Claims of the existence of other moons of Earth—that is, of one or more natural satellites other than the Moon that orbit Earth—have existed for some time. Several candidates have been proposed, but none have been confirmed. Since the 19th century, scientists have made genuine searches for more moons, but the possibility has also been the subject of a number of dubious non-scientific speculations as well as a number of likely hoaxes.
Although the Moon is Earth's only natural satellite, there are a number of near-Earth objects (NEOs) with orbits that are in resonance with Earth. These have been called, inaccurately but provocatively, "second", "third" or "other" moons of Earth.
2016 HO3, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth. As it orbits the Sun, 2016 HO3 appears to circle around Earth as well. It is too distant to be a true satellite of Earth, but is the best and most stable example of a quasi-satellite, a type of near-Earth object. They appear to orbit a point other than Earth itself, such as the orbital path of the NEO asteroid 3753 Cruithne. Earth trojans, such as 2010 TK7, are NEOs that orbit the Sun (not Earth) on the same orbital path as Earth, and appear to lead or follow Earth along the same orbital path.
Other small natural objects in orbit around the Sun may enter orbit around Earth for a short amount of time, becoming temporary natural satellites. To date, the only confirmed example has been 2006 RH120 in Earth orbit during 2006 and 2007, though further instances are already 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 Earth.
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, and within Earth's atmosphere. Petit published another paper on his 1846 observations in 1861, basing the second moon's existence on perturbations in movements of the actual Moon. This second moon hypothesis was not confirmed either.
In 1898 Hamburg scientist Dr. Georg Waltemath announced that he had located a system of tiny moons orbiting 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 of its next appearances. "Sometimes, it shines at night like the sun but only for an hour or so."
E. Stone Wiggins, a Canadian weather expert, ascribed the cold spring of 1907 to the effect of a second moon, which he said he had first seen in 1882 and had publicized the find in 1884 in the New York Tribune when he put it forward as probable cause of an anomalous solar eclipse of May of that year. He said it was also probably the "green crescent moon" seen in New Zealand and later in North America in 1886, for periods of less than a half-hour each time. He said this was the "second moon" seen by Waltemath in 1898. Wiggins hypothesized that the second moon had a high carbon atmosphere but could be seen occasionally by its reflected light.
The existence of these objects put forward by Waltemath (and Wiggins) was discredited after the absence of corroborating observation by other members of the scientific community. Especially problematic was a failed prediction that they would be seen 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 "second moon" that he had seen 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 be the first person in history to view it as it crossed the Sun.
In 1926 the science journal Die Sterne published the findings of amateur German astronomer W. Spill, who claimed to have successfully viewed a second moon orbiting Earth.
In the late 1960s John Bargby claimed to have observed over ten small natural satellites of Earth, but this was not confirmed.
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.
In early 1954 the United States Army's Office of Ordnance Research commissioned Clyde Tombaugh, discoverer of Pluto, to search for near-Earth asteroids. The Army issued a public statement 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. At The Pentagon, a general who heard the news reportedly asked whether the satellites were natural or artificial. Tombaugh denied the alleged discovery in a letter to Willy Ley, and 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 meteor conference in Los Angeles in 1957, Tombaugh reiterated that his four-year search for natural satellites had been unsuccessful. In 1959, he 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.
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, 2014 OL339, 2013 LX28, 2016 HO3 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 the orbit of a satellite of Earth fundamentally depends on the gravity of the Earth–Moon system, whereas the orbit of a quasi-satellite would negligibly change if Earth and the Moon were suddenly removed because a quasi-satellite is orbiting the Sun on an Earth-like orbit in the vicinity of Earth.
Earth possesses one known trojan, a small Solar System body caught in the planet's gravitationally stable L4 Lagrangian point. This object, 2010 TK7 is roughly 300 metres across. Like 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 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 [4,200 km], 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.”
In April 2015, an object was discovered orbiting the Earth, and initially designated 2015 HP116, but more detailed investigation quickly showed the object to be the Gaia spacecraft, and the object's discovery soon was retracted.
On October 3, 2015, a small object, temporarily designated WT1190F, was found to be orbiting the Earth every ~23 days, and had been orbiting since at least late 2009. It impacted the Earth on November 13, 2015 at 06:18:34.3 (±1.3 seconds) UTC.
On February 8, 2016 a 0.5-meter object was discovered orbiting the Earth with a period of 5 days and given the temporary designation XC83E0D, and most likely lost. The object was later identified with the lost artificial satellite SR-11A, or possibly its companion SR-11B, which were launched in 1976 and lost in 1979.
On April 8, 2016, an object, named S509356, was discovered with an orbital period of 3.58 days. Although it has the typical area-to-mass ratio (m^2/kg) of satellites, it has a color typical of S-type asteroids. It has not yet been identified with any known artificial satellites, because there are no known satellites currently orbiting Earth in between 2.99 and 3.79 days.
On December 8, 2017, the object YX205B9 was discovered with an orbital period of 21 days, on an eccentric orbit taking it from slightly beyond the geocentric satellite ring to almost twice the distance of the Moon. While its orbit remains poor, its absolute magnitude (25.1) makes an artificial satellite unlikely.
It is a real weather and magnet moon, and whenever it is about to cross the earth's course it disturbs the atmosphere and surface of the earth, producing storms, rain, tempests, magnetic deviations and earthquakes...