When asteroids suffer natural impacts in space, debris flies off from the point of impact. The tail of particles that form can help determine the physical characteristics of the asteroid. NASA’s Double Asteroid Redirection Test mission in September 2022 gave a team of scientists including Rahil Makadia, a Ph.D. student in the Department of Aerospace Engineering at the University of Illinois Urbana-Champaign, a unique opportunity—to observe the evolution of an asteroid’s ejecta as it happened for the first time.
“My work on this mission so far has been to study the heliocentric changes to the orbit of Didymos and its smaller moon Dimorphos—the target of the DART spacecraft,” said Makadia. “Even though it hit the secondary, there are still some changes in the entire system’s orbit around the sun because the entire system feels the consequences of the impact. The ejecta that escapes the system provides an extra boost in addition to the impact. So, to accurately determine where the system will be in 100 years, you need to know the contribution of the ejecta that escaped the system.”
The team observed a 33-minute change in the orbit after DART’s impact. Makadia said, if there were no ejecta, the period change would have been less than 33 minutes. But because some ejecta escaped the gravitational pull of Dimorphos, the orbit period change is higher than if there were no ejecta at all.
These three panels capture the breakup of the asteroid Dimorphos when it was deliberately hit by NASA’s 1,200-pound Double Asteroid Redirection Test mission spacecraft on September 26, 2022. Hubble Space Telescope had a ringside view of the space demolition derby. The top panel, taken 2 hours after impact, shows an ejecta cone of an estimated 1,000 tons of dust. The center frame shows the dynamic interaction within the asteroid’s binary system that starts to distort the cone shape of the ejecta pattern about 17 hours after the impact. The most prominent structures are rotating, pinwheel-shaped features. The pinwheel is tied to the gravitational pull of the companion asteroid, Didymos. In the bottom frame Hubble next captures the debris being swept back into a comet-like tail by the pressure of sunlight on the tiny dust particles. This stretches out into a debris train where the lightest particles travel the fastest and farthest from the asteroid. The mystery is compounded when Hubble records the tail splitting in two for a few days.
The study, published in the journal Nature, focused on the Hubble Space Telescope’s measurements of the ejecta, beginning 15 minutes after the impact to 18 ½ days after the impact. The images showed the exact evolution of the tail and how it evolved over time.
You can watch a video that condenses images from 18 ½ days down to 19 seconds.
“After a few days, the primary force acting on these ejecta particles becomes solar radiation pressure,” Makadia said. “The photons emitted from the sun exert an acceleration on these small particles, and they evolve into a straight tail in an anti-solar direction.
“There have been cases in which it was determined that a natural impact caused the observed active asteroid. But because this one was very much intended, we could have telescopes pointed at it before and after the impact and study its evolution.”
He said they’ll use the data about how this ejecta evolves to understand how the entire system’s orbit changes as well.
“Now that we have this treasure trove of data, we can make educated guesses about other tails we might observe,” Makadia said. “Depending on what kind of particles are in the tail and their sizes, we can figure out how long ago that impact happened. And we’ll be able to understand the ejecta that escape the system and change the entire system’s heliocentric orbit.”
Makadia, who earned his B.S. in 2020 from UIUC, said almost all of his work is computational.
“To calculate where an asteroid will be on a given date, we need to propagate all the possible locations that the asteroid could be at an initial time, not just one nominal solution. That requires a lot of computational power and understanding of how orbits are affected by small forces, like solar radiation pressure as well as gravity from all kinds of sources within the solar system.
“I developed simulations to study the heliocentric changes when I first started working on my Ph.D. to make sure we have a propagator that can impart all these impulses that are coming from the escaping ejecta. Now I’m developing an orbit determination tool so once we do have enough observations, we can extract this information about the heliocentric change to the system.”
About the project, Makadia said, “This is 100 percent the most exciting thing in my life. It’s absolutely real but so astonishing. Even now, whenever people ask about it, it sounds like I’m talking about a movie plot rather than an actual thing that happened.”
The NASA/Johns Hopkins University Applied Physics Laboratory Double Asteroid Redirection Test team which includes Rahil Makadia, his adviser Siegfried Eggl, and Bhaskar Mondal who is another one of Eggl’s Ph.D. students, is receiving the 2023 AIAA Award for Aerospace Excellence. The award states it is “In recognition of humanity’s first time purposely changing the motion of a celestial object by a team of protectors of our home planet.”
The study, “Ejecta from the DART-produced active asteroid Dimorphos,” by Jian-Yang Li, et al., is published in the journal Nature. DOI: 10.1038/s41586-023-05811-4