Un-Earthing planetary defense: Studying extraterrestrial bodies could help defend Earth from future impact threats (op-ed)

Wendy K. Caldwell is a mathematician/planetary scientist at Los Alamos National Laboratory. She is the lead author of Los Alamos National Laboratory’s Psyche research and a member of the DART Investigation Team, a multi-agency international collaboration. She is also a regular human being who volunteers her time acting, dancing, directing, and choreographing for local performing arts organizations. Caldwell contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

This summer, NASA will launch its first mission to a metallic asteroid, 16 Psyche, located in the main asteroid belt between the orbits of Mars and Jupiter. Previous missions have explored rocky and icy asteroids, but Psyche’s composition is widely believed to consist of a considerable amount of metal. Of course, in today’s click-bait culture, googling Psyche will take you down the proverbial internet rabbit hole of stories about how it is worth more than the global economy. As enticing as that idea is, we are not going to scrap it and sell it for parts. 

From a scientific perspective, though, Psyche is priceless.

The Psyche mission: Visiting a metal asteroid

In school, we learn that Earth has a layered structure: crust, mantle and core (inner and outer). Billions of years ago, as Earth and other planets were forming, collisions between solid bodies were more frequent than today. Some of these collisions resulted in accretion, helping to build up these layers and leading to larger solid bodies that became planets. Others resulted in disruption, in which planetesimals were blown apart before they could fully form into planets. The metallic asteroids are thought to be their remnants, cores of would-be planets that were stripped of their outer layers through high-velocity impacts. 

Psyche is the largest of these remnants, with an approximate diameter of 140 miles (225 kilometers). Psyche may hold answers to questions about the early solar system, including how planets form, or, in Psyche’s case, fail to fully form. The NASA mission will be equipped with instruments for measuring various properties of Psyche, and these data can provide more insight into the asteroid's composition, including how much of it is metallic and how much porosity (empty space) is present. 

The mission is not expected to arrive at the asteroid until 2026. Until it begins collecting data, computational modeling can help us understand Psyche by exploring the feasibility of different material compositions for it. Psyche has two large impact structures in its southern hemisphere. Simulating the formation of these craters using computational models helps to probe how such large and relatively shallow craters could have formed on Psyche. Simulations we have run using Los Alamos National Laboratory’s extensive computational resources indicate Psyche may be a rubble-pile configuration, consisting of large boulders held together by gravitational forces. In our simulations, impacts into rubble piles created crater shapes similar to those found on Psyche: large in diameter, relatively shallow in depth. (See simulation video here).

At Los Alamos, we are interested in solid bodies for another application: planetary defense. If a large body were on an Earth-crossing trajectory, the more we knew about the object, the greater the likelihood of successful deflection attempts to prevent a catastrophic impact. Perhaps the most commonly known event was the Chicxulub impact, which wiped out the dinosaurs. From that point in time, about 65 million to 66 million years ago, Earth's environment was changed forever. 

Before panic sets in, note that the frequency of such catastrophic impacts ranges from tens of thousands to hundreds of millions of years. To further clarify, Psyche is not on an Earth-crossing trajectory and does not pose a threat. Data from the Psyche mission, however, can give us a better idea of how well our observational and modeling tools determine material properties of distant objects. Such information would be crucial to successfully deflecting any future potentially hazardous objects. 

These objects are characterized as being large enough to pose a threat — about 99 feet (30 meters) across, for a city, and about 460 feet (140 m) across, for a region — and on a path to come within 4.6 million miles (7.4 million km) of Earth’s orbit. The notion of "close" in space is quite different than here on Earth. An object deemed close to Earth's orbit is approximately within the distance that Earth travels through space in three days. So, when you hear of a close approach, the object is often still extremely far away and often too small to pose a threat, thanks in large part to Earth's atmosphere, which provides an extra layer of protection against incoming objects.

Related: Potentially hazardous asteroids (images)

Currently, an ongoing planetary defense mission may hold the answers to the question of whether we can deflect an asteroid with a kinetic impactor — essentially a big cannonball. NASA's Double Asteroid Redirection Test (DART) mission, launched in November 2021, will travel to the Didymos binary asteroid system, in which a smaller body, Dimorphos, orbits the larger Didymos. Like Psyche, Dimorphos is far away, limiting Earth-based observations of its material properties. Further, like Psyche, Dimorphos does not pose a threat to Earth.

The spacecraft will impact Dimorphos at a velocity greater than 13,320 mph (21,440 kph), and the subsequent transfer of energy is expected to alter the orbit of the asteroid. Data will be collected before, during and after impact, and those data will be used to evaluate the efficacy of the kinetic impactor method for planetary defense. 

We are currently using the same modeling techniques we employed to understand Psyche to model impacts on Dimorphos. Of course, there are some notable differences. Dimorphos, which has a diameter of about 558 feet (170 m), is considerably smaller than Psyche. As a result, Dimorphos can be modeled in more detail than Psyche, based on the computational cost of large simulations. We modeled Psyche using the nickel-iron alloy Monel, based on the properties of ore from the Sudbury impact crater in Canada. The ore is believed to have come from the impactor that formed the crater, which means Monel is essentially "sky metal" that likely has similar properties to metallic asteroids. 

Dimorphos, on the other hand, is a rocky body, so we use igneous geologic materials like basalt, which has similar properties to lunar samples, in our models. Because the DART mission has had years of planning, we know exactly what the spacecraft impactor is: we know size, shape, material, velocity and a range of probable impact angles. Thus, Dimorphos models can be more finely tuned to represent the actual impact. The impact into Psyche was billions of years ago, so modeling choices were more difficult to make.

The good news is that both of these missions will result in additional data that we can use to update our models for more accurate results in the future. The data will also help us determine the uncertainties of our models and could highlight areas in which we could reduce those uncertainties. Science is never finished: we are always reevaluating our methods, conducting new experiments, gathering new data and forming new hypotheses to test. But science does get better: new information leads to more accurate models, renders some hypotheses incorrect and indicates that others align with our current understanding of the field. Even a finding such as "that didn’t work at all" has huge scientific value. Knowing what does not work can point us in the right direction for what may work.

When the Psyche mission launches this summer, and when the DART spacecraft crashes into Dimorphos this fall, be sure to raise a glass to science. 2022 is going to be the most exciting year for planetary science since I entered the field as a graduate student. For the first time ever, a mission will begin a trip to the main asteroid belt to visit a metallic asteroid, potentially opening a window to the early days of the solar system. Shortly after that mission launches, the DART spacecraft will reach Dimorphos on a one-way trip to an epic crash that could pave the way for a new era in planetary defense. Science is about trial and error, and although we do not always get it right, we never stop trying.

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