Tag Archives: solar system

ABC Asteroid XYZ


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.



Ceres, the largest asteroid, first discovered in 1801 by Giuseppe Piazzi, and visited by the Dawn spacecraft in February, 2015. Those bright spots remain a mystery, and Dawn continues taking pictures of Ceres from its closest approach, which continues the next few months. Image from NASA JPL via Dawn.

NASA’s new 2016 budget includes money for two asteroid missions. One of those missions continues an effort underway for years, the detection of near-Earth objects (NEOs), asteroids that pose a risk of colliding with our home planet. The other mission proposes to land on a near-earth asteroid, pick up a large boulder from its surface, and redirect the boulder into an orbit around the Moon. Once in a stable lunar orbit, manned missions would visit the asteroid fragment and retrieve pieces to return to Earth for detailed study.

Both missions complement each other and serve multiple roles. The detection mission will help protect Earth from a potentially cataclysmic collision with an asteroid. It’s happened many times in the past, and it continues to happen now. The biggest recent impact came in 2013 over Chelyabinsk, Russia, from an asteroid estimated at a mere 18 meters in diameter.

The detection mission would also help NASA find a nearby asteroid from which it might pluck a piece. That mission to “redirect” a piece of an asteroid into orbit around the Moon would build our understanding of what asteroids consist of and how they might contribute valuable minerals for industry. Perhaps even more important, the “redirect” mission involves landing on an asteroid and moving a piece of it onto a different trajectory, skills we would need if we ever detected an asteroid on a collision course with Earth and wanted to deflect it away.

How many asteroids are out there? How big are they? How many are on a course that might bring them into contact with Earth one day? What are they made of and does their composition mean we might benefit by capturing one and mining it? Could we deflect an asteroid on a collision course with Earth? What spacecraft missions have already studied asteroids, and what have we learned?

Stay tuned.