In 2022, NASA set a landmark for space exploration by intentionally crashing a spacecraft into an asteroid, testing whether the strike could change the smaller body’s orbit around its larger companion.
The Double Asteroid Redirection Test (DART) mission’s headline result was already clear: it shortened the orbital period of the Didymos–Dimorphos pair by a remarkable 33 minutes.
New observations, however, point to an even broader effect. The collision appears to have adjusted the trajectory of the entire Didymos–Dimorphos system as it travels through space.
As a result, this 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.
Why NASA carried out DART for planetary safety
DART was undertaken with planetary safety in mind. The Solar System contains countless sizeable rocky bodies and, although none are currently known to be on a likely collision course with Earth in the near future, preparing for that possibility remains a priority.
The Didymos–Dimorphos target and what DART needed to change
The concept behind DART was simple. Its target was a gravitationally bound pair of asteroids: the larger object, Didymos, is about 780 metres across (2,560 feet), while the smaller, Dimorphos, measures roughly 160 metres across (525 feet). Because Dimorphos is the smaller body, shifting it was expected to be more achievable.
The Didymos system was selected partly because its orbital period had been measured extremely well, meaning even small changes could be detected. For DART to meet its goal, the impact needed to alter Dimorphos’s motion sufficiently to change its orbital period around Didymos.
Researchers had anticipated a shift of about 7 minutes, which made the observed 33-minute reduction particularly thrilling.
How debris from the DART impact could shift the system’s path around the Sun
Even so, the asteroid pair is only one piece of a much larger dynamical picture: the Solar System as a whole. Makadia and colleagues set out to determine whether DART changed not only Dimorphos’s period around Didymos, but also the combined system’s large-scale path around the Sun.
Because Didymos and Dimorphos are gravitationally linked, they revolve around a shared centre of mass, known as a barycentre. When DART hit Dimorphos, it didn’t merely jolt the smaller asteroid; it also blasted material off into space.
Scientists had predicted that this escaping debris would carry momentum away, producing a minute recoil that could slightly modify the motion of the Didymos–Dimorphos pair in its heliocentric orbit.
Measurements of the DART effect on Didymos and Dimorphos
In the years following the September 2022 impact, the system has been tracked closely. Makadia’s team examined data from 22 stellar occultations, 5,955 ground-based measurements of the system’s position, three navigation measurements from the DART spacecraft itself, and nine ground-based distance measurements.
Taken together, the observations indicate that the collision did provide a subtle shove to the Didymos–Dimorphos system, reducing its orbital speed by about 11.7 micrometres per second – about 42 millimetres per hour (roughly the width of an Apple Watch).
In the space environment, though, even a barely perceptible change can build into a sizeable displacement. Across ten years, a steady difference of 11.7 micrometres per second would add up to around 3.69 kilometres.
That suggests that, on timescales relevant to planetary defence-years or decades of warning, if we are fortunate-even a small deflection could potentially move a dangerous asteroid onto a path that safely misses Earth.
What ESA’s Hera mission will add
Upcoming work should sharpen the picture of what occurred during the collision. The European Space Agency’s Hera spacecraft, due to reach the Didymos system later this decade, will inspect the crater created by DART and make detailed measurements of the asteroids’ masses and internal structure.
Even with what is known today, the milestone is extraordinary: for the first time, humans have changed the course of a natural object moving 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 has been published in Science Advances.
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