Kordylewski cloud

The Kordylewski clouds, sometimes called the lunar libration clouds, are concentrations of dust that exist at the L₄ and L₅ Lagrangian points of the Earth–Moon system. They were first reported by Polish astronomer Kazimierz Kordylewski in 1961, who observed the clouds from the Tatra Mountains in former Czechoslovakia. The clouds are likely composed of trapped dust particles from the interplanetary dust cloud. Dust particles remain for decades, forming large, rapidly-evolving bands within the clouds. Eventually, perturbations from the Sun lead to their escape.

Following Kordylewski's discovery, inconsistent observations by other astronomers led to their existence becoming controversial. Attempts to observe the sparse clouds were complicated by their exceedingly dim nature, making them difficult to discriminate against gegenschein and atmospheric airglow even in very dark skies. Observations from the ground, air, and space reported both positive and negative detections, and a 1991–92 encounter from the Hiten spacecraft failed to find the clouds. In 2018, they were tentatively confirmed to exist by a team of Hungarian astronomers through polarimetry. Due to their elusiveness they are sometimes nicknamed ghost moons.

Following French astronomer Frédéric Petit's spurious report of a second moon of Earth in 1846, other astronomers began searching for potential undiscovered moons. Between 1953 and 1956, a team headed by Clyde Tombaugh planned to search for small natural satellites near the Moon's Lagrange points—dynamically stable regions of space—but were prevented by poor weather. In 1951, Polish astronomer Kazimierz Kordylewski began his own search for Trojan satellites at the lunar L₄ and L₅ Lagrange points. He was also unsuccessful, but in 1956 Josef Witkowski suggested to Kordylewski to instead search for faint, diffuse dust clouds.

The clouds were first observed with the naked eye by Kordylewski in October 1956, at the Skalnaté pleso Observatory in the Tatra Mountains of former Czechoslovakia. Even with very dark skies, the clouds were difficult to observe. They appeared as slight brightenings near the lunar L₄ and L₅ points at least 2° in diameter and one to two magnitudes fainter than the brightest gegenschein. On 6 March and 6 April 1961, Kordylewski successfully photographed two distinct clouds at the lunar L₅ point from Kasprowy Wierch's summit observatory. The photographs were taken using a Jupiter 3 Leica camera, with an exposure time of 11 minutes on March and 12 minutes in April. Kordylewski photometrically analyzed the photographs and published his results in the journal Acta Astronomica in 1961, and an International Astronomical Union circular announced the clouds' discovery on 23 May of that year.

Further attempts to detect the Kordylewski clouds were conflicting and controversial. Ground-based observations of the clouds are complicated by their exceedingly dim nature, making observations sensitive to weather, gegenschein, and airglow. Following Kordylewski's announcement on 1961, other professional and amateur astronomers attempted to observe the clouds, initially without success. On 4 January 1964, astronomer J. W. Simpson and his colleagues R. G. Miller and G. Gardner observed the L₅ Kordylewski cloud. Thence until 1967, the team took about 100 photographs of the cloud. In 1966, NASA organized an airborn observations campaign, reporting detections of "circular or elliptical nebulous patches" at both Lagrange points on four flights. Other astronomers reported negative detections through optical or radar observations. From 1962 to 1963, an attempt by the United States Geological Survey to photograph the clouds from Chacaltaya, Bolivia gave inconclusive results. A photographic search for the L₅ cloud was conducted from March 1966 to March 1967 by astronomer Robert Roosen at the McDonald Observatory failed to find any clouds. Astronomers C. Wolff, L. Dundelman, and L. C. Haughney attempted to aerially photograph the clouds, flying well away from land over the Pacific Ocean to minimize light pollution. The team did not detect any clouds.

Later observation attempts from space were conducted; space-based observations have the advantage of avoiding atmospheric airglow. In 1975, researcher J. R. Roach analyzed photographic data collected from 1969 to 1970 by the sixth Orbiting Solar Observatory telescope (OSO-6). The imagery was taken in green visible light, revealing clouds near both Lagrange points that appeared to librate around each point. A team of researchers led by R. H. Munro analyzed data taken by the coronograph aboard the Skylab space station, aiming to detect potential forward scattered sunlight by the clouds. No clouds could be distinguished against the solar coronal background. In 1991–1992, the Japanese Hiten spacecraft made single looping passes around the lunar L₄ and L₅ points, failing to detect the dust clouds with its dust counting instrument.

With mixed observational results, several astronomers expressed skepticism of the Kordylewski clouds' existence. In 1969, Roosen and Wolff published an article arguing against the existence of dust clouds within the Earth–Moon system, asserting on theoretical grounds that any such clouds would be unstable and destroyed by perturbations from the Sun or from the Moon's orbital eccentricity. Instead, they suggested that reported positive detections may be due to passing interplanetary dust clouds. In 1970, Naosuke Sekiguchi computed the behavior of dust, stating that dust tends to disperse from the lunar Lagrange points and suggested that positive detections may have been transient dispersing clouds. A similar analysis conducted by GP. Horedt, meanwhile, was inconclusive regarding dust behavior near the Lagrange points. Other astronomers suggested the possibility that the Kordylewski clouds quickly vary in structure over time as an explanation to conflicting ground observations. Successful reported observations of a cloud at the L₅ point are around three times more common than those for the L₄ point.

The Kordylewski clouds were tentatively confirmed in 2018 by a team of astronomers led by Judit Slíz-Balogh. The team first developed computer models to simulate the dynamical behavior of dust particles at the L₅ point, including predictions of what the simulated cloud would appear like in polarimetric observations from Earth. Polarimetric observations of the area around the L₅ point were then conducted over several months in 2017 at a private observatory in Badacsonytördemic, Hungary. As a control, the same region of sky was photographed when thin cirrus clouds and contrails passed overhead or when the L₅ point was not in view. Using a CCD camera with three linearly polarizing filters attached to its lens, the team successfully photographed L₅ features with polarization characteristics consistent with light scattered by dust clouds. When compared against the control photographs, the polarization characteristics differed from those expected of clouds, contrails, or zodiacal dust. Slíz-Balogh's team then compared their photographs of the clouds to their earlier computer models, finding that the photographed cloud structures matched predictions. The team published their confirmation of the clouds' existence in the Monthly Notices of the Royal Astronomical Society in 2018.