1. More information is needed to defend against intermediate-size asteroids that could cause severe regional damage to our planet. Imagecredit: ESA - P.Carril

2. An artist's concept of the DART mission to intentionally strike thebinary asteroid system called Didymos (Greek for “twin”) and test planetarydefense. Image Credit: ESA

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NASA’s DART Targets Twinsfor Asteroid DefenseBy john F. Kross

Taking a page from a science fiction moviescript, NASA has approved a mission to de-flect an asteroid to further planetary de-fense. The mission, dubbed the DoubleAsteroid Redirection Test (DART), takes aimat an intermediate-size object that couldcause regional damage if it collided withEarth. “DART would be NASA’s first missionto demonstrate what’s known as the kineticimpactor technique—striking the asteroid toshift its orbit—to defend against a potentialfuture asteroid impact,” said Lindley John-son, planetary defense officer at NASAheadquarters. To acquire the know-howneeded to tackle potential threats, NASAcreated the Planetary Defense CoordinationOffice (PDCO) in 2016 to ramp up the track-ing and characterization of potentially haz-ardous objects and coordinate a response.In concert, the U.S. space agency approvedthe promotion of DART from concept devel-opment to the preliminary design phase inJune 2017.

Weighing 500 kilograms, the refrigerator-size DART spacecraft would target a binaryasteroid system called Didymos (Greek for“twin”) composed of a larger asteroid, Didy-mos A, about 780 meters in size, and Didy-mos B, its 160-meter wide sidekick. OnlyDidymos B would be in DART’s crosshairs asthe twins make a distant approach (10.9million kilometers) from Earth in 2022 and2024. After launch, DART would fly to thebinary system using its targeting system tozero in on Didymos B, most likely in 2024. “Abinary asteroid is the perfect natural labora-tory for this test,”said Tom Statler, program scientist for DART at NASA. “The fact thatDidymos B is in orbit around Didymos A makes it easier to see the results ofthe impact.”

DART would slam into the space rock aboutnine-times faster than a bullet—six kilome-ters per second—allowing Earth-based observatories to see the crash and shift in the orbit of Didymos B around its largertwin. “We intend to… change the orbital period of Didymos… seven minutes ormore… and we will be able to see that fromthe ground,” predicted Cheryl Reed, DART’sproject manager at the Johns Hopkins University Applied Physics Laboratory(JHUAPL).

The sudden impact would change the mutual orbit of the two objects, but causeonly a minor shift in the heliocentric orbit of the system. Although the Didymos twinspose no threat, in principle, even a smallnudge to a menacing asteroid could changeits speed and trajectory enough to miss ourplanet.

“DART is a critical step in demonstrating wecan protect our planet from a future aster-oid impact,” explained DART investigationteam coleader, Andy Cheng at JHUAPL.“With DART, we can show how to protectEarth from an asteroid strike with a kineticimpactor by knocking the hazardous objectinto a different flight path that would notthreaten the planet.”

About 100 tons of cosmic material falls onto the Earth daily,mostly in the form of harmless dustand small meteorites that break up inthe atmosphere.

Most of the 1,000 or so near-Earth asteroidslarge enough to cause a global disaster havebeen spotted and tracked. However, far lessis known about intermediate-size asteroids

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that could cause severe regional damageto our planet. “Since we don’t know that

much about their internal structure or compo-sition, we need to perform this experiment on areal asteroid,” said Andy Cheng at Johns HopkinsUniversity Applied Physics Laboratory (JHUAPL).

Autonomous Navigation and Ion Thrusters The DART (Double Asteroid Redirection Test)spacecraft features a simple single string designand thruster-only control, but carries sophisti-cated instruments and propulsion technologies.On board, a high-resolution imager derived fromthe New Horizons LORRI camera, called the Didymos Reconnaissance and Asteroid Camerafor Op-Nav (DRACO), would support navigation

and pinpoint the impact site in geologic context.As a technology demonstration mission, DARTwould also integrate advanced technologies suchas electric propulsion and autonomous naviga-

tion. Precise, self-directed navigation is requiredto ensure the accuracy of the kinetic impact.

In addition, the autonomous systemwould test fuel management logic soft-

ware to tweak the timing of coursecorrections and optimize the

limited fuel onboard.

DART would steer to itsviolent rendezvous

using the NASA

Evolutionary Xenon Thruster-Commercial (NEXT-C)solar electric propulsion system, a more advancedand powerful version of the ion thruster used onthe Dawn spacecraft. Electric propulsion providesgreater flexibility to DART’s mission timeline andlaunch window, as well as decreases launch costs.DART could ride as a shared payload on a com-mercial rocket carrying a communications satelliteand be inserted into a geostationary transfer orbitbefore thrusting its ion engine toward Didymos.

Originally, the DART project, a joint effort betweenNASA and the JHUAPL, was a complementary partof the Asteroid Impact and Deflection Assessment(AIDA) mission in collaboration with the EuropeanSpace Agency (ESA) and the German AerospaceCenter. ESA agreed to build a separate AsteroidImpact Mission (AIM) spacecraft to orbit the aster-oid pair and observe DART’s impact. However, inlate 2016, the AIM probe was defunded by ESA tohelp pay for ExoMars and other projects. Euro-pean space officials are still weighing a future rolefor a scaled-back, cheaper version of the AIM mis-sion called AIMlight. Despite AIDA’s abrupt finale,DART gained approval from NASA as a solo mis-sion, though it is not a specific budget item in theFiscal Year 2018 budget. If fully funded, however,DART would be the first off-world demonstrationof planetary defense, just in case the sky reallydoes fall one day.

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