The Double Asteroid Redirection Test was designed to answer a practical question in planetary defense: if a dangerous asteroid were found early enough, could a spacecraft change its path by hitting it? In September 2022, NASA tested that idea by steering a spacecraft into Dimorphos, a small asteroid moonlet orbiting the larger asteroid Didymos.

The answer was yes. According to NASA’s October 11, 2022 mission update, the impact shortened Dimorphos’ orbit around Didymos from 11 hours and 55 minutes to 11 hours and 23 minutes. NASA described it as the first time humanity had purposely changed the motion of a celestial object.

That does not mean Earth was in danger. Neither Dimorphos nor Didymos posed a threat before or after the test. The point of choosing a binary asteroid system was measurement. By watching how the smaller body moved around the larger one, telescopes on Earth could detect a small change in orbit far more clearly than they could detect a tiny shift in a lone asteroid’s path around the Sun.

A crash with a purpose

DART was a kinetic impactor. It did not carry explosives. It changed the asteroid’s motion by transferring momentum through a high-speed collision. The spacecraft struck Dimorphos on September 26, 2022, at about 14,000 miles per hour, or about 22,530 kilometers per hour, according to NASA’s mission account.

The target was well chosen for an experiment. Didymos is the larger body, while Dimorphos is the smaller companion orbiting it. Before impact, astronomers already knew Dimorphos’ orbital period very well. That gave the mission a clean before-and-after test: if DART worked, the timing of the pair’s mutual events, as seen in light curves from Earth, would shift.

NASA had set a minimum threshold of 73 seconds for a successful change in orbital period. The observed shift was far larger. Early NASA reporting gave the change as 32 minutes with an uncertainty of about plus or minus 2 minutes. Later peer-reviewed work refined the measurement.

The measured change was about 33 minutes

In a 2023 Nature paper led by Cristina Thomas of Northern Arizona University, the DART team reported that the impact changed Dimorphos’ orbital period by minus 33.0 minutes, with a three-sigma uncertainty of 1.0 minute. The result came from Earth-based light-curve observations and radar, using two independent approaches that reached the same value.

That number is the core of the mission’s success. DART did not simply make a crater or produce a flash on impact. It measurably changed the motion of a natural body in space. In the language of planetary defense, the experiment demonstrated that a kinetic impactor can deflect an asteroid, at least for an object and impact geometry like this one.

The change was small in ordinary human terms. Dimorphos was not blasted away. It still orbits Didymos. But asteroid deflection does not require cinematic destruction. For planetary defense, a tiny velocity change applied years before a predicted encounter could accumulate into a large miss distance by the time the object reaches Earth’s neighborhood.

The debris mattered

The spacecraft’s momentum was only part of the story. Images from space telescopes and ground observatories showed a plume and tail of material thrown from Dimorphos after impact. That ejected material, often called ejecta, acted like recoil. As debris streamed away in one direction, Dimorphos received an extra push in the other.

In a second 2023 Nature paper led by Andrew Cheng of the Johns Hopkins Applied Physics Laboratory, researchers estimated the momentum transfer from the impact. They found an instantaneous reduction in Dimorphos’ along-track orbital velocity of 2.70 millimeters per second, with a 0.10 millimeter per second uncertainty. Depending on assumptions about Dimorphos’ density and mass, the momentum enhancement factor ranged from 2.2 to 4.9.

That enhancement factor matters because it says the escaping debris contributed substantially to the deflection. DART was not just a billiard-ball collision. The asteroid’s physical response, including how much material was excavated and how it escaped, amplified the effect.

This is also why one successful test does not settle every future case. Asteroids differ in size, density, shape, spin, porosity and surface strength. A loose rubble-pile asteroid may respond differently from a more coherent body. A future mission would need to understand the target well enough to predict how an impact would behave.

The impact itself was a technical test

DART also had to prove it could hit a small object autonomously. Dimorphos was only about 160 meters across, and the spacecraft was closing fast. In a 2023 Nature paper led by R. Terik Daly of the Johns Hopkins Applied Physics Laboratory, the mission team reconstructed the impact and reported that DART struck Dimorphos successfully, demonstrating the autonomous targeting and impact sequence needed for a kinetic-impact test.

The distinction is important. Planetary defense is not only physics. It is also detection, navigation, timing, spacecraft operations, observation and follow-up measurement. DART was a full-scale demonstration of one piece of that chain, not a complete asteroid-defense system by itself.

That is why the mission was aimed at a harmless asteroid pair. It allowed researchers to test the method without risk. The experiment changed Dimorphos’ orbit around Didymos, not the path of a dangerous asteroid headed for Earth.

What “first time” means

Humans had struck small bodies before DART. NASA’s Deep Impact mission, for example, sent an impactor into comet Tempel 1 in 2005 to study the comet’s interior. But DART was different in intent and result. It was designed as a deflection test, and the orbital change was the measurement that mattered.

That is the careful meaning of the milestone. DART was the first full-scale test of asteroid deflection technology, and NASA reported that it was the first time humans had purposely changed the motion of a celestial object. The later Nature papers supplied the more precise measurement and the physical explanation for why the change was larger than a simple direct-hit calculation would suggest.

The test also changed how planetary defense can be discussed. Before DART, kinetic impact was a plausible method supported by modeling and mission design. After DART, there was a real asteroid, a real spacecraft and a measured orbital shift.

What remains unknown

The mission did not show that any asteroid can be deflected at any time. Warning time remains central. A small push is useful only if applied early enough. The method also depends on the asteroid’s properties and on knowing its orbit accurately.

The European Space Agency’s Hera mission is intended to examine the Didymos-Dimorphos system in detail after DART. NASA’s 2022 update noted that Hera would survey both bodies, study the crater or reshaping left by the collision, and help measure Dimorphos’ mass more precisely. Those details matter because the size of the momentum transfer depends partly on the target’s mass and structure.

For now, the essential result is already clear. A human-built spacecraft deliberately hit an asteroid. Telescopes measured the aftermath. The orbit changed. For the first time, planetary defense had moved from an idea tested mainly in models to a demonstrated change in the motion of a natural object in space.