ABC Asteroid XYZ

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Spacecraft NEAR Shoemaker executed a soft landing on this asteroid, Eros, in 2000.

In “Asteroids Good and Bad” we touched on NASA’s plans to detect asteroids as well as to eventually land on one and “redirect” a piece of it into orbit around the Moon.

Why the interest in these space rocks?

Asteroids have pounded Earth for billions of years, and they’re not done with their mission, as the Chelyabinsk meteor rudely reminded us over Russia in 2013. With no warning it slammed into Earth’s atmosphere and exploded 28 miles up. Even that far away the relatively small 18-meter diameter asteroid fragment’s 500 kiloton equivalent explosion injured over 1,200 surprised people going about their business below it.

So what are asteroids? They consist of rocks and minerals, and come in three varieties: chondrite (C-class) made of clay and silicate rocks; stony (S-class) made of silicate and nickel-iron rocks; and metallic (M-class) consisting of mostly nickel-iron rock. They’re irregularly shaped, usually pitted or covered with craters.

Size matters, especially if one is headed our way. Most asteroids range from thirty feet to 330 miles in diameter, and none have any atmosphere. They orbit the Sun, rotating or tumbling along. More than half-a-million are known, but millions more are out there – exactly how many is unknown – and size matters.

Most asteroids orbit our Sun between Mars and Jupiter in the main asteroid belt. Between 1 and 2 million asteroids larger than 1 kilometer in diameter may exist in the main belt, yet most are still a million or more kilometers away from their nearest sibling. Just 150 have a companion moon, a few have two moons, and some asteroids occur as a pair of relatively equal-sized bodies. Astronomers have even spotted triple asteroids tumbling along together.

Although most asteroids may remain in the main belt, Jupiter’s massive gravity flings them out across the solar system from time to time. At present, 10,003 are known to approach Earth close enough to be considered potential collision hazards. These asteroids are called “Near-Earth Objects” or NEOs, and 861 NEOs have a diameter of 1 kilometer or more. Thus far scientists have tracked over 1,400 “Earth-crossers” that pose a significant threat. But as the surprise arrival of the asteroid over Chelyabinsk, Russia demonstrated, we’ve not yet located every potential collider.

How often do astronomers expect an asteroid to hit Earth? What evidence do we have that asteroids have hit Earth in the past? Where else can we find most asteroids other than in the main belt? How do asteroids get named, and did you know there’s an asteroid named for a cat called Dr. Spock? That’s weird.

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