One spacecraft’s journey to test Earth’s planetary defenses

A NASA spacecraft intentionally crashed into an asteroid’s moon to see how it affects the motion of an asteroid in space. The Double Asteroid Redirection Test, or DART, collided with Dimorphos on September 26 at 7:14 p.m. ET. Learn more about the spacecraft, its final destination and what it means.

The Double Asteroid Redirection Test was the first time that humanity sent a spacecraft to measurably change the orbit of a celestial body. The mission was also the world’s first full-scale demonstration of deflection technology that could protect the planet.

The DART spacecraft headed for Dimorphos, a small moon orbiting the near-Earth asteroid Didymos. The asteroid system poses no threat to our planet, making it a perfect target to test out a kinetic impact — which may be needed if an object is ever on track to hit Earth.

The double-asteroid system is visible through ground-based telescopes as a single point of light, but DART provided our first look — in exquisite detail — at Dimorphos. The mission will allow scientists to have a better understanding of the size and mass of each asteroid, which is crucial to our understanding of near-Earth objects.

Near-Earth objects are asteroids and comets with an orbit that places them within 30 million miles (48.3 million kilometers) of Earth. Detecting the threat of near-Earth objects that could cause grave harm is a primary focus of NASA and other space organizations around the world.

No asteroids are currently on a direct impact course with Earth, but more than 27,000 near-Earth asteroids exist in all shapes and sizes.

The valuable data collected by DART will contribute to planetary defense strategies, especially the understanding of what kind of force can shift the orbit of a near-Earth asteroid that could collide with our planet.

The DART vehicle launched aboard a Falcon 9 rocket in November 2021. With its large solar arrays, DART was about the size of a school bus. Its total mass was roughly 1,260 pounds (570 kilograms) when it smacked into Dimorphos.

The spacecraft had two Roll-Out Solar Arrays, called ROSA. The flexible wings were lighter than traditional solar arrays, despite the fact that each one measured 27.9 feet (8.5 meters) long. Solar cells and reflective concentrators within the arrays provided DART with three times more power than standard solar arrays on other spacecraft. These rollable arrays have been tested and installed outside of the International Space Station, but DART marked the first time NASA used them on a planetary spacecraft.

Another first aboard DART was its Evolutionary Xenon Thruster—Commercial propulsion system, or NEXT-C. Testing this technology, which enhances fuel efficiency, on DART will help researchers assess how it performs for possible inclusion on future deep space missions. The solar-powered system relies on an ion drive that bombards xenon gas with electrically charged atoms.

The spacecraft guided itself using the Small-body Maneuvering Autonomous Real Time Navigation, or SMART Nav system. This system synced with DART’s eyes, the Didymos Reconnaissance and Asteroid Camera for Optical navigation, or DRACO, to allow the spacecraft to identify the double-asteroid system and distinguish which space object it was supposed to impact.

What’s more, DRACO served as a high-resolution camera that measured the size and shape of its target in order to determine where it would impact the asteroid’s moon — all while capturing stunning images of the two asteroids that streamed to Earth in real time at a rate of one image per second.

Acting as DART’s videographer was the Italian Space Agency’s Light Italian CubeSat for Imaging of Asteroids, or LICIACube. This briefcase-size CubeSat hitched a ride with DART to space and detached from the spacecraft on September 11.

On the CubeSat are two cameras called LUKE (LICIACube Unit Key Explorer) and LEIA (LICIACube Explorer Imaging for Asteroid). Together, they collected images and helped guide LICIACube on its journey. The little satellite traveled behind DART to record what happened.

Astronomers discovered Didymos more than two decades ago. It means "twin" in Greek, a nod to how the asteroid forms a binary system with the smaller asteroid, or moon.

Didymos is roughly 2,560 feet (780 meters) across. Meanwhile, Dimorphos is 525 feet (160 meters) in diameter, and its name means "two forms."

Dimorphos was chosen for this mission because its size is comparable to asteroids that could pose a threat to Earth. An asteroid the size of Dimorphos could cause “regional devastation” if it impacted Earth.

About four hours before the impact event on September 26, the spacecraft became autonomous. At that time, DART actually targeted Didymos because it still wasn’t able to see tiny Dimorphos yet. The spacecraft was still 56,000 miles (90,000 kilometers) away from the asteroid.

About 50 minutes before impact, Dimorphos came into view and DART slightly transitioned its field of view to focus on the little moon. Slowly, Dimorphos shifted from being a tiny point of light and grew larger and larger within the frame of DRACO’s camera.

Scientists were able to see the moonlet for the first time, determining its shape and structure and how rocky it is. Each image revealed more detail as DART moved closer.

The SMART Nav system guided DART until about two minutes prior to impact.

The spacecraft accelerated to more than 13,421 miles per hour (21,600 kilometers per hour) when it collided with Dimorphos — effectively ending the DART mission.

The spacecraft is about 100 times smaller than Dimorphos, so it didn't obliterate the asteroid or blow it into pieces. Instead, DART tried to change the asteroid's speed and path in space. The mission team has compared the spacecraft’s impact on the tiny asteroid to a golf cart crashing into one of the great pyramids — it hit with enough energy to leave an impact crater.

Three minutes after impact, LICIACube flew by Dimorphos to capture images and video of the ejecta cloud as it sprayed up off the asteroid and maybe even spotted the impact crater. The mini satellite also glimpsed Dimorphos’ opposite hemisphere, which DART didn’t get to see before it was obliterated. The CubeSat turned to keep its cameras pointed at Dimorphos as it flew by.

The video, while not immediately available, will be streamed back to Earth in the weeks and months following the collision.

This kinetic impact is just one way to redirect asteroids that may pose a threat to Earth, but it's the one scientists believe to be the “most technologically mature.”

The fast impact will only change Dimorphos' speed as it orbits Didymos by 1%, which doesn't sound like a lot — but it will change the moon's orbital period.

The nudge will shift Dimorphos slightly and make it more gravitationally bound to Didymos — so the collision won't change the binary system's path around Earth or increase its chances of becoming a threat to our planet.

Dimorphos completes an orbit around Didymos every 11 hours and 55 minutes. After the impact, that may change between 73 seconds and 10 minutes. Follow-up observations will determine how much of a shift will have occurred.

Astronomers will use ground-based telescopes to observe the binary asteroid system and see how much the orbital period of Dimorphos changed, which will determine whether DART was successful. Scientists can compare observations prior to the impact with those taken after.

Telescopes around the world observed Dimorphos as it passes in front of and behind the asteroid. Measuring the brightness of Didymos and how it changes can help astronomers determine how the orbital period has changed.

Space-based telescopes such as Hubble, Webb and NASA's Lucy mission also observed the event.

The first full-frame images from LICIACube will return and be processed a couple of days after the impact. The satellite’s unique perspective will provide a view of the impact and Dimorphos that scientists wouldn’t be able to see any other way.

To survey the aftermath of the impact, the European Space Agency's Hera mission will launch in 2024. The spacecraft, along with two CubeSats, will arrive at the asteroid system in 2026, about four years after DART completes its mission. Once there, Hera will study both asteroids, measure physical properties of Dimorphos, and examine the DART impact crater and the moon's orbit, continuing with the overall aim of establishing an effective planetary defense strategy.