The search for Earth's many moons
What, Earth's many moons?" "I thought that our planet only has one moon?" I can imagine the incredulous head-scratching the title to this article will generate.
Yes, Earth does technically have only one moon, and by technically, I mean one permanent, natural moon. However, it would appear that Earth has had, and will continue to have, numerous other moons orbiting it, even if only temporarily.
MOON'S ORIGINS
When Earth was first formed approximately 4.5 billion years ago, scientists theorize that our young, newlyformed planet was in collision with a Mars-sized, protoplanet (given the name Theia, one of the Titans in Greek mythology) which ejected vast amounts of debris from both bodies out into space, where over time, it began to orbit the Earth. While it is theorized that the bulk of Theia was absorbed into the still-malleable Earth, the orbiting ejecta from the collision eventually coalesced into the satellite we now refer to as the moon.
It is also theorized by some astronomers that there may have been a smaller companion moon that formed at the same time as well, and that a subsequent collision between this smaller moon and our moon may explain why the near-side (the one facing Earth) of the moon is low, flat, and dominated with volcanic scars, while the far-side (the one we never see from Earth) is mainly mountainous and heavily cratered. However, as other lunar processes could explain the different lunar terrains of the two sides, the theory of a secondary companion moon remains, at this point, purely hypothetical.
BACK IN TIME
Let's step back in history a couple of centuries to when the French astronomer, Frederic Petit (1810-1865), first raised the possibility that our planet had more than one moon. He claimed to have observed, on the night of March 21, 1846, a second moon orbiting the Earth at a distance of only 11.4 km (the height at which most airlines currently fly). Needless to say, his claim was quickly dismissed by his peers.
As an aside, the famous French novelist, poet and playwright, Jules Verne (1828-1905), of Twenty Thousand Leagues Under the Sea fame, was so inspired by all the talk of a second moon orbiting the Earth that he wrote From the Earth to the Moon in 1865. In 1898, another astronomer. Dr. George Waltemath (1840-1915), from Hamburg, Germany, also reported having observed, not only a second moon, but also a system of tiny moons, orbiting the Earth at a distance of 1,030,000 km. As his observations could not be
independently confirmed by other astronomers around the world, Waltemath's claim of a second moon orbiting Earth, like that of Petit, was ultimately considered false, as was his later claim of having discovered a third moon (which he named "Waltemath's moon") orbiting 427, 290km above Earth.
OTHER MOONS ORBITING EARTH?
Despite both these claims being labelled false, interest in the possibility of a second moon, and perhaps numerous moons orbiting Earth, did not entirely disappear. Enough interest within the scientific community remained that, decades later, the famed discoverer of the planet Pluto in 1930, Clyde Tombaugh (1906-1997) was asked in 1954 to conduct a search for additional moons orbiting Earth.
After a five-year study, Tombaugh released a report in 1959, stating that he had found nothing to suggest that there were any other moons attending our planet. In the 1960s, it was suggested that Earth had at least 10 small natural satellites orbiting it that may have broken off from a larger parent body; lacking adequate confirmation by the majority of astronomers, this claim was deemed groundless.
WHAT'S ORBITING EARTH?
Over the course of the following decades, a number of mysterious objects were discovered to be in orbit around our planet. Further study indicated that these objects — referred to as "temporary moons", "captured moons", or "mini-moons" — were only in orbit around Earth for a few months, leading some astronomers to speculate that Petit, Waltemath and others may have, indeed, spotted one or more of these temporary moons.
Temporary moons are described as relatively small asteroids captured by Earth's gravitational field, becoming, at least for a short period of time, a natural satellite of Earth. These objects eventually either get thrown back out into space and return to a solar orbit, or end up crashing through the Earth's atmosphere and exploding.
'THE GREAT PROCESSION'
The earliest known reference to "temporary moons" occurred in 1913, when, on the night of Feb. 9, a massive parade of meteors appeared in the sky over Canada, the Northeast United States, Bermuda, and as far south as Brazil.
A subsequent report of the event, labelled "The Great Procession of 1913", speculated that, as there appeared to be no apparent radiant (point of origin), the meteoric parade, said to have lasted 5 minutes, must have been the result of a short-lived temporary moon that, upon entering the Earth's atmosphere, exploded.
Two large fragment streams were observed, which disintegrated and travelled at varying rates, streaking across the night sky from the westnorthwest to the southeast. It was also reported that a second large, white, tail-less "fireball" followed the earlier meteor procession around 2 a.m., Feb. 10.
Typically, when meteors strike the Earth's atmosphere at a relatively shallow angle, the meteors, referred to as "Earth-grazers", tend to break apart, with the fragments usually traveling along the same path across the sky. It was concluded that the 1913 event object was most likely a temporary moon that had gone into orbit around the Earth at some point in time, before striking the atmosphere and disintegrating.
'QUASI-MOONS'
There is another group of objects that orbit Earth, but which are more affected by the sun's gravity than that of the Earth. Referred to as "quasi-moons" or "quasisatellites'', these bodies, while being mostly outside our planet's gravitational influence, and not directly following Earth's orbit, take 365 days to orbit the sun just like Earth, thereby making them appear to be in orbit around it.
Quasi-satellites are what are called "co-orbiting objects'' - objects that orbit at a similar distance as Earth from the sun, rather than the Earth itself. They occupy a specific type of orbital configuration with a planet (in this case, Earth), whereby they can remain close to the planet over the course of numerous orbital periods. This closeness and similar orbital period makes it appear they are orbiting the planet.
'EARTH TROJANS'
There is also a class of objects referred to as "Earth trojans", small celestial bodies, mainly asteroids, which, while sharing the orbit of Earth, remain in a stable orbit 60 degrees ahead and/or 60 degrees behind Earth along its orbital path around the sun. Earth trojans occupy a position near one of the Lagrange points L1 - L5 of the Earth - moon system.
Lagrange points are places in the solar system where the gravitational pull and orbital motion of any two planetary bodies establishes points of gravitational stability or equilibrium, whereby objects sent to such points either stay there naturally, or require minimal energy expenditure to do so. The JWST is currently in a stable orbit around the sun-earth L2 point. To date, there are only two known Earth trojans - 2010 TK7 (the first Earth trojan discovered), a 5 km dia asteroid oscillating between the sun-earth L3 and L4
Lagrange points, and 2020 XL5, a 1.2 km dia asteroid oscillating around the sun-earth L4 Lagrange point.
There is another group of objects that orbit Earth.
'GHOST MOONS'
Finally, there are the "ghost moons", also known as "Kordylewski Clouds", after the Polish astronomer, Kazimierz Kordylewski (1903 1981), who, in 1956, claimed to have discovered vast concentrations of dust at the Trojan points of the Earthmoon system.
These "ghost clouds", as the name suggests, are ethereal dust clouds too faint to be observed with the naked eye, orbiting the Earth along the same orbital path as the moon. The clouds are composed of millions of individual micron-sized dust particles that reflect the sun's light. It wasn't until 2018 that the existence of these massive ethereal clouds was confirmed. Due to their tenuous nature, many astronomers do not class them as "moons" or "satellites", even though they do orbit the Earth.
THIS WEEK'S SKY
Mercury (mag. +1.7, in Pisces - the Fish) will pass in front of the sun (inferior conjunction) on April 11, is too close to the sun to be seen, currently only at 7 degrees separation.
Venus (mag. -3.9, in Pisces) sits just on the eastern horizon at dawn, and is not observable.
Mars (mag. +1.2, in Aquarius - the Water-bearer), at only 2 degrees above the southeast horizon at dawn, is likewise unobservable this week.
Jupiter remains the only bright planet visible over the course of the coming week, becoming visible shortly after 8 p.m., 21 degrees above the western horizon as dusk yields to darkness, before sinking towards the horizon and setting around 10:25 p.m. Look for a thin, waxing crescent moon to the upper right of Jupiter on Apr. 10.
Saturn (mag. +1.1, in Aquarius) sits next to Mars, 2 degrees above the southeast horizon at dawn, and, like Mars, is not observable.
Uranus (mag. +5.8, in Aries - the Ram), at only 14 degrees above the western horizon at dusk, is not observable. Distant Neptune (mag. +8.0, in Pisces) is within 20 degrees of the setting Sun, and is not observable.
COMET ALERT
Comet 12P/pons - Brooks is currently located in the constellation of Aries - the Ram. At mag. +4.7, the comet can be seen with the use of binoculars to the lower right of Jupiter in the western evening sky once it has darkened. By the end of the week, it will have moved into the constellation of Taurus - the Bull, heading towards its closest approach to the Sun (perihelion) on Apr. 21. The comet has been nicknamed the "Mother of All Dragons", due, in part, to its greenish colour, the result of the presence of diatomic carbon molecules (duos of carbon atoms stuck together) in the comet's head.
Until next week, clear skies.