If someone were to ask you “how many moons does the Earth have?”, you would answer “One”. And you would be correct–most of the time. But not all of the time…..
When it comes to moons in our solar system, Earth is kind of an oddball. Most of the planets have a number of moons. Jupiter and Saturn, the gravitational powerhouses among our planets, have over 60 each. Uranus has almost 30, and Neptune has around 15. Mars has two, and Pluto has five. Mercury and Venus have none.
Earth’s moon, Luna, is unique in several ways. Nearly all of the other moons in our solar system are actually asteroids (or in the case of Pluto, Neptune, and Uranus, Kuiper Belt objects) that wandered too close to a planet, were captured by gravity, and became locked in orbit. Our Moon, however, is the result of a chance encounter, billions of years ago when there were many more planets circling the Sun than there are today. At that time, Earth shared its orbit with a Mars-sized fellow planet, now called Thea, and when they collided, a large chunk of the Earth’s mantle was blown off into space, settled into orbit around us, and eventually coalesced to form the Moon. As a result of its unusual birth, our Luna has one of the largest sizes relative to the planet that it orbits than any other moon in the solar system–only Pluto’s moon Charon has a higher ratio.
From the 1840’s to the 1960’s, there were around half a dozen claims from people, some of them professional astronomers, who declared they had observed small moons, up to 400 miles across, in orbit around the Earth. One of these was even given the proposed name “Lilith”. But none of these observations was ever confirmed. In the 1950’s, the US Air Force began one of the first searches for Near-Earth Asteroids which had the potential to collide with us. Astronomer Clyde Tombaugh, who had recently discovered Pluto, ran the project. Flying saucer “investigator” Donald Keyhoe produced a series of magazine articles claiming that the project had secretly discovered one or two small moons orbiting the Earth at about 300 miles altitude, but had covered up the find (presumably because the space aliens had a UFO base there). Tombaugh had to make a number of announcements that no new Earth moonlets had been found.
For many years, the fact that Earth had only one Moon–and it was not a captured asteroid–was something of a puzzle. Venus and Mercury were close to the Sun in a zone where there aren’t many asteroids, so it was no surprise that they had no moons. But Earth’s orbit is criss-crossed by a fairly large number of asteroids, and it seemed odd that none of them had been captured by our planet’s gravity to become a second moon. Mathematical models and computer simulations often had Earth capturing passing asteroids and holding them in orbit at least temporarily. The three-body gravitation of the Sun, Earth, and Luna made them unstable, however, and they would always, sooner or later, be perturbed and ejected from Earth orbit. According to these models, Earth should have a virtually continuous (on the galactic time scale) traveling companion, as asteroids, ranging in size from several feet to several miles, would be grabbed by gravity, orbit for a time before being kicked away, then be replaced by another. Astronomers began to refer to them as “mini-moons”.
Then, it was announced that indeed such a “temporary mini-moon” had been found. Back in September 2006 a team from the University of Arizona noticed an object in a very slow polar orbit around the Earth. Measuring around 15 feet long (about the size of a typical midsize car), it was at first thought to be just an empty piece of the Saturn V rocket that took one of the Apollo spaceships to the Moon, but calculations of its mass showed it to be a rocky asteroid. It was given the designation RH120. Just 13 months after it was first detected, while at the farthest point in its orbit, the mini-moon was perturbed by gravitational forces and was ejected from Earth orbit, to once again become just another tiny Near-Earth Asteroid sweeping around the Sun. But our little mini-moon will return–calculations show that RH120’s orbit brings it into an approach to Earth every 22 years, close enough to probably be captured. The next close approach will be in August 2028, when we will likely have another temporary second moon.
Indeed, astronomers have now found that in the game of gravity wells, nothing is permanent, and all sorts of odd things can go on. Mars currently has two moons, Phobos and Deimos. These are captured asteroids. But the orbit of the larger moon, Phobos, is decaying, and in about ten million years (a mere eyeblink on the galactic time scale) it will crash into the planet, and Mars will have only one moon. Many of the moons of Jupiter and Saturn are also just temporary visitors: they will either be kicked out of orbit to wander around the Sun on their own, or will be drawn in and crushed inside the gas giants. In the Kuiper Belt, where there are a large number of small objects interacting with each other, gravitational captures are virtually inevitable. Neptune’s large moon Triton revolves in a “backwards” orbit around the planet compared to the planet’s other moons, indicating that it is probably a captured Kuiper Belt object. Pluto’s moon Charon is almost half its own diameter, an example of a Kuiper Belt object being captured by another Kuiper Belt object. So far, there have been no observed examples of a moon orbiting a moon which itself orbits a planet. Such a three-tier system is entirely possible, though the complex gravitational interactions would make it unstable and temporary. In January 2016, measurements of gravitational disturbances in the orbits of Kuiper Belt objects led a team of astronomers to conclude that there must be another large planet orbiting the sun, beyond Pluto, which is causing those perturbations. The hunt is now on to find the predicted “Planet Nine”.
In addition to its temporary little “mini-moon” traveling companions, Earth has a number of odd objects in nearby space, some of which have been (incorrectly) called “second moons”.
In October 1986, a Scottish researcher working in Australia found a near-Earth asteroid measuring about three miles across, and named it 3753 Cruithne, after an ancient Irish king. As with all near-Earth objects, Cruithne’s orbit was measured to see if it presented any collision danger. But instead of a simple smooth ellipse, the asteroid’s orbit was pushed and pulled so much by gravitational interactions that its complex path was not fully calculated until 1997. Cruithne’s orbit was incredibly convoluted, with a pattern that sometimes brought it close to Mars and sometimes to Venus. From our point of view on Earth, over a period of 770 years the asteroid appears to move almost completely around the night sky, then suddenly stop and go almost completely around in the other direction before reversing again–a phenomenon known as a “horseshoe orbit”. But the odd part is that Cruithne, despite its wobblings and wanderings, takes almost exactly the same amount of time to orbit the Sun as Earth does. The press dubbed it “our second moon”, but that label is not correct, since Cruithne orbits the Sun, not the Earth. But its odd synchronized pathway around our planet has earned it the label “quasi-orbital satellite”. Since then, similar Sun-centered “quasi-satellites” have been found around Mars, Jupiter, and Saturn, and two more around the Earth.
Like most asteroids, Cruithne’s orbit is not stable. Some calculations seem to suggest that in roughly 8,000 years, the asteroid will approach Venus–not close enough to be captured, but perhaps enough to “slingshot” around the planet and be accelerated, likely being tossed out beyond Mars.
Another odd asteroid was discovered in 2010. Called TK7, it is a nondescript space rock about 1000 feet long–the size of an aircraft carrier. What makes it unusual is its location: at some time in the past, the asteroid happened to move through a “Lagrange point”–a mathematical location where the gravitational pull of the Earth, the Moon and the Sun are all equally balanced by centrifugal force. TK7 is lodged in Lagrange Point 4, just ahead of the Earth in its orbit. Calculations indicate that it may have been caught in the L4 point for some 1500 years. Similar Lagrange satellites were already seen near Mars, Jupiter, Neptune, and some of Saturn’s moons. Such asteroids are now given the name “Trojans”. Since they do not actually orbit their planet, they are not “moons”.