World-first in planetary defence

10 July 2017

DART and Didymos A and B | Credit: NASA/Johns Hopkins APL

The NASA-approved Double Asteroid Redirection Test (DART) is moving from concept development to preliminary design phase – beginning the first mission to demonstrate an asteroid deflection technique.

The target for DART is an asteroid that will have a distant approach to Earth in October 2022, and then again in 2024. The asteroid is called Didymos (Greek for ‘twin’) because it is an asteroid binary system that consists of two bodies: Didymos A, about one-half mile (780 meters) in size, and a smaller asteroid orbiting it called Didymos B, about 530 feet (160 metres) in size. DART would impact only the smaller of the two bodies, Didymos B.

Said Lindley Johnson, planetary defence officer at NASA Headquarters in Washington: “DART would be NASA’s first mission to demonstrate what’s known as the kinetic impactor technique – striking the asteroid to shift its orbit – to defend against a potential future asteroid impact. This approval step advances the project toward a historic test with a nonthreatening small asteroid.”

The Didymos system has been closely studied since 2003. The primary body is a rocky S-type object, with composition similar to that of many asteroids. The composition of its small companion, Didymos B, is unknown, but the size is typical of asteroids that could potentially create regional effects should they impact Earth.

After launch, DART would fly to Didymos and use an APL-developed onboard autonomous targeting system to aim itself at Didymos B. Then the refrigerator-sized spacecraft would strike the smaller body at a speed roughly nine times faster than a bullet (approximately 3.7 miles per second, or 6 kilometres per second).

Earth-based observatories would be able to see the impact and the resulting change in the orbit of Didymos B around Didymos A, allowing scientists to better determine the capabilities of kinetic impact as an asteroid mitigation strategy. The kinetic impact technique works by changing the speed of a threatening asteroid by a small fraction of its total velocity, but it does so well before the predicted impact; and this small nudge will add up over time to a big shift of the asteroid’s path away from Earth.

Double Asteroid Redirection Test (DART) | Credit: NASA/

Small asteroids hit Earth almost daily, breaking up harmlessly in the upper atmosphere. Objects large enough to do damage at the surface are much rarer. Objects larger than 0.6 miles (1 kilometre) in diameter – large enough to cause global effects – have been the focus of NASA’s ground-based search for potentially hazardous objects with orbits that bring them near the Earth, and about 93 percent of kilometre-sized objects have already been found.

DART would test technologies to deflect objects in the intermediate size range – large enough to do regional damage yet small enough that there are many more that have not been observed and could someday hit Earth. NASA-funded telescopes and other assets continue to search for these objects, track their orbits and determine if they are a threat.

To assess and formulate capabilities to address these potential threats, NASA established its Planetary Defense Coordination Office (PDCO) in 2016, which is responsible for finding, tracking and characterising potentially hazardous asteroids and comets coming near Earth, issuing warnings about possible impacts, and assisting plans and coordination of U.S. government response to an actual impact threat.

DART is being managed by APL in Laurel, Maryland. The project would be overseen by the Planetary Missions Program Office at Marshall Space Flight Center in Huntsville, Alabama. DART also is supported by teams from the Goddard Space Flight Center, Greenbelt, Maryland; Johnson Space Center, Houston, Texas; and the Jet Propulsion Laboratory, Pasadena, California.

Credit: Johns Hopkins Applied Physics Laboratory,

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