DART's smashing success

On September 26, the DART impactor smashed into Dimorphos, the moonlet asteroid orbiting Didymos. The collision was a successful planetary defense test mission.

The DART mission (Double Asteroid Redirection Test) was a joint venture between NASA and the Johns Hopkins Applied Physics Laboratory (APL). Frank Laipert, who joined the Nabla Zero Labs team this month as a Mission Design and Navigation Expert, was a member of the DART mission design and navigation team. (Congratulations, Frank and everyone who made the success possible!)

About the spacecraft

DART was a small and simple spacecraft, which included a camera, a solar array, antennae, and an ion thruster. It launched on a SpaceX Falcon 9 rocket on November 24, 2021.

About the navigation

From the mission’s inception to the successful collision, people and computers worked hard to chart a trajectory and make adjustments to guide the spacecraft toward its target. As Laipert says, “The task of impacting an object only hundreds of meters across is probably one of the hardest navigation challenges there is.” In addition to the small size of the target, uncertainty in Didymos’ orbit posed navigation challenges.

After launch, the team tracked the spacecraft’s trajectory using data from NASA’s Deep Space Network antennas and, as DART got closer to the Didymos system, images from the camera on board. The navigation team compared that data to the planned path and made 6 maneuvers to keep DART on track toward Didymos.

Closer to Didymos and Dimorphos, the camera captured the first images that distinguished the two asteroids. Prior to this, they appeared as one shape in photographs. Approximately 4 hours before impact, the spacecraft’s SMARTnav system took over navigation. Using the photographs, SMARTnav zeroed in on the center of Dimorphos and autonomously navigated DART to a perfect hit!

Why the Didymos system?

A key reason the mission targeted Didymos system is that it is a binary asteroid system. Dimorphos orbits Didymos, and as Dimorphos passes in front of Didymos (from the perspective of Earth), the level of light decreases. This allows scientists to calculate Dimorphos’s speed. Further, because Dimorphos’ orbit around Didymos is significantly shorter than Didymos’ (or another asteroid’s) orbit around the sun, scientists could measure the effect of the collision on the orbit quickly, rather than waiting years to see the results.

Before impact, Dimorphos’ orbit around Didymos took 11 hours and 55 minutes. NASA’s benchmark of success was a change in orbital period of 73 seconds or more. The collision shortened Dimorphos’ orbit to 11 hours and 23 minutes. This means the mission attained an orbital change of 25 times the minimum that was required to deem the experiment a success!

What the success means

For Laipert, because DART was a relatively low-cost mission for NASA, the team “always felt like a scrappy underdog.” He says, “The fact that everything went so perfectly when everyone’s eyes were on us felt a bit like a miracle.”

For the American public, the mission’s success means their tax dollars are supporting the sort of space research they care about. 62% of respondents to a 2018 Pew Research survey answered that monitoring asteroids and other objects that could impact Earth should be a top NASA priority.

The Didymos system had (and still has) zero risk of colliding with Earth and no known large asteroids (greater than 140 m in diameter) are expected to hit us within the next 100 years. However, should one be discovered that might collide with our planet, the DART mission’s success means that kinetic impact deflection might just keep us safe.