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  Night Sky

Enjoy the starry night skies
Be a considerate neighbor. Reduce nighttime glare.
Shield your outside lights downward.
Let the stars light up the night.

October 2015 night sky

—Charlie Christmann

Earth’s moons

Yes, you read that right. I said “moons.” I know, you look up at night and you see only one big moon looming above your head, so where are the other moons? If you think about it, having only one moon is a bit unusual in our solar system. Mars has two (at least for the next few million years), Jupiter has 67 that we have found so far, Saturn has 53 known moons, Uranus has 27 so far, and Neptune has 13. Even the dwarf planets have their own moons: Pluto has five, Eris has one, and Haumea has two. Mercury and Venus are the exception with no known moons at all. Then, there was one object found orbiting Earth in the early 2000s that set off alien invasion alarms in the UFO community.

So why should Earth have only one, and a relative large one at that? It seems that the terrestrial planets, those inside the asteroid belt, did not start out with a moon. Earth most likely acquired its big moon when we collided with a Mars-sized planet not long after the Earth was formed. That collision blasted a large chunk of the Earth into orbit where some of it coalesced into the moon. Mars most likely captured its moons from the asteroid belt. Mars’ closest moon, Phobos, is gradually spiraling inward by about five feet per century. Phobos will create a new large crater on Mars’ surface in fifty million years.

A second “moon” was discovered in 1986 in a very weird orbit around Earth. Its name is 3753 Cruithne. The orbit had a kidney bean shape, not the usual ellipse, with a period of 770 years. This unusual orbital shape was a real mystery until 1997, when many observations finally determined its real orbit. It only appears to be a moon of Earth. In reality, it orbits the sun in a highly elliptical orbit about the sun crossing both Mars’ and Venus’ orbits. It is also inclined to the Earth’s orbit by twenty degrees. It just so happens that its orbital period almost matches Earth’s: 364 days. The appearance of orbiting Earth is really an illusion due to the motions of both bodies around the sun.

Cruithne is expected to remain in its orbit for a few thousand to tens of thousands of years. As it passes the planets, its orbit is perturbed slightly. In 2058, Cruithne will come within 8.5 million miles of Mars. In about eight thousand years, Cruithne will have another close encounter, this time with Venus. All these close encounters will alter its orbit in unpredictable ways. Though no one knows for sure, there is a possibility it could be captured by Earth someday to become a real moon.

Earth most likely has several temporary moons, like 2006 RH120, discovered in 2006. While normally orbiting the sun, it does come close to Earth every twenty years where it can be temporarily captured. It was in Earth’s orbit from September 2006 to June 2007 before being flung back into solar orbit.

2010 SO16, like 3753 Cruithne, also orbits the sun, but seems to have a horseshoe-shaped orbit around Earth with a period of about 350 years. Simulations predict its orbit is unusual, remaining in its current orbit for at least 120,000 years and possibly for more than a million years.

Finally, there is 2002 AA29 in an almost identical orbit with Earth, taking 361 day to orbit the sun. It also appears to temporarily orbit Earth in a spiral shape as we catch up to it, then pass it, in our orbit every 95 years.

J002E3 is a mystery object. Discovered by amateur astronomer Bill Yeung in 2002, it was first thought to be a small asteroid orbiting Earth. However, spectrographic studies found that its surface was covered in titanium dioxide—white paint. Astronomers think the object entered Earth orbit very recently since the combined gravity of the Earth and Moon would have quickly ejected an object in such a highly elliptical orbit. UFO conspiracy theorists had a field day. After some hard searching and simulations, NASA found the object was the spent third stage of the November 1969 Apollo 12 mission. They intended to send the booster into solar orbit, but due to some problems, it entered a looping orbit about Earth that is not stable. J002E3 likely entered Earth orbit in 2002 and left again in June 2003. It will return for a few swings around Earth every forty years or so before returning to a solar orbit.

Astronomers think Earth may have several of these temporary-type moons in orbit at any one time. They are just so small and so fleeting that they are hard to find. But if you have a telescope, stay vigilant; you might just discover a new moon.

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