NASA's DART Mission Changed the Didymos-Dimorphos System's Solar Orbit

NASA's DART mission and planetary defence

In 2022, NASA achieved a historic first when it intentionally crashed a spacecraft into an asteroid, testing whether the collision could change that object's orbit around a larger companion asteroid.

The Double Asteroid Redirection Test (DART) mission was already known to have been an outstanding success: it shortened the orbital period of the Didymos and Dimorphos asteroid pair by a remarkable 33 minutes.

Fresh measurements, however, have uncovered an even more significant result. The collision changed the overall route travelled by the Didymos-Dimorphos system through space.

It is the first occasion on which humans have directly modified the orbit of a natural body 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 was undertaken to support planetary safety. The Solar System contains many large rocky bodies and, although no known asteroid is expected to strike Earth in the near future, humanity wants to be ready should that possibility arise.

The mission's concept was simple. Its target was two asteroids held together by gravity: the larger object, Didymos, is approximately 780 metres wide (2,560 feet), while the smaller one, Dimorphos, measures around 160 metres across (525 feet). As the smaller member of the pair, Dimorphos was the easier asteroid to shift.

This particular system was selected partly because scientists had characterised its orbital period extremely well, allowing any alteration to be measured readily. For DART to be successful, its impact needed to divert Dimorphos sufficiently to change the time it took to orbit its companion.

The science team had anticipated a shift of roughly 7 minutes. The eventual 33-minute reduction was therefore enormously exciting.

Didymos-Dimorphos orbital path around the Sun

Yet the asteroid pair forms only one part of the wider Solar System. Makadia and colleagues set out to establish whether DART had changed not only Dimorphos's orbital period around Didymos, but also the larger-scale path followed by both objects around the Sun.

As Dimorphos and Didymos are gravitationally linked, they circle a common centre of mass, called a barycentre. When DART hit Dimorphos, the collision did more than push the smaller asteroid: it flung debris out into space.

Scientists had predicted that this escaping material, carrying momentum away from the system, would produce a minute recoil and slightly change the Didymos-Dimorphos pair's movement around the Sun.

Since the collision in September 2022, the asteroid system has been closely observed by scientific instruments. Makadia's team examined data from 22 stellar occultations, 5,955 ground-based observations of the system's position, three navigation measurements taken by the DART spacecraft, and nine ground-based distance measurements.

Collectively, the evidence showed that the impact did give the Didymos-Dimorphos system a very small shove, reducing its orbital velocity by around 11.7 micrometres per second - approximately 42 millimetres per hour, or roughly the width of an Apple Watch.

In space, though, even an extremely small push can ultimately produce a substantial positional difference. Across a decade, a change of 11.7 micrometres per second would build up to about 3.69 kilometres.

For the timescales that matter in planetary defence - years or decades of warning, if we are fortunate - this suggests that even a slight nudge could move a dangerous asteroid far enough to avoid Earth safely.

Hera will examine the DART impact

Later missions should offer a still more detailed understanding of the collision. The European Space Agency's Hera spacecraft, due to reach the Didymos system later this decade, will investigate the crater created by DART and precisely assess the asteroids' masses and internal structure.

Even so, the achievement to date is extraordinary. Humanity has, for the first time, altered the trajectory 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.