Earth's Second Moon

Objects Claimed to Be Moons of Earth

Earth only has one moon, but its gravity does capture asteroids from time to time.
Earth only has one moon, but its gravity does capture asteroids from time to time. Anne Helmenstine

 Time after time, claims have been made that Earth has more than one moon. Starting in the 19th century, astronomers have sought these other bodies. While the press might refer to some of the discovered objects as our second (or even third) moon, the reality is that the Moon or Luna is the only one we have. To understand why, let's be clear on what makes a moon a moon.

What Makes the Moon a Moon

In order to qualify as a true moon, a body must be a natural satellite in orbit around a planet.

Because a moon must be natural, none of the artificial satellites or spacecraft orbiting the Earth may be called a moon. There's no restriction on the size of a moon, so although most people think of a moon as a round object, there are small moons with irregular shapes. The Martian moons Phobos and Deimos fall into this category. Yet even without a size restriction, there really aren't any objects that orbit the Earth, at least not long enough to matter.

Quasi-satellites of Earth

When you read in the news about mini-moons or second moons, usually this refers to quasi-satellites. While quasi-satellites don't orbit the Earth, they are near the planet and orbit the Sun about the same distance away as us. Quasi-satellites are considered to be in 1:1 resonance with Earth, but their orbit isn't tied to the gravity of the Earth or even the Moon. If the Earth and Moon suddenly vanished, the orbits of these bodies would be largely unaffected.

Examples of quasi-satellites include 2016 HO3, 2014 OL339, 2013 LX28, 2010 SO16, (277810) 2006 FV35, (164207) 2004 GU9, 2002 AA29, and 3753 Cruithne.

Some of these quasi-satellites have staying power. For example, 2016 HO3 is a small asteroid (40 to 100 meters across) that loops around Earth as it orbits the Sun.

Its orbit is tilted a bit, compared with that of Earth, so it appears to bob up and down with respect to Earth's orbital plane. While it's too far away to be a moon and doesn't orbit the Earth, it has been a close companion and will continue to be one for hundreds of years. In contrast, 2003 YN107 had a similar orbit, but left the area over a decade ago.

3753 Cruithne

Cruithne is noteworthy for being the object most often called Earth's second moon and the one most likely to become one in the future. Cruithne is an asteroid about 5 kilometers (3 miles) wide that was discovered in 1986. It's a quasi-satellite that orbits the Sun and not the Earth, but at the time of its discovery, its complex orbit made it appear that it might be a true moon. Cruithne's orbit is affected by Earth's gravity, though. At present, the Earth and the asteroid return to about the same position relative to each other each year. It won't collide with Earth because its orbit is inclined (at an angle) to ours. In another 5,000 years or so, the asteroid's orbit will change. At that time, it might truly orbit the Earth and be considered a moon. Even then, it will only be a temporary moon, escaping after another 3,000 years.

Trojans (Lagrangian Objects)

Jupiter, Mars, and Neptune were known to have trojans, which are objects that share the orbit of the planet and remain in the same position with respect to it. In 2011, NASA announced the discovery of the first Earth trojan, 2010 TK7. In general, trojans are located at Lagrangian points of stability (are Lagrangian objects), either 60° ahead of or behind the planet. 2010 TK7 precedes the Earth in its orbit. The asteroid is about 300 meters (1000 feet) in diameter. Its orbit oscillates around Lagrangian points L4 and L3, bringing it to its closest approach every 400 years. The closest approach is about 20 million kilometers, which is over 50 times the distance between the Earth and the Moon. At the time of its discovery, it took the Earth about 365.256 days to orbit the Sun, while 2010 TK7 completed the journey in 365.389 days.

Temporary Satellites

If you're okay with a moon being a temporary visitor, then there are small objects transiently orbiting the Earth that might be considered moons. According to astrophysicists Mikael Ganvik, Robert Jedicke, and Jeremie Vaubaillon, there is at least one natural object around 1-meter in diameter orbiting the Earth at any given time. Usually these temporary moons remain in orbit for several months before escaping again or falling to Earth as a meteor.

References and Further Reading

Granvik, Mikael; Jeremie Vaubaillon; Robert Jedicke (December 2011). "The population of natural Earth satellites". Icarus218: 63. 

Bakich, Michael E. The Cambridge Planetary Handbook. Cambridge University Press, 2000, p. 146,