Planetary defense is a challenge that concerns all of humanityas our planet is constantly exposed to the risk of impacts with potentially dangerous asteroidsand to prevent or mitigate this risk, scientists are studying different techniques to divert asteroids from their original trajectory and away from Earth, somewhat like what happened with Dimorphos and NASA’s DART mission.
One of these techniques is kinetic impaction, which involves launching a spacecraft into an asteroid at high speed, thereby transferring some of its momentum and changing its orbit.
To test the effectiveness of this technique, NASA launched the DART mission in 2021 (Double Asteroid Redirection Test), the first large-scale demonstration of asteroid redirection technology. The mission’s goal was to intentionally hit a small asteroid called Dimorphos, which orbits a larger asteroid called Didymos, and change its orbital period. This binary asteroid system poses no threat to Earth, but was chosen because it offers a natural laboratory to study the effects of kinetic impact.
There DART mission he reached his goal on September 26, 2022, when the spacecraft slammed into Dimorphos at a speed of about 6.6 kilometers per second. The impact created a crater about 40 meters in diameter on the surface of the asteroid, which has a diameter of about 160 meters, and ejected a large amount of debris into space. The impact also changed Dimorphos’ orbit around Didymos, reducing its orbital period from 11 hours 55 minutes to 11 hours 23 minutes, according to measurements taken by ground-based telescopes in the days after the collision.
This change was much greater than that predicted by NASA, which had defined a minimum success as a reduction in the orbital period of at least 73 seconds. However, what surprised scientists even more was the fact that Dimorphos’ orbit continued to change even after the impact, in an unexpected and inconsistent way. A new research conducted by a group of high school students at the Thacher School in California showed that Dimorphos’ orbital period increased gradually until November 6, 2022, reaching a value higher than that measured immediately after the impact.
What the research carried out on Dimorphos debris says
The students used their school’s 0.7-meter telescope to observe the binary asteroid system before and after the impact, and calculated Dimorphos’ orbital period based on the change in its reflected brightness. They found that the orbital period had increased from 11 hours and 23 minutes to 11 hours and 34 minutes within a month and a half. This result was in contrast to official NASA measurements, which indicated a reduction in the orbital period to 11 hours and 21 minutes over the same time interval.
“We got a slightly higher number, a variation of 34 minutes,” Dr Jonathan Swift, teacher and director of the Thacher School observatory, told New Scientist. “It’s annoyingly inconsistent.” The students presented their work at the American Astronomical Society meeting in Albuquerque in June 2022, and in a paper accepted for publication in the Research Notes of the American Astronomical Society.
How do you explain this discrepancy between the students’ measurements and those of NASA? And why did Dimorphos’ orbit continue to change after the impact? Scientists do not yet have a definitive answer, but they hypothesize that the phenomenon is linked to the distribution and movement of debris ejected by the impact. This debris, in fact, may have interacted with Dimorphos in different ways, influencing its momentum and orbit.
One possibility is that some debris fell onto Dimorphos’ surface, transferring additional momentum to it and slowing it down even more. Another possibility is that some debris remained in orbit around Dimorphos, forming some sort of cometary ring or tail. This debris may have undergone the Yarkovsky effect, that is, a push caused by the temperature difference between the illuminated side and the shadowed side of the object. This push may have changed the orbit of the debris and, by reaction, that of Dimorphos.
These hypotheses are supported by some images obtained from the Hubble Space Telescope, which observed the binary asteroid system on 4 and 8 October 2022. The images show a tail of material ejected from the surface of Dimorphos after the impact, which changes shape in time. Notably, the tail splits into two distinct parts, suggesting that the debris has different dynamics.
“If you hit a pile of rubble with a spacecraft, a large amount of material will be ejected and fly away from the object. We see it in the first images after the collision. The ejected material carries momentum with it,” Dr. Cristina Thomas, of Northern Arizona University, said in an earlier press conference. “The period change we observed is not only the result of momentum transfer from the colliding spacecraft, but also an additional momentum gain due to the motion of the projectile.”
To better understand what happened to Dimorphos after the impact, scientists will have to wait for the arrival of the European Space Agency’s Hera mission, scheduled for the end of 2026. Hera will be the first mission to visit a binary asteroid system and examine up close the crater created by DART and the surrounding debris. Hera will also be accompanied by two small CubeSat satellites, called APEX and Juventas, which will make additional measurements on Dimorphos’ shape, mass, density and gravitational field.
The DART mission demonstrated that kinetic impaction is an effective technique for deflecting asteroids from their original orbit, but it also revealed that the system is much more complex than previously thought. The impact not only changes the asteroid’s orbit in the short term, but also in the long term, due to interactions with the ejected debris. This behavior was not expected, but in fact this was the first time humanity had literally moved a celestial body. For this reason, scientists must continue to study the phenomenon and monitor the binary asteroid system to better understand its dynamics and implications for planetary defense.
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