The collision of giant asteroids with the Earth, capable of causing global extinction, has been part of the premise of more than one Hollywood blockbuster. But this idea was not only in the minds of science fiction screenwriters; Several scientists, and of course NASA itself, have taken very seriously the possibility of an asteroid over 100 meters in size colliding with our planet and causing a significant level of destruction, surpassing the explosion of a nuclear bomb. Among the most interesting proposals and experiments of recent years, DART stands out.
In September 2022, NASA announced that its Double Asteroid Redirect Test (or DART, Double Asteroid Redirection Test) was successful when a space probe collided with a 160-meter-diameter asteroid called Dimorphos to demonstrate that asteroid deflection was possible through a kinetic impact. As the name of the DART mission indicates, Dimorphos is part of a binary system. This means that Dimorphos is orbiting another much larger asteroid called Didymos, measuring 780 meters. An asteroid with its own moon.
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A team of astronomers from the INAOE in Tanantintla, Pueblo, aims to study near-Earth asteroids to prevent future threats. With historic telescopes like the Schmidt Camera, these “asteroid hunters” continue to expand the world’s catalog.
While DART first indicated that a man-made device managed to alter the orbit of a celestial object, more research is needed to determine whether this kinetic impact method would be sufficient to protect Earth from a potentially dangerous near object. We now have the first measurement of DART-induced changes in our hands, published this week in the journal Achievements of science. It turned out that the impact of DART not only changed the orbit of Dimorphos, but also slightly changed the heliocentric orbit of the binary system to which it belongs.
A kamikaze probe
Before the DART Dimorphos mission, it took 11 hours and 55 minutes to complete one orbit around Didymos. After the impact, its orbit was reduced by about 33 minutes. Currently, the orbital period is 11 hours and 22 minutes.
And what else. When NASA’s spacecraft collided with Dimorphos at more than 6 kilometers per second, it didn’t just transfer its own momentum to the asteroid. The impact ejected a large amount of material from the surface of the body, creating a cloud of debris that was ejected into space (which would apparently lead to the first artificial meteor shower). This ejected material also carried momentum, amplifying the effect of the impact
The phenomenon is measured by a parameter known as the moment-magnification factor (momentum enhancement factor). This compares the total thrust transmitted (shock plus debris) to that which would be from the ship. According to the study, the material ejected during the crash provided about the same amount of extra momentum as the impact of the spacecraft itself. This result confirms that when an impactor collides with an asteroid, the ejected fragments can greatly amplify the effect of the collision.
How did Didymos influence
The spacecraft of the DART mission did not directly hit the main asteroid Didymos, but its small satellite Dimorphos. However, both form a binary system, so they are gravitationally bound and revolve around a common center of mass, the barycenter.
When the DART hit the Dimorphos, it transferred the momentum and also ejected a large amount of debris. This shock changed the speed and shape of the orbit of Dimorphos, which began to revolve closer and faster around Didymus. When the orbit of one of the two bodies is modified, the gravitational dynamics of the entire system changes slightly, thus the trajectory of the center of mass and the relative motion of Didymos are also adjusted. The effect was extremely small.
Detect a small change
To measure the change in the trajectory of the entire system around the Sun, the researchers analyzed a database of astronomical observations with nearly 6,000 measurements of the system’s position obtained over 29 years, mostly by amateur astronomers. Radar observations and measurements of stellar eclipses were also taken into account, a very precise method consisting of recording the moment at which an asteroid passes in front of a star.
With these measurements, the scientists were able to estimate the change in velocity of the asteroid system after the DART impact. The collision resulted in a change in velocity of -11.7 ± 1.3 micrometers per second in the direction of its motion around the Sun.
This may seem insignificant, but even such a small change is enough to gradually change the orbit of a planet or asteroid. Calculations show that the semi-major axis of the orbit (average distance to the Sun) has decreased by about 360 meters, which slightly shortens the period of its return. In practical terms, the Didymos system now completes each revolution around the Sun about 150 milliseconds earlier.
Other tracks
The analysis also allowed us to estimate the mass and density of the two asteroids. The results show that Dimorphos is much less dense than its companion Didymos. This suggests that Dimorphos may have been a “heap of rubble,” that is, a body composed of loose fragments held together by gravity rather than solid rock. This type of structure is common among small asteroids.
In addition, the analysis shows that Didymos is almost 200 times more massive than Dimorphos, which explains why the impact caused a much larger change in the moon’s orbit than in the trajectory of the entire system.
Does it serve to protect the planet?
Step aside, Bruce Willis! Thanks to DART, humanity has shown that we can change the trajectory of an object at a distance of more than 700 meters. In addition, the study confirms that targeting a secondary object can be particularly effective in binary systems. A change in orbit can transfer some of the momentum to the whole system.
However, much work remains to be done before NASA can conclude that the kinetic impact method is viable as a model for protecting the planet from large asteroids.
The researchers hope to refine these measurements when the European Space Agency’s Hera mission arrives at the Didymos system in the coming years. The spacecraft will study the crater left by DART, accurately measure asteroid masses and provide navigational data that will improve orbit calculations. With these observations, it will be possible to understand in more detail how asteroids react to a human-induced impact.
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