Just in time for Halloween, a team of Hungarian astronomers and physicists has reported new evidence of two dust clouds, or "ghost moons," orbiting Earth at a distance of roughly 250,000 miles (400,000 kilometers).
In a paper published in the journal Monthly Notices of the Royal Astronomical Society, the research team explains how the elusive "Kordylewski clouds" — first detected nearly 60 years ago by Polish astronomer Kazimierz Kordylewski — coalesce in what are known as Lagrange points. These regions of space occur where the force of gravity balances out between two celestial bodies, like Earth and the moon. Our Earth-moon system has five such Lagrange points, with L4 and L5 offering the best gravitational equilibrium for the formation of ghost moons.
"L4 and L5 are not completely stable, as they are disturbed by the gravitational pull of the Sun. Nonetheless they are thought to be locations where interplanetary dust might collect, at least temporarily," the Royal Astronomical Society reports in a statement. "Kordylewski observed two nearby clusters of dust at L5 in 1961, with various reports since then, but their extreme faintness makes them difficult to detect and many scientists doubted their existence."
An artist’s impression of the Kordylewski cloud in the night sky — with its brightness greatly enhanced — at the time of the observations. (Image: G. Horváth/Royal Astronomical Society)
To reveal the ghostly apparitions orbiting Earth, the researchers first used computer simulations to model how the dusty satellites might form and best be detected. They eventually settled on using polarized filters, as most scattered or reflected light is "more or less polarized," to detect the faint clouds. After using a telescope to capture a series of exposures in the L5 region, they were thrilled to observe two dust clouds consistent with Kordylewski's observations six decades earlier.
"The Kordylewski clouds are two of the toughest objects to find, and though they are as close to Earth as the Moon, are largely overlooked by researchers in astronomy," study co-author Judit Slíz-Balogh says. "It is intriguing to confirm that our planet has dusty pseudo-satellites in orbit alongside our lunar neighbor."
A mosaic pattern of the polarization angle around L5, represented by a white dot. Bright red pixels indicate the Kordylewski dust cloud's central region; the dark lines are satellite tracks. (Image: J. Slíz-Balogh/Royal Astronomical Society)
Much like traditional ghosts, the shapes of these clouds may change over time, the researchers note in their paper, depending on factors such as solar-wind disturbances or even debris from objects like comets becoming trapped at the Lagrange points. Perhaps more importantly, the fairly stable points of L4 and L5 present intriguing possibilities for siting future space missions.
"These points are suitable for spacecraft, satellite or space telescope parking with minimal fuel consumption," the researchers write, pointing out that neither L4 nor L5 currently host any spacecraft. Additionally, the Lagrange points "can be applied as transfer stations for the mission to Mars," they add, "or other planets, and/or to the interplanetary superhighway."