Other moons of Earth
Claims have existed for many decades that Earth might possess other natural satellites besides the Moon. Several candidates have been proposed, but not proven, and the Moon remains Earth's only known natural satellite.
Several genuine scientific searches for more moons were undertaken in the 19th and 20th centuries, but the field has also been the subject of several non-scientific proposals and possible hoaxes. These possible hoaxes, which were about objects of specific size and orbits, were poorly founded and all have been disproven.
Although the Moon is Earth's only known natural satellite, there are a number of near-Earth objects with orbits that are in resonance with Earth. These have even, perhaps glibly, been referred to as "second moons". Quasi-satellites, such as 3753 Cruithne, orbit the Sun in 1:1 resonance with Earth and may appear to orbit a point outside the Earth from a corotating reference frame. Earth trojans, such as 2010 TK7, follow the same orbital path as Earth, either trailing or following, in the vicinity of the Earth–Sun Lagrangian points.
Small objects in orbit around the Sun can also temporarily fall into orbit about the Earth, becoming temporary natural satellites. The only confirmed one was 2006 RH120, which was in Earth orbit in 2006–2007. Additional examples of small, temporarily captured asteroids are predicted.
The first major claim of another moon of Earth was made by French astronomer Frédéric Petit, director of the Toulouse Observatory, who in 1846 announced that he had discovered a second moon in an elliptical orbit around the Earth. It was claimed to be reported by Lebon and Dassier also at Toulouse, and Lariviere at Artenac Observatory, during the early evening of March 21, 1846. Petit proposed that this second moon had an elliptical orbit, a period of 2 hours 44 minutes, with 3,570 km (2,220 mi) apogee and 11.4 km (7.1 mi) perigee. This claim was soon dismissed by his peers. The 11.4 km (37,000 ft) perigee is similar to the cruising altitude of most modern airliners. Petit published another paper on his 1846 observations 15 years later (1861), basing the second moon's existence on perturbations in movements of the existing moon. This second moon hypothesis was not confirmed either. Nevertheless, Petit's proposed moon became a major plot point in Jules Verne's 1870 science fiction novel Around the Moon.
In 1898 Hamburg scientist Dr. Georg Waltemath announced that he had located a system of tiny moons orbiting the Earth. He had begun his search for secondary moons based on the hypothesis that something was gravitationally affecting the Moon's orbit.
Waltemath described one of the proposed moons as being 1,030,000 km (640,000 mi) from Earth, with a diameter of 700 km (430 mi), a 119-day orbital period, and a 177-day synodic period. He also said it did not reflect enough sunlight to be observed without a telescope, unless viewed at certain times, and made several predictions as to when it would appear. "Sometimes, it shines at night like the sun but only for an hour or so." However, after the failure of a corroborating observation of Waltemath's moons by the scientific community, these objects were discredited. Especially problematic was a failed prediction that they would be observable in February 1898. The August 1898 issue of Science mentioned that Waltemath had sent the journal "an announcement of a third moon", which he termed a wahrhafter Wetter und Magnet Mond ("real weather and magnet moon"). It was supposedly 746 km (464 mi) in diameter, and closer than the moon that he had described previously.
In 1918, astrologer Walter Gornold, also known as Sepharial, claimed to have confirmed the existence of Waltemath's moon. He named it Lilith. Sepharial claimed that Lilith was a 'dark' moon invisible for most of the time, but he claimed to have viewed it as it crossed the sun.
William Henry Pickering (1858–1938) studied the possibility of a second moon and made a general search ruling out the possibility of many types of objects by 1903. His 1922 article "A Meteoritic Satellite" in Popular Astronomy resulted in increased searches for small natural satellites by amateur astronomers. Pickering had also proposed the Moon itself had broken off from Earth.
After he had discovered Pluto, the United States Army's Office of Ordnance Research commissioned Clyde Tombaugh to search for near-Earth asteroids. The Army issued a public statement in March 1954 to explain the rationale for this survey. Donald Keyhoe, who was later director of the National Investigations Committee on Aerial Phenomena (NICAP), a UFO research group, said that his Pentagon source had told him that the actual reason for the quickly initiated search was that two near-Earth objects had been picked up on new long-range radar in mid-1953. In May 1954, Keyhoe asserted that the search had been successful, and either one or two objects had been found. The October 1955 issue of Popular Mechanics magazine reported:
Professor Tombaugh is closemouthed about his results. He won't say whether or not any small natural satellites have been discovered. He does say, however, that newspaper reports of 18 months ago announcing the discovery of natural satellites at 400 and 600 miles out are not correct. He adds that there is no connection between the search program and the reports of so-called flying saucers.
At a 1957 meteor conference in Los Angeles, Tombaugh reiterated that his four-year search for natural satellites had been unsuccessful. In 1959 Tombaugh issued a final report stating that nothing had been found in his search.
In 2011, planetary scientists Erik Asphaug and Martin Jutzi proposed a model in which a second moon would have existed 4.5 billion years ago, and later impacted the Moon, as a part of the accretion process in the formation of the Moon.
Quasi-satellites and trojans
Although no other moons of Earth have been found to date, there are various types of near-Earth objects in 1:1 resonance with it, which are known as quasi-satellites. Quasi-satellites orbit the Sun from the same distance as a planet, rather than the planet itself. Their orbits are unstable, and will fall into other resonances or be kicked into other orbits over thousands of years. Quasi-satellites of Earth include 2010 SO16, (164207) 2004 GU9, (277810) 2006 FV35, 2002 AA29 and 3753 Cruithne. Cruithne, discovered in 1986, orbits the Sun in an elliptical orbit but appears to have a horseshoe orbit when viewed from Earth. Some went as far to nickname Cruithne "Earth's second moon".
The key difference between a satellite and a quasi-satellite is that a natural Earth satellite's orbit fundamentally depends upon the gravity of the Earth–Moon system whereas the orbit of a quasi-satellite would negligibly change if the Earth and Moon were suddenly removed since a quasi-satellite is orbiting the Sun on an Earth-like orbit in the vicinity of the Earth.
Earth possesses one known trojan asteroid, a small Solar System body caught in the planet's gravitationally stable L4 Lagrangian point. The object, 2010 TK7 is roughly 300 metres long. Like the quasi-satellites, it orbits the Sun in a 1:1 resonance with Earth, rather than Earth itself.
On 14 September 2006, an object estimated at 5 meters in diameter was discovered in near-polar orbit around Earth. Originally thought to be a third stage Saturn S-IVB booster from Apollo 12, it was later determined to be an asteroid and designated as 2006 RH120. The asteroid re-entered Solar orbit after 13 months and is expected to return to Earth orbit in 21 years.
Computer models by astrophysicists Mikael Granvik, Jeremie Vaubaillon, and Robert Jedicke of Cornell University suggest that these "temporary satellites" should be quite common; and that "At any given time, there should be at least one natural Earth satellite of 1-meter diameter orbiting the Earth." Such objects would remain in orbit for ten months on average, before returning to solar orbit once more, and so would make relatively easy targets for manned space exploration. "Mini-moons" were further examined in a study published in the March issue of Icarus.
"It would seem that the bodies had been traveling through space, probably in an orbit about the sun, and that on coming near the earth they were promptly captured by it and caused to move about it as a satellite."
And later in 1916, William Frederick Denning surmised:
"The large meteors which passed over Northern America on February 9, 1913, presented some unique features. The length of their observed flight was about 2600 miles, and they must have been moving in paths concentric, or nearly concentric, with the earth's surface, so that they temporarily formed new terrestrial satellites.”
- The writer Jules Verne learned of Petit's 1861 proposal and made use of the idea in his 1865 novel, From the Earth to the Moon. This fictional moon was not, however, exactly based on the Toulouse observations or Petit's proposal at a technical level, and so the orbit suggested by Verne was mathematically incorrect. Petit died in 1865, and so was not alive to offer a response to Verne's fictional moon. The explosive popularity of Verne's book in the 19th century triggered many amateur astronomers to search for other moons around Earth.
- Eleanor Cameron's Mushroom Planet novels for children (starting with the 1954 The Wonderful Flight to the Mushroom Planet) are set on a tiny, habitable second moon called Basidium in an invisible orbit 50,000 miles (80,000 km) from Earth.
- The 1963 Tom Swift, Jr. juvenile novel, Tom Swift and the Asteroid Pirates, has a moon Nestria, also called Little Luna, which was originally an asteroid and was moved into Earth orbit at 50,000 miles (80,000 km) altitude. It was claimed for the United States and a research base was established there by Swift Enterprises.
- Samuel R. Delany's 1975 novel Dhalgren features an Earth which mysteriously acquires a second moon.
- In Haruki Murakami's novel 1Q84, a second moon, irregularly shaped and green in color, is visible to some characters in the story.
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