A method of decoding the interior of asteroids could help target asteroid deflection missions

NASA hit the mark 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 impacted 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 toward Earth.

Now MIT researchers have a tool that could improve the aim of future asteroid targeting missions. The team has developed a method to map an asteroid’s internal structure, or density distribution, based on how the asteroid’s spin changes when it encounters more massive objects like Earth.

Knowing how densities are distributed within an asteroid could help scientists plan the most effective defenses. For example, if an asteroid is composed of relatively light and uniform matter, a DART-like spacecraft could be oriented differently than if it were deflecting an asteroid with a denser, less balanced interior.

“If you knew the density distribution of the asteroid, you could hit it in just the right spot to actually make it move away,” says Jack Dinsmore ’22, who developed the new asteroid-mapping technique as an MIT physics major.

The team is keen to apply the method to Apophis, a near-Earth asteroid estimated to pose a significant hazard should it impact. Scientists have ruled out the likelihood of a collision on Apophis’ next flybys for at least a century. In addition, their forecasts become fuzzy.

“Apophis will miss Earth in 2029, and scientists have cleared her for their next encounters, but we can’t clear her forever,” says 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, assistant professors in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), explain their new method in a study published today in the Monthly Bulletins of the Royal Astronomical Society.

Spinning cooked versus raw

The origin of the team’s asteroid mapping method emerged from an MIT course Dinsmore attended last year and was taught by de Wit. Class 12.401 (Essentials of Planetary Sciences) introduces the basic principles and formation mechanisms of planets, asteroids and other objects in the solar system. As a final project, Dinsmore researched how an asteroid behaves during a close encounter.

In class, he wrote code to simulate different shapes and sizes of asteroids and how their orbital and spin dynamics change when affected by the gravitational pull of a more massive object like Earth.

“I was initially just trying to ask what happens when an asteroid passes by Earth? Does he react at all? Because I wasn’t sure,” Dinsmore recalled. “And the answer is that in a way it depends very much on the shape and the physical properties of the asteroid.”

This initial finding raised 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 makeup? To get an answer, Dinsmore continued the project with de Wit through the MIT Undergraduate Research Opportunities Program (UROP), which allows students to conduct their own research with a faculty member.

Diving deeper into the dynamics of a close encounter, he and de Wit wrote more complex code that they used to simulate a zoo of different asteroids, each with a different size, shape, and internal composition, or density distribution. They then ran the simulation forward to see how each asteroid’s spin should wobble, or shift, as it passes close to an object of a given mass and gravitational pull.

“It’s similar to how you can tell the difference between a raw egg and a cooked one,” says de Wit. “When you spin the egg, the egg reacts and spins differently depending on its internal properties. The same is true for an asteroid in a close encounter: you can understand what’s happening inside just by noticing how it responds to the strong gravitational forces it experiences during a flyby.”

A tight game

The team is presenting their findings in a new software “toolkit” they’re calling AIME for Encounters’ Asteroid Interior Mapping (the acronym also means “love” in French). The software can be used to reconstruct an asteroid’s internal density distribution from observations of its rotation change during a close encounter.

The researchers say that if scientists can make more detailed measurements of asteroids and their spin dynamics during close encounters, those measurements could be used to improve AIME’s reconstructions of asteroid interiors.

Their best chance, they say, might come with Apophis. During the upcoming close encounters, de Wit and Dinsmore hope astronomers will turn their telescopes on the space rock to measure its size, shape and rotational evolution as it flies by. They could then feed these measurements into AIME to find a match – a simulated asteroid with the same size, shape and spin dynamics as Apophis, also relating to a specific internal density distribution.

“Then you could use AIME to publish a density map that most likely represents the interior of Apophis,” says Dinsmore.

“Understanding the internal properties of asteroids helps us to understand to what extent close encounters might be of concern and how to deal with them, as well as where they formed and how they got here,” adds de Wit. “With this frame, there is now a new way to look inside an asteroid.”

This story is republished with permission from MIT News (web.mit.edu/newsoffice/), a popular website covering news about MIT research, innovation, and teaching.