NASA hit the bullseye in late September with DART, the Double Asteroid Redirection Test, which flew a spacecraft directly into the heart of a nearby asteroid. The one-way kamikaze mission slammed into the stadium-sized space rock and successfully reset the asteroid’s orbit. DART was the first test of a planetary defense strategy, demonstrating that scientists could potentially deflect an asteroid heading towards Earth.
MIT researchers now have a tool that could improve the aim of future asteroid targeting missions. The team developed a method to map an asteroid’s interior structure, or density distribution, based on how the asteroid’s rotation changes when it makes close encounters with more massive objects like Earth.
Knowing how density is distributed inside an asteroid could help scientists plan the most effective defense. For example, if an asteroid were made of relatively light, uniform matter, a DART-like spacecraft might aim differently than if it deflected an asteroid with a denser, less balanced interior.
“If you know the density distribution of the asteroid, you could hit it in the right place to make it actually move away,” says Jack Dinsmore ’22, who developed the new asteroid mapping technique as MIT undergraduate majoring in physics.
The team is eager to apply the method to Apophis, a near-Earth asteroid that is believed to pose a significant hazard if it were to impact. Scientists have ruled out the likelihood of a collision in future Apophis flybys for at least a century. Beyond that, their predictions become hazy.
“Apophis will miss Earth in 2029, and scientists have cleared it for its next encounters, but we can’t clear it forever,” said Dinsmore, who is now a graduate student at Stanford University. “So it’s good to understand the nature of this particular asteroid, because if we ever need to redirect it, it’s important to understand what it’s made of.”
Dinsmore and Julien de Wit, an assistant professor in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS), detail their new method in a study published today in the Royal Astronomical Society Monthly Notices.
Spinning boiled versus raw
The seeds for the team’s asteroid mapping method grew from an MIT course Dinsmore took last year, taught by de Wit. The class, 12.401 (Essentials of Planetary Sciences), introduces the basic principles and mechanisms of formation of planets, asteroids, and other solar system objects. As a final project, Dinsmore explored the behavior of an asteroid during a close encounter.
In class, he wrote code to simulate different shapes and sizes of asteroids and how their orbital and rotational dynamics change when influenced by the gravitational pull of a more massive object like Earth.
“Initially, I just tried to ask what happens when an asteroid passes by Earth? Does it react at all? Because I wasn’t sure,” Dinsmore recalled. “And the answer is, it does, in a way that depends very heavily on the shape and physical properties of the asteroid.”
This initial realization prompted another question: Could the dynamics of an asteroid’s close encounter be used to predict not only its shape and size, but also its internal composition? To get an answer, Dinsmore pursued the project with de Wit, through MIT’s Undergraduate Research Opportunity Program (UROP), which allows students to conduct original research with a faculty member.
He and de Wit delved deeper into the dynamics of a close encounter, writing more complex code, which they used to simulate a zoo of different asteroids, each with a different size, shape, and internal composition, or a density distribution. . They then ran the simulation to see how each asteroid’s rotation should oscillate or shift as it passes near an object of a certain mass and gravitational pull.
“It’s similar to how you can tell the difference between a raw egg and a boiled egg,” de Wit offers. “If you spin the egg, the egg reacts and spins differently depending on its inner properties. The same goes for an asteroid in a close encounter: you can figure out what’s going on inside just by looking how it reacts to the strong gravitational forces it experiences during an overflight.”
A close match
The team presents its findings in a new software “toolkit” it calls AIME, for Asteroid Interior Mapping from Encounters (the acronym also translates to “amour” in French). The software can be used to reconstruct the internal density distribution of an asteroid, from observations of its spin change during a close encounter.
The researchers say that if scientists can take more detailed measurements of asteroids and their rotational dynamics during close encounters, these measurements could be used to improve AIME’s reconstructions of asteroid interiors.
Their best chance, they say, could come with Apophis. During his next close encounters, de Wit and Dinsmore hope astronomers will point their telescopes at the space rock to measure its size, shape and rotational evolution as it passes. They could then feed those measurements into AIME to find a match – a simulated asteroid with the same size, shape and rotational dynamics as Apophis, which also relates to a particular interior density distribution.
“Then, with AIME, you could publish a heatmap that most likely depicts the interior of Apophis,” Dinsmore explains.
“Understanding the interior properties of asteroids helps us understand how concerning close encounters might be, and how to deal with them, as well as where they formed and how they got here,” adds de Wit. “Now, with this frame, there’s a new way to see inside an asteroid.”
This research was supported, in part, by the MIT UROP office.