The DART mission -
deflecting an asteroid

DART (Double Asteroid Redirection Test) was a NASA planetary defence mission designed to test whether a spacecraft could change an asteroid's orbit by kinetic impactor - that is, by crashing into it at high speed. The principle is simple: momentum is transferred from spacecraft to asteroid, altering the asteroid's velocity by a tiny amount. Applied far enough in advance of a predicted Earth encounter, even a tiny velocity change compounds into a large positional shift.

The test proved successful. The impact changed Dimorphos's orbital period around its parent body Didymos by 33 minutes, reducing it from 11 hours 55 minutes to 11 hours 22 minutes. The mission's minimum success threshold had been 73 seconds. The actual result was more than 25 times that minimum.

The target was chosen with care. Dimorphos is a 160-metre moonlet orbiting a larger asteroid, Didymos, which measures approximately 780 metres across. The binary system gave the mission two advantages that a single asteroid could not.

First, measuring the result is straightforward. Dimorphos orbits Didymos in a roughly 12-hour cycle. Ground-based telescopes can observe the brightness of the combined system dip when Dimorphos passes in front of or behind Didymos and time those events precisely. Comparing the timing before and after the impact reveals any change in the orbital period - no spacecraft needed to observe the result. The change in period was confirmed by multiple independent observatory networks within days of the impact.

Second, deflecting Dimorphos posed no risk of redirecting Didymos toward Earth. The change to Dimorphos's trajectory around Didymos caused an entirely negligible shift in Didymos's path around the Sun - far too small to affect its close approach distances. The binary system allowed a genuine, full-scale impact test in a controlled and safe configuration.

DART launched on 23 November 2021 from Vandenberg Space Force Base in California aboard a SpaceX Falcon 9. The 10-month cruise to the Didymos system was largely uneventful. On 26 September 2022, DART's onboard SMART Nav guidance system took over from ground controllers and steered the spacecraft autonomously toward Dimorphos using real-time images from its DRACO camera.

The final images, transmitted back to Earth in the last seconds before impact, showed Dimorphos's rocky surface in increasing detail as DART closed to zero distance at approximately 22,530 km/h. Contact was confirmed in real time at the Johns Hopkins Applied Physics Laboratory in Maryland, which built and operated the spacecraft.

LICIACube, a small Italian cubesat that had separated from DART 15 days earlier, flew past Dimorphos on a close approach trajectory timed to capture images of the impact aftermath. Its cameras documented the expanding ejecta plume - a cloud of rock and dust blasted off the surface by the impact, stretching thousands of kilometres into space.

Before impact, Dimorphos completed one orbit of Didymos every 11 hours 55 minutes. After the impact, that period dropped to 11 hours 22 minutes - a change of 33 minutes. The result was confirmed by multiple independent teams using ground-based photometry within two weeks of the impact.

The magnitude of the change exceeded predictions. Scientists had modelled the expected period change from pure momentum transfer - spacecraft mass times velocity divided by Dimorphos's mass - and arrived at a figure of perhaps 7-10 minutes. The additional 20-plus minutes of change came from the ejecta plume. As hundreds of tonnes of rock were blasted off Dimorphos's surface, that material carried momentum away from the asteroid. The recoil from the ejection added substantially to the deflection force. The ejecta acted as a momentum amplifier.

This ejecta amplification effect - called the momentum transfer efficiency or beta factor - had been predicted theoretically but never previously measured for a real impact. DART's result confirmed that the amplification is significant and measurable, improving models of how kinetic impactors will perform against different asteroid types.

DART changed the status of kinetic impactor deflection from theoretical to tested. Before September 2022, planetary scientists could model the technique and argue it should work. After September 2022, they had measured it working on a real asteroid in deep space.

The key lesson is that ejecta amplification is real and substantial. A kinetic impactor does not just push - it also excavates. The momentum of the excavated material adds to the deflection. This means kinetic impactors may be more effective than the most conservative models had assumed.

The result also clarified the conditions required for success. With decades of warning, a spacecraft of DART's size and speed could deflect an asteroid of Dimorphos's dimensions away from Earth. Larger asteroids require proportionally more energy - but the principle holds. Lead time is the most important variable. A mission launched 20 years before a predicted impact has far more flexibility than one launched 2 years before.

ESA's Hera spacecraft launched in October 2024 and is currently travelling to the Didymos system on a trajectory that will bring it into orbit around Didymos in late 2026. Hera will conduct the detailed post-impact survey that DART could not - no follow-up spacecraft was included in the original DART mission design.

Hera's objectives include mapping the crater left by DART on Dimorphos's surface, measuring Dimorphos's mass precisely (currently estimated only from orbital dynamics), and characterising the asteroid's internal structure using radar. Two small cubesats carried by Hera - Milani and Juventas - will operate in close proximity to Dimorphos and attempt to land on its surface.

The mass measurement is particularly important. The momentum transfer efficiency depends on the ratio of spacecraft momentum to asteroid mass. DART's beta factor was calculated using estimated masses; Hera will provide a direct measurement. That data will allow the DART results to be extrapolated to different asteroid sizes and compositions with much greater confidence.