Dust in the (Solar) Wind

On Earth, no natural phenomenon is quite as predictable as gravity. Even a child playing on a beach knows that the sand she’s excavating will remain in her trowel until she dumps it, held steady by this powerful force.

But on small, low-gravity celestial bodies like asteroids, the rules we know so well no longer apply—at least, not in the ways that we’re used to. And that’s a problem for the scientists like Christine Hartzell, who studies regolith, the dusty or pebbly material found on the surfaces of these bodies.

“Gravity is so weak on the surface of these bodies that our intuition fails,” says Hartzell, an assistant professor of aerospace engineering at the University of Maryland. “And there’s a large degree of uncertainty in which other forces are important, and how strong they are.”

Hartzell is a participating scientist on the OSIRIS-REx mission currently orbiting Bennu, an asteroid between Earth and Mars with an average diameter of almost 500 meters that NASA says could one day pose an impact hazard to Earth. The mission is scheduled to take a sample from the asteroid next year and return it to Earth in 2023.

Asteroids like Bennu are remnants of the early solar system: essentially chunks of material that did not become planets. Regolith samples from asteroids and other small celestial bodies are critical for researchers to better understand how the solar system began, and how it has evolved since.

In the absence of strong gravitational influences, electrostatic forces that would be considered weak to negligible on Earth can hold outsized importance in space. Hartell studies these electrostatic forces to advance our understanding of the natural evolution of asteroids and help inform the design of sampling methods and instruments on future asteroid exploration missions.

Electrostatic forces occur when oppositely charged particles interact with each other. This causes regolith particles to behave curiously in three ways: First, they cause dusty particles that rub against each other to stick together, or clump. Second, dust exposed to the flow of charged particles from solar wind plasma can detach (or “loft” away) from the surface, drawn to opposite charges in the solar wind flowing past. Third, particles can levitate after being kicked up by a small meteorite impact or blasted by a visiting spacecraft, because the electrostatic forces on those particles cancel out any gravitational pull.

And it’s possible that it’s not just tiny dust particles that may behave unusually—but larger grains, due to the extremely weak effects of gravity on asteroids.

Though Hartzell’s work has demonstrated these forces in laboratory experiments, many questions remain about what they look like on an asteroid, and how the presence of a spacecraft in close proximity to an asteroid’s surface might change the environment.

Whether lofting occurs depends on the strength of the forces causing particles to stick together and, by extension, to other objects, such as spacecraft surfaces and optics. Hartzell is developing an experimental method to measure this cohesion that could potentially be used for actual sampling on a future exploratory mission. She suggests that charged plates could be used to attract dust samples, then drop them into sample collectors or directly onto analysis instruments by removing the plate’s charge.

More likely, however, is that the method could help characterize the surface of a site intended for longer-term use—asteroid mining, for instance. Early planning stages would involve understanding the chemistry and behavior of any dusty surface, including how its cohesive properties may affect the function of tools like drill bits.

Harnessing electrostatic forces to control dusty particles might also mean cleaner, better functioning solar panels on Mars. An electrostatic dust shield could use coils embedded in solar arrays to “bounce” dust grains off the surface via alternating electrical charges.

But for now, Hartzell is conducting a lot of creative lab experiments and lab-based modeling, with one goal in mind: “We want to keep the spacecraft safe during operations,” she says.