Is the Moon alone?- the search for other satellites of Earth

For millenia, from the earliest of civilizations to now, people have looked up into the sky at night, and saw, along with the milky way's faint smudge, and the occasional meteor shower or comet, the only thing in the sky that lit up the night around them: the Moon.

Unlike the other planets, however, since the Moon was first studied in detail, it's been known to orbit the Earth (although not exactly found by the same methods, as in the Geocentric model.)

For a slightly shorter time, people have also looked up and wondered if the Moon was accompanied by anything else orbiting the Earth. Here are a few of the more notable discoveries:

The first, and most popular claim of another moon, was in 1846, when Frederic Petit made a 'not-so-tiny' claim of another moon in orbit of Earth. According to him, the moon orbited Earth about every 2 3/4 hours, and orbited only 11.4 kilometers (7.1 miles) off the Earth's surface, based on visual perturbations of the Moon's orbit. While his theory was grounded through research, however, we know this definitely not to be true: First of all, if the moon orbits only 7 miles off the surface of Earth, then here's what it would look like:

Badly/quickly-drawn picture of Earth showing the proposed orbit of the satellite to-scale (Top) [MS paint ftw]

Not only this an unrealistic orbit, and not only are the crust deviations in Earth's surface enough to have this place hit Mt. Everest or something else in the Himalayas, but it's so close that not only would it orbit inside the atmosphere, but it orbits low enough that average commercial jets should have collided with it by now.

Even with all these inaccuracies, the asteroid still managed to get mentioned in Jules Verne's Around the Moon.

5 decades later, another astronomer proposed a slightly-more-believable, but also problem-ridden theory: He proposed not one, but a moon with moons orbiting it, orbiting Earth- at 1,030,000 kilometers (640,000 miles) away from Earth, and 700 kilometers (430 miles) wide, that orbits the Earth in 119 days.

Here's the a-bit-more-realistic orbital diagram:

Diagram showing the Earth, and the orbit of the Moon (to scale) and

the orbit of the 2nd proposed moon, twice as far away.

His explanation for why it hadn't been seen before, however, was a bit of a stretch:
He claimed that the moon was very dim, except for an hour when it shines like the Sun. Wouldn't it have been noticed before, however?

Soon, after a prediction that it would become exceedingly bright in February 1898, unsurprisingly missed, the moon was found to be nonexistent.

As if that wasn't enough, he proposed another, slightly larger and closer moon in August 1898. Here's the same diagram with this other moon:

Diagram showing slightly-smaller orbit of the 3rd moon.

This moon was rejected so predictably and unsurprisingly that the fact that I had to say this fact was pretty much pointless because the moon's nonexistence was already assumed by the reader long before I even started on this long, drawn-out sentence.

While numerous other (fake) moons were proposed during the rest of the early to mid 20th century, let's just move on to ones with more scientific grounding:


Date: February 9th, 1913, 7:00 PM
All over eastern and North America, people are leaving their late-night jobs and returning home, having dinner, and going to bed, in various stages. Over most of the east coast, it's overcast, nothing out of the regular. In Canada and the northern USA, however, it's fairly clear.

Date: February 9th, 1913, 8:50 PM
Most of the Eastern United States and Canada is asleep, with only a few avid astronomers, insomniacs, and night-shift workers awake. The night is still continuing as usual, with nothing special appearing to happen.

Date: February 9th, 1913, about 9:15 PM
A few minutes after the clock hits 9:00, people begin to notice a series of bright fireballs traveling towards them from the northwest. As another minute ticks by, the observers count between 5 and 10 of these fireballs traveling slowly across the sky, taking about 40 seconds to pass by. As more of these go by, more keep coming, bringing the total from 10, to 20, to 30, to 50. By the end of 5 minutes, the observers across the Northeastern USA and southern Canada were nearly done witnessing the procession, but they were just about to be in for the most spectacular part:

Date: February 9th, 1913, about 9:20 PM
The trail is nearly finished, but at the back is a bright, white, huge fireball traveling slowly behind them. While the other fireballs slowed down and sped up, eventually disintegrating, this fireball kept burning brightly for nearly a minute, eventually passing over the horizon, bringing an end to that night 's chaos.

Date: February 10th, 1913, 2:20 AM
Observers jolted awake by the meteors who stayed awake all night to see if anything else would happen were not disappointed: 5 hours later, a dimmer meteor procession came over the area, although not nearly as bright as the original- possibly the debris coming a full revolution around. Based on this, the meteorites were traveling 8,020 kilometers (4,980 miles) per hour, or nearly 2 kilometers per second.

Date: February 10th, 1913, the following morning.
As newspapers and emergency phone lines are piled with reports of 'the end of the world', people struggle to find out what actually happened: Many reported a loud, thundering noise, and many more thought it to be unidentified flying objects.

Current analysis shows that the procession could have been the orbital decay of a temporary satellite captured by the Earth, or possibly even the last remnants of a temporary ring around Earth! However, we may never know exactly what did cause it.




Jump forward 93 1/2 years, and in Tuscon, Arizona, at the Catalina Sky Survey telescope, astronomers find a new asteroid, designated 2006 RH120, that is unlike any other asteroid found before:
It was a moon of Earth.
The asteroid, 3-6 meters across, typically orbited the Sun near Earth, but in September of 2006, made a very close approach to Earth and was captured into orbit of it.

By June 2007, however, perturbations by the Moon and Earth threw the asteroid back into orbit of the Sun, where it is now.


Here are a couple of other honerable mentions that don't really count as 'satellites' exactly:


In 2010, the WISE team found a Trojan asteroid- an asteroid that orbits near a planet's Lagrangian points, where they can have a stable orbit without gravitational pushes and pulls from another planet- designated 2010 TK7. These sort of bodies are common around Jupiter and Saturn- but this one was found near Earth. It is the first, and only asteroid to be a Trojan of Earth, although it doesn't exactly 'orbit' Earth, it shares an orbit with it around the Sun.

Then there's the slightly-more complicated type of minor planet, called a quasi-satellite. These asteroids orbit the Sun at the exact same amount of time as Earth, but don't share an orbit. Currently, there are 5 of these: 3753 Cruithne (see image below), 2002 AA29, (164207) 2004 GU9, (277810) 2006 FV35, and 2010 SO16. All of these asteroids will probably move out of this orbit soon, but for the moment technically 'orbit Earth'.

Diagram showing the orbit of Cruithne coinciding with the orbit of Earth
Credit: Various, including the Celestia program, GDFL, and
Wikipedia user Jecowa.

So, to summarize: Searches for other moons around earth have mostly turned out not to be very fruitful, and while some objects may get into temporary orbits of Earth, most never stay. While Mars may have two, Jupiter may have over 60, and many more orbiting other planets, Earth will always have a single, special satellite that we call the Moon.