Scientists using data from NASA's Parker Solar Probe have assembled a comprehensive picture of the structure and behavior of the large cloud of space dust that swirls through the innermost solar system - and the new insight offers clues to similar clouds around stars across the universe.

Research teams led by Jamey Szalay of Princeton University and Anna Pusack of University of Colorado, Laboratory for Atmospheric and Space Physics took advantage of Parker Solar Probe's flight path - an orbit that swings it closer to Sun than any spacecraft in history - to get the best direct look yet at the most active region of the zodiacal cloud, a dusty cloud of grains shed from passing comets and asteroids. The teams published their findings Sep. 9 in the Planetary Science Journal.

"Every stellar system has a zodiacal cloud, and we actually get to explore ours and understand how it works," said Szalay. "Understanding the evolution and dynamics of our zodiacal cloud will allow us to better understand every zodiacal observation we've seen around any other stellar system."

Built and operated by the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, Parker Solar Probe does not carry a dedicated dust counter that would give it accurate readings on grain mass, composition, speed and direction. But as dust grains pepper the spacecraft along its path, the high-velocity impacts create plasma clouds. These impact clouds produce unique signals in electric potential that are picked up by several sensors on the probe's FIELDS instrument, which is designed to measure the electric and magnetic fields near the Sun.

Gathering data from Parker Solar Probe's first six orbits around the Sun, the researchers saw impacts that were consistent with the two primary dust populations in the cloud. The first population makes up the bulk of the zodiacal cloud: most of the grains are being slowly pulled in toward the Sun over thousands to millions of years; then, as the swirling cloud gets denser, the larger grains collide and create fragments that, if small enough, are pushed out of the solar system in all directions by pressure from sunlight. This second population of smaller fragments are called beta-meteoroids.

"There are two populations of material that are part of the same story on how the cloud evolves, and with Parker Solar Probe we were able get the closest look yet at the most intense region," Szalay said. "It was really exciting that we got to measure not just the cloud itself, but the way it loses material."

But it was readings from Parker Solar Probe's fourth and fifth orbits - which were a step closer than the first three - that really got the researchers' attention. As the spacecraft sped away from the Sun, it picked up an enhancement in dust detections that didn't match the two-component model, a tip that another dust population may be in the area, less than a third of Earth's distance from the Sun.

One idea was that the spacecraft flew through a "tube" of materials called a meteoroid stream, causing a spike in dust impacts - much like how we see a meteor shower when Earth moves through one of these streams. But the closest streams didn't seem to have enough material to cause the enhancement.

So the team went in a different direction: One of these meteoroid streams - most likely the Geminids stream, which each December causes one of the most intense meteor showers at Earth - was colliding at high speeds into the inner zodiacal cloud itself. These impacts with zodiacal dust produce large quantities of beta-meteoroids that don't blast off in random directions, but are focused into a narrow set of paths.

"The idea is that a meteoroid stream is colliding with and spraying out material all along its orbit in a tangential direction, and we've called that a beta-stream," Szalay said. "What's exciting about this concept is that it's a fundamental process that would be occurring not only at every meteoroid stream in our solar system, but with every meteoroid stream to varying degrees in every dust cloud in the universe."

Fortuitously, during Parker Solar Probe's fourth orbit, the FIELDS instrument was configured to indicate from which direction meteoroids were hitting the spacecraft. By looking at this data, Pusack and colleagues also concluded the enhancement is consistent with a Geminids beta-stream, and found considerable substructure in this signature that may allow researchers to better characterize this interesting phenomenon.

"The data show distinct dust impacts hitting the spacecraft from behind, indicating particles that would have to catch up and exceed the speed of the spacecraft," said Pusack.

The researchers added that these measurements have already helped them to better understand the collisional evolution of our solar system's dust cloud, in a region previously unexplored by any spacecraft, and may also very well have provided direct observations of how a meteoroid stream collides with, and erodes, our zodiacal cloud.

"The amazing thing about all of this is that our mission is just getting started," said Nour E. Raouafi, Parker Solar Probe project scientist at APL. "In only three years since launch, we have already learned so much about the environment near the Sun, from the behavior of the zodiacal cloud, to the structure of the solar wind that streams throughout the solar system, to the processes that heat and accelerate the solar wind itself. And as this durable spacecraft speeds even closer to the Sun - actually flying through its atmosphere - we can't wait to see what other discoveries await us."

Launched Aug. 12, 2018, Parker Solar Probe recently completed its ninth solar orbit, which took it to within 6.5 million miles of the Sun's surface. The probe will ultimately come to within 3.8 million miles of the solar surface, making three close passes in 2024 and 2025 at speeds exceeding 430,000 miles per hour - and its heat shield facing temperatures higher than 2,500 degrees Fahrenheit.

Parker Solar Probe is part of NASA's Living with a Star program to explore aspects of the Sun-Earth system that directly affect life and society. The program is managed by NASA's Goddard Space Flight Center for the Heliophysics Division of NASA's Science Mission Directorate. APL manages the Parker Solar Probe mission for NASA. The FIELDS instrument suite is led by the University of California, Berkeley.

Research Report: "Collisional Evolution of the Inner Zodiacal Cloud" b

Research Report: "Dust Directionality and an Anomalous Interplanetary Dust Population Detected by the Parker Solar Probe"

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