Kordylewski cloud
Template:Short description Template:Use dmy dates
The Kordylewski clouds, sometimes called the lunar libration clouds,<ref name="Munro1975"/> are concentrations of dust that exist at the Template:L4 and Template: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.<ref name="TWC-20181105"/>
History
Following French astronomer Frédéric Petit's spurious report of a second moon of Earth in 1846, other astronomers began searching for potential undiscovered moons.<ref name="PlanetaryHandbook"/>Template:Rp From 1953 to 1956, a team of astronomers led by Clyde Tombaugh planned to search for small natural satellites near the Moon's Lagrange pointsTemplate:Mdashdynamically stable regions of spaceTemplate:Mdashbut were prevented by poor weather.<ref name="Tombaugh1959"/>Template:Rp In 1951, Polish astronomer Kazimierz Kordylewski began his own search for Trojan satellites at the lunar Template:L4 and Template:L5 Lagrange points. This search was also unsuccessful, but in 1956 Josef Witkowski suggested to Kordylewski to instead search for faint, diffuse dust clouds.<ref name="BEA"/>Template:Rp
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 faint clouds were observed with difficulty. They appeared as slight brightenings near the lunar Template:L4 and Template:L5 points at least 2° in diameter and 1–2 magnitudes fainter than the brightest gegenschein.<ref name="S&T196108"/>Template:Rp<ref name="BEA"/>Template:Rp On 6 March and 6 April 1961, Kordylewski successfully photographed two distinct clouds at the lunar Template:L5 point from Kasprowy Wierch's summit observatory. The photographs were taken using a Jupiter 3 f/1.5 50 mm Leica camera, with an exposure time of 11 minutes on March and 12 minutes in April. The photographs were photometrically analyzed, with Kordylewski publishing his results on Acta Astronomica in 1961<ref name=Kordylewski/> and an International Astronomical Union circular announcing the clouds' discovery on 23 May of that year.<ref name="IAUC1760"/>
Subsequent observations and debate
Further attempts to detect the Kordylewski clouds were often conflicting and controversial.<ref name="Munro1975"/> Ground-based observations of the clouds are complicated by their exceedingly dim nature, making observations sensitive to weather, gegenschein, and airglow.<ref name=Wang2022/>Template:Rp Following Kordylewski's announcement on 1961, other professional and amateur astronomers attempted to observe the clouds, initially without success.<ref name="Sliz-Balogh2022"/>Template:Rp On 4 January 1964, astronomer J. W. Simpson and his colleagues R. G. Miller and G. Gardner observed the Template:L5 Kordylewski cloud. Thence until 1967, the team took about 100 photographs of the cloud.<ref name="Simpson1967"/><ref name="Sliz-Balogh2022"/>Template:Rp In 1966, NASA organized an airborn observations campaign, reporting detections of "circular or elliptical nebulous patches" at both Lagrange points on four flights.<ref name="Vansyek1969"/> Other astronomers reported negative detections through optical or radar observations.<ref name="Roosen1969"/> From 1962 to 1963, a United States Geological Survey attempt to photograph the clouds from Chacaltaya, Bolivia gave inconclusive results.<ref name="Wolff1967"/> A photographic search for the Template:L5 cloud was conducted from March 1966 to March 1967 by astronomer Robert Roosen at the McDonald Observatory failed to find any clouds.<ref name="Roosen1968"/>Template:Rp 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.<ref name="Wolff1967"/>
Later observation attempts from space were conducted; space-based observations have the advantage of avoiding atmospheric airglow.<ref name="Wolff1967"/> 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,Template:Efn revealing clouds near both Lagrange points that appeared to librate around each point.<ref name="Roach1975"/> 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.<ref name="Munro1975"/>Template:Rp In 1991–1992, the Japanese Hiten spacecraft made single looping passes around the lunar Template:L4 and Template:L5 points, failing to detect the dust clouds with its dust counting instrument.<ref name=Laufer2007/>
With mixed observational results, several astronomers expressed skepticism of the Kordylewski clouds' existence.<ref name=Wang2022/>Template:Rp 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.<ref name="Roosen1969"/> In 1970, astronomer 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.<ref name="Horedt1973"/>Template:Rp Other astronomers suggested the possibility that the Kordylewski clouds quickly vary in structure over time as an explanation to conflicting ground observations.<ref name="Moeed1997"/><ref name=Wang2022/>Template:Rp Additionally, successful reported observations of a cloud at the Template:L5 point are around three times more common than those for the Template:L4 point.<ref name="Sliz-Balogh2022"/>Template:Rp
Current status
The Kordylewski clouds were tentatively confirmed in 2018 by a team of astronomers led by Judit Slíz-Balogh.<ref name="TPS20240521"/> The team first developed computer models to simulate the dynamical behavior of dust particles at the Template:L5 point, including predictions of what the simulated cloud would appear like in polarimetric observations from Earth.<ref name="Sliz-Balogh2018"/>Template:Rp Polarimetric observations of the area around the Template:L5 point where 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 Template:L5 point was not in view. Using a CCD camera with three linearly polarizing filters attached to its lens, the team successfully photographed Template:L5 features with polarization characteristics consistent with light scattered by dust clouds.<ref name="Sliz-Balogh2019"/>Template:Rp When compared against the control photographs, the polarization characteristics differed from those expected of clouds, contrails, or zodiacal dust.<ref name="Sliz-Balogh2019"/>Template:Rp 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.<ref name="Sliz-Balogh2019"/>Template:Rp<ref name="RAS20181025"/>
A followup observation campaign was led by Slíz-Balogh on 31 October 2021 and 3 July 2022, targeting both the Template:L4 and Template:L5 points.<ref name="Sliz-Balogh2023"/>Template:Rp Using the same methods and location as the 2017 observations, the Template:L4 and Template:L5 clouds were successfully photographed.<ref name="Sliz-Balogh2023"/>Template:Rp
Properties
Structure and variability
Template:CSS image crop The Kordylewski clouds are large and heterogeneous structures,<ref name="Sliz-Balogh2018"/>Template:Rp<ref name="Sliz-Balogh2023"/>Template:Rp with ground-based observations suggesting an apparent diameter of several degrees.<ref name="Moeed1997"/>Template:RpTemplate:Efn The clouds have an elongated, relatively dense "core" about 25,000 km in size and aligned parallel to the Moon's orbital plane.<ref name="Sliz-Balogh2018"/>Template:RpTemplate:Efn A diffuse series of dusty bands or blobs extend from the center perpendicular to the ecliptic plane, giving the clouds a striped appearance in polarized light.<ref name="Sliz-Balogh2018"/>Template:Rp<ref name="Sliz-Balogh2023"/>Template:Rp Simulations of the clouds suggest an asymmetry, with the Template:L5 cloud's core being denser than that of the Template:L4 cloud.<ref name="Sliz-Balogh2022"/>Template:Rp
Observations and modelling indicate that the Kordylewski clouds' structures change over time,<ref name="Wang2022"/>Template:Rp and their structures and densities are influenced by the rate at which dust is trapped at the Lagrange points.<ref name="Sliz-Balogh2019"/>Template:Rp The structure of the clouds vary over timescales as short as a few days,<ref name="Sliz-Balogh2018"/>Template:Rp and they may be transient, ephemeral features.<ref name="Sliz-Balogh2019"/>Template:Rp Modelling by Slíz-Balogh and collaborators indicate that the structure of the clouds is also controlled by different populations of trapped dust. Dust populations are trapped at different times and with different velocities, with populations older than 20–25 days forming bands. The evolution of these populations lead to bands appearing, disappearing, or changing in density.<ref name="Sliz-Balogh2018"/>Template:Rp Modelling by Nathan R. Boone and Robert A. Battinger in 2021 demonstrated that the density of the clouds may also vary with respect to solar perturbations as it causes structures within the clouds to expand and collapse.<ref name="Boone2021"/>Template:Rp
Dynamics and source
The Kordylewski clouds are located near the lunar Template:L4 and Template:L5 points,<ref name="Sliz-Balogh2022"/>Template:Rp which are 60° ahead and behind the Moon along its orbit, respectively.<ref name="Salazar2012"/> The clouds are not centered exactly on the Template:L4 and Template:L5 points; ground observations have noted the clouds displaced by 6–10° or more away from those points.<ref name=Covington/>Template:Rp<ref name="Winiarski1989"/>Template:Rp
In the restricted three-body problem, the five Lagrange points represent points of equilibrium.<ref name="LagrangePoints"/>Template:Rp Although points Template:L1–Template:L3 are unstable, the Template:L4 and Template:L5 points are stable so long as the mass ratio of the primary and secondary is large enough: particles at these two points can become trapped.<ref name="LagrangePoints"/>Template:Rp The mass ratio between the Earth and the Moon is great enough to ensure theoretical stability of the lunar Template:L4 and Template:L5 points.<ref name="Sliz-Balogh2022"/>Template:Rp However, gravitational tides from the Sun and solar radiation pressure—relevant for dust particles—disrupt the stability of the lunar Template:L4 and Template:L5 points.<ref name="Gimeno2024"/>Template:Rp Nevertheless, numerical simulations suggest that dust particles near these points are able to be temporarily captured into the Kordylewski clouds, potentially remaining for decades in small "islands of stability".<ref name="Gimeno2024"/>Template:Rp<ref name="Sliz-Balogh2018"/>Template:Rp
The Kordylewski clouds are likely supplied by dust from interplanetary space.<ref name="Gimeno2024"/>Template:Rp The interplanetary dust cloud near Earth is quite homogeneous; despite this, models result in the Template:L5 cloud trapping up to 9% more dust than the Template:L4 cloud. This modelled asymmetry is similar to asymmetries in other trojan populations, such as between the two Jupiter trojan asteroid camps, but its exact cause is still unknown.<ref name="Sliz-Balogh2022"/>Template:Rp Captured dust populations are initially evenly distributed within the clouds, forming bands after 20–25 days. Mean-motion resonances with the Moon may contribute to the formation of the bands.<ref name="Sliz-Balogh2024"/>Template:Rp After becoming destabilized, dust particles then escape from the Earth–Moon system back into interplanetary space.<ref name="Sliz-Balogh2022"/>Template:Rp
Exploration
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In October 1991, the Hiten spacecraft executed a looping halo trajectory around the lunar Template:L4 point; it then executed a similar looping trajectory around the lunar Template:L5 point in January 1992. Hiten was en route to the Moon as a part of its extended mission, but the excursions to the lunar Template:L4 and Template:L5 points were planned to search for the Kordylewski clouds using its Munich Dust Counter (MDC) instrument.<ref name="Uesugi1996"/>Template:Rp No increase in dust impact rates were detected, though this could be due to the MDC's insensitivity to low-velocity dust impacts or from Hiten missing the clouds entirely.<ref name="Wang2022"/>Template:Rp<ref name=Laufer2007/>
Proposed missions
Exploration of the Kordylewski clouds was suggested as early as 1963 by Arvydas Kliore, proposing either a flyby or rendezvous mission.<ref name="Wang2022"/>Template:Rp At the International Electric Propulsion Conference in 1988, Keith Ryden and collaborators proposed a rendezvous mission to the Kordylewski clouds using ion propulsion. Such a spacecraft would weigh a total of Template:Cvt, of which Template:Cvt would be xenon propellant. The probe would be parked in low-Earth orbit, before continuously raising its orbit in a spiral trajectory and rendezvousing with the Template:L5 cloud.<ref name="Ryden1988"/> In 1997, N. S. Moeed and J. C. Zarnecki proposed reactivating the Giotto spacecraft, an ESA probe originally designed to flyby and study Halley's comet, for an extended mission intercepting the Template:L4 cloud. Though some of its instruments were damaged by dust impacts whilst travelling through Halley's coma, the proposal sought to use its Dust Impact Detector System to study the Template:L4 cloud.<ref name="Moeed1997"/>Template:Rp Giotto was set for its second encounter with Earth on 1 July 1999.<ref name="ESA19990630"/> The proposal called for a correction burn six months before encounter to deflect the spacecraft towards the Template:L4 cloud. Approaching at a velocity of about 3.4 km/s, Giotto would traverse the cloud in just under three hours.<ref name="Moeed1997"/>Template:Rp Giotto was not reactivated for its second Earth encounter.<ref name="ESA1July"/>
In 2021, a team of researchers led by Peng Wang proposed a coordinated space- and ground-based observation campaign for the Kordylewski clouds. After injection into a highly eccentric orbit, the spacecraft would conduct multiple flybys of both Kordylewski clouds, sampling them with a dust counter. Ground-based observations could be used to guide the spacecraft toward the clouds' locations. A lunar flyby is then executed, setting the spacecraft to the Template:L4 point. After rendezvousing with the Template:L4 point, it enters a halo orbit, further exploring the cloud's structure and dynamics.<ref name="Wang2022"/>Template:Rp
Due to their dynamical stability, the Template:L4 points have attracted attention for their potential to support space infrastructure for both civilian and military applications. However, the same mechanisms that trap dust in the Kordylewski clouds can also trap artificial space debris, potentially leading to a hazardous space environment within the clouds in the future.<ref name="Boone2021"/>Template:Rp In contrast, the risk of collision from natural debris in the Kordylewski clouds appears to be low. Modelling from researchers Nathan R. Boone and Robert A. Battinger in 2021 concluded that the likelihood of a Kordylewski cloud particle substantially threatening a spacecraft is Template:Val%.<ref name="Boone2021"/>Template:Rp
See also
- Sodium tail of the Moon
- Zodiacal light
- Trojan moons of Saturn
Notes
References