbit around the sun, according to NASA's in-depth page on Didymos. 

The larger asteroid of Didymos probably has a rocky surface covered in boulders, although NASA wrote that it likely lacks the regolith, or fine dust, that covers Earth's moon and other asteroids. It also lacks any atmosphere, rings, or magnetosphere. 

Additional resources

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at:

Vicky Stein
Vicky Stein

Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter.