What to Know About Asteroids Dimorphos, Didymos That NASA Will Smash Into

NASA's planetary defense mission has launched from Space Launch Complex 4 at Vandenberg Space Force Base, California, which is tasked with the aim of slamming into an asteroid and altering its orbit. The target of the Double Asteroid Redirection Test (DART) spacecraft is the binary asteroid system Didymos and Dimorphos.

DART will reach the asteroid Didymos and its smaller moonlet Dimorphos when they are around 6.8 million miles away from Earth, slamming into the smaller body. Through this impact, NASA scientists will assess if a kinetic impact could redirect an asteroid that is on a collision course with Earth.

The closest the orbit brought the Didymos and Dimorphos double asteroid system to Earth was within 4.5 million miles in 2003. It presents no impact threat to Earth. The fact that the asteroid system occasionally passes close to Earth was the reason they were chosen as a target for DART. In 2123, it will come to within 3.6 million miles of our planet.

The asteroid Didymos was discovered on April 11, 1996, by researcher Joseph Montani of Spacewatch at Kitt Peak National Observatory in Tucson, Arizona. Montani suggested the asteroid's name, twin in Greek.

The discovery that Didymos was orbited by its own moonlet followed when researchers spotted echoes in observations of the asteroid made by NASA's Goldstone Solar System Radar, located in California. These suspicions were confirmed by observing how the brightness of the object changed over time and through further radar images taken by the Arecibo Observatory in Puerto Rico taken on Nov. 23, 2003.

didymos and dimorphos
An illustration of the binary asteroid system Didymos and Dimorphos. The moonlet Dimorphos is the target of the DART mission which will test if a kinetic impact can redirect an asteroid. JHU/APL/NASA

The parent body of the binary asteroid system, Didymos, has a diameter of 780 meters, about half a mile. Its moonlet, Dimorphos—of which the name derives from the Greek word meaning "having two forms"—has a much smaller diameter of 160 meters (about 500 feet).

The orbit of the system ranges from just outside the orbit of Earth, about 93 million miles, to around 211 million miles, just beyond the orbit of Mars. The system completes an orbit of the sun in 2.11 years.

Most asteroids formed from the material around the sun during the formation of the solar system, meaning that studying these unspoiled objects is a great way of investigating the evolution of the solar system's planetary bodies.

While there are currently several formation methods suggested for binary asteroids, the rapid rotation of Didymos gives scientists a clue as to how it may have come to possess its own moonlet.

As Didymos rotated it is believed that it could have flung off material that eventually coalesced into a moonlet, a process called rotational fission.

The reason that Didymos is believed to have begun spinning so rapidly is that infrared light was emitted unevenly by its surface, which astronomers have thus far been unable to image in detail, as it was warmed by the sun.

Because photons of infrared light carry momentum, emitting them caused a "kick" that sped up the rotation of Didymos. Over a period of millions of years, this resulted in the asteroid building up enough rotational speed to start flinging off the material that eventually formed DART's target, Dimorphos.

DART will reach the binary asteroid system between September 26 and October 1, 2022 when it will impact the moonlet. This will cause a change in velocity of its orbit around its larger companion Didymos of just 1 percent.

The team will then assess how this change alters the orbital period of the larger body by several minutes, a change that could be sufficient to divert an asteroid on a trajectory that really threatens Earth.

DART mission
An artist's illustration of the DART spacecraft approaching its target. The mission will aim to alter an asteroid's path through space. NASA / Johns Hopkins APL