DART had a surprising effect on its target, according to recent observations

After NASA’s DART mission collided with the Dimorphous asteroid in September 2022, scientists determined that the impact caused tons of rock to be ejected from the surface of the small asteroid. But more importantly, the DART effect changed the orbital period of Dimorphos, causing it to decrease by about 33 minutes.

However, a group of researchers measured the orbital period about a month later and discovered that it had increased to 34 minutes, one minute longer than the first measurements. Although it was a single DART effect, some of the force continued to slow the asteroid’s orbit, and astronomers don’t yet know what that mechanism is.

“We found that no mechanism previously introduced for this system could explain this large change in period, and drag from impact projectiles is an unlikely explanation,” the researchers wrote in their paper, which was published as a preliminary edition. arXiv. “Further observations of the Didymus (65803) system are needed to confirm our results and to further understand this system after effect.”

The purpose of DART was to test how asteroids respond to shocks. When the first data was released after the collision, the change in the orbital period was great news, because this type of kinetic effect is a planetary defense technique, in which a spacecraft deliberately collides with a potentially dangerous asteroid to change its trajectory. Data from DART helps both NASA and the European Space Agency prepare for the possibility of having to redirect an asteroid away from eventual impact with Earth.

Andy Rifkin, co-lead of the DART investigation team at Johns Hopkins Applied Physics Laboratory (APL), said in December 2022 when the first DART data was released: “We know the initial experiment worked. Now we can start applying that knowledge.” .

The DART weighed 610 kg (1,340 lb) and crashed into Dimorphos at 22,530 km/h (approximately 14,000 mph). DART drilled a hole on the surface of Dimorphos that ejected more than 900,000 kilograms (990 US tons) of debris into space. He noted that the impact of DART on Dimorphos also altered the trajectory of the moon’s parent asteroid, Didymus.

Scientists estimate that the DART effect dislodged more than a million kilograms (two million pounds) of dusty rock out into space, or enough to fill six or seven railroad cars. The DART science team continues to analyze its data, as well as new information about the asteroid’s moon composition and ejecta characteristics, to see how much DART’s initial hit moved the asteroid, and how much bounce it generated.

But now, another group of researchers, led by Taylor Godebsky and Elizabeth Hildridge, used the 0.7-meter telescope at the Thacher Observatory on the campus of the Thacher School in Ventura County, California, to make their observations.

They measured the change in the post-collision period in observations taken about 20 to 30 days after the initial data, and their results suggest that the system’s period may have shortened during this short period.

One idea was that because the debris cloud was so large and changed over time, it might affect the orbit of Dimorphos. In a study released in March 2023, astronomers tracked the evolution of the debris cloud from the collision for a month, and found that as the debris expanded outward, structures began to form, such as clumps, spirals, and long tails pushed away by the sun’s radiation. . But Godebski and Hildridge’s team don’t think the debris could be responsible for the change they observed.

Interestingly, the researchers said, even before the DART impact, the dimorphosity period was observed to change slowly. But, the researchers write, even this amount of change could not explain the difference, “as it was four times too small to explain the difference we see.”

“Therefore, whatever effect caused the orbital decay before the collision cannot explain the discrepancy we observe; this includes the binary YORP effect, reciprocal tides, differential Yarkovsky force, nodal precession, and mass loss,” they said.

This group of researchers and the DART team will continue to monitor and study the impact of DART. It will be interesting to see whether or not the orbital period continues to decrease, and how this might affect the use of kinetic impactors.

In addition, another spacecraft will be launched in 2024 to study Dimorphos more closely. The European Space Agency’s HERA mission is supposed to reach Didymos and Dimorphos in December 2026. HERA will conduct a detailed study of Dimorphos to understand more deeply how the impact affects it.