NASA DART Changed an Asteroid System’s Orbit Around the Sun

In 2022, NASA achieved a historic first when it intentionally crashed a spacecraft into an asteroid to test whether the object’s orbit around its larger companion asteroid could be changed.

The Double Asteroid Redirection Test (DART) mission had already proved spectacularly effective, cutting the orbital period of the Didymos and Dimorphos asteroid pair by an extraordinary 33 minutes.

Fresh measurements, however, have uncovered an even more significant result: the collision also modified the Didymos-Dimorphos system’s overall route through space.

It is the first occasion on which humans have directly changed the orbit of a natural object around the Sun.

"This work adds the capability of deflecting a binary asteroid system in its heliocentric orbit to the list of novel technologies demonstrated by the DART mission," writes a team led by aerospace engineer Rahil Makadia of the University of Illinois at Urbana-Champaign.

DART and planetary safety

NASA carried out the DART mission to support planetary defence. The Solar System contains numerous large rocky bodies and, although no known asteroid is expected to strike Earth in the near future, humanity wants to be ready should such a threat emerge.

DART’s concept was simple. Its target was a gravitationally linked asteroid pair: the larger body, Didymos, is approximately 780 metres across, while the smaller one, Dimorphos, is about 160 metres across. As the smaller member of the pair, Dimorphos was the easier object to shift.

Scientists selected this system partly because its orbital period had been exceptionally well established, allowing any alteration to be measured readily. For DART to be considered successful, its collision needed to divert Dimorphos sufficiently to change the time it took to orbit its companion asteroid.

The scientific team had anticipated a shift of roughly 7 minutes, making the eventual 33-minute reduction immensely exciting.

How DART changed the Didymos-Dimorphos system

Yet the asteroid pair forms only one part of the wider Solar System. Makadia and colleagues sought to establish whether DART had altered not only Dimorphos’s orbital period around Didymos, but also the larger-scale trajectory of both objects around the Sun.

As Dimorphos and Didymos are gravitationally bound, they travel around a common centre of mass, called a barycentre. When DART hit Dimorphos, it did more than push the smaller asteroid: it also blasted debris out into space.

Scientists had predicted that this escaping material, which took momentum away from the system, would create a minute recoil and marginally change the movement of the Didymos-Dimorphos pair around the Sun.

Since the collision in September 2022, researchers have closely tracked the asteroid system using a range of instruments. Makadia’s team examined information from 22 stellar occultations, 5,955 ground-based observations of the system’s position, three navigation measurements taken by the DART spacecraft itself, and nine ground-based distance measurements.

Collectively, the findings showed that the collision did give the Didymos-Dimorphos system a slight shove, reducing its orbital speed by around 11.7 micrometres per second – approximately 42 millimetres per hour, or roughly the width of an Apple Watch.

A small nudge with major consequences

In space, even an exceptionally small push can produce a substantial positional difference over time. Across a decade, a velocity change of 11.7 micrometres per second would build up to around 3.69 kilometres.

For the timescales that matter in planetary defence – years or decades of warning, if humanity is fortunate – this suggests that even a minor deflection might move a dangerous asteroid safely away from Earth.

Further missions should offer a more detailed understanding of the impact. The European Space Agency’s Hera spacecraft, which is due to reach the Didymos system later this decade, will inspect the crater created by DART and closely measure the asteroids’ masses and internal structure.

Nevertheless, the achievement to date is remarkable. Humanity has, for the first time, changed the course of a natural object travelling through the Solar System.

"By demonstrating that asteroid deflection missions such as DART can effect change in the heliocentric orbit of a celestial body," the researchers write, "this study marks a notable step forward in our ability to prevent future asteroid impacts on Earth."

The research was published in Science Advances.