Earth is not alone…

Continuing the lunar trend after my last post, which highlighted new multimedia from NASA that reviewed the stages of the Moon’s development and a tour of the Moon’s most notable features, now I’d like to share some more recent news about Earth’s only natural satellite…or so we thought.

Leading theories about the creation of the Moon is that an object about half the size of Earth (roughly Mars-sized) collided with the Earth roughly 4.5 billion years ago. This cataclysmic, near-Earth-ending impact (models show that an impactor only 10% larger would have completely destroyed the planet) ejected a large amount of debris that settled into a ring system around Earth. As the Earth cooled and gravity pulled the planet’s mass into its roughly spherical shape (technically an “oblate spheroid“), pieces of debris in that ring system began to collide and combine, eventually collecting almost all of the debris and forming the Moon. But now, as NPR reports, an international collaboration of researchers from the University of Hawaii, University of Finland, and Observatoire de Paris have written a computer simulation that calculates how many small asteroid-like objects may actually be in captured orbits around Earth. Their results, published in a recent issue of the scientific journal Icarus, conclude that at any given time, Earth may have one or more smaller captured objects, or additional “moons”, in orbit around it. The idea of a captured moon is not a new one, most scientists agree that the two irregularly-shaped moons of Mars, Phobos and Deimos, are most likely captured asteroids whose orbits were disturbed while out in the asteroid belt and were captured by the Red Planet after they strayed too close to it. And of course having a single moon would make Earth unique, of the 6 planets which have moons (Mercury and Venus lack them), Earth is the only planet with a singular moon, or so we thought. But even if these other natural satellites are in orbit around earth, will we consider them moons or will the International Astronomical Union (IAU) make some new kind of classification of “dwarf moons”?

The moons of Mars, Phobos (left) and Deimos (right) are thought to be captured asteroids. Credit: Encylcopedia Britannica

Speaking of the good ol’ IAU, as I detailed in one of my earliest posts, “Alas, poor Pluto“, part of the 2006 decision that de-planetized Pluto defined that a “planet” must clear its orbit of debris. I also mentioned that such a vague definition could leave open an argument for Jupiter to be demoted as well since it has Trojan asteroids which share its orbit. Well now, some could argue that Earth might not technically be a planet anymore either. A new study by NASA’s Wide-field Infrared Survey Explorer (WISE) mission located a Trojan asteroid that shares Earth’s orbit. As NASA explains, “Trojans are asteroids that share an orbit with a planet near stable points in front of or behind the planet. Because they constantly lead or follow in the same orbit as the planet, they never can collide with it. In our solar system, Trojans also share orbits with Neptune, Mars and Jupiter. Two of Saturn’s moons share orbits with Trojans.” Details about the tiny Trojan, designated 2010 TK7, were published in the July 28 issue of Nature. 2010 TK7, seen in the star field below, is only 1,000 feet (300 meters) in diameter is in a highly irregular orbit as it follows Earth around the Sun (click here for an animation of the little guy’s odd orbit). Thankfully though, the smart folks at NASA have done their research and the asteroid’s orbit is well-defined and for at least the next 100 years, so we know that it will not come closer to Earth than 15 million miles (24 million kilometers), which is more than 50 times the distance from the Earth to the Moon. So no end of the world asteroid impacts…from this asteroid, but I’ll leave that conversation for another post.

Earth’s only known Trojan asteroid, 2010 TK7, is shown here, in the green circle, among a field of stars. The asteroid was found by NASA’s WISE mission. Credit: NASA

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