system about 4.6 billion years ago. Most of these fragments of ancient
space rubble - sometimes referred to by scientists as minor planets -
can be found orbiting the Sun in a belt between Mars and Jupiter. This
region in our solar system, called the Asteroid Belt or Main Belt,
probably contains millions of asteroids ranging widely in size from
Ceres, which at 940 km in diameter is about one-quarter the diameter of
our Moon, to bodies that are less than 1 km across. There are more than
20,000 that are numbered.
Earlier this month, a skyscraper-sized asteroid passed within 50,000 miles of Earth — a galactic hair’s breadth separating the planet from an impact like one that flattened 800 square miles of Siberian tundra in 1908.
IMAGE BY NASA
Then there’s an asteroid spotted in 2004 and called Apophis. Astronomers originally thought it might hit Earth in 2029. Then they decided that it couldn’t. Finally they moved back the clock to 2036.
The uncertainty is understandable, but not exactly reassuring. And even if Apophis misses, some other rock big enough to put a serious dent in Earth and everything living here will take dead aim for us someday. It’s just a matter of time. Some researchers put the odds of a civilization-wrecker at one in the next 300,000 years, others at 1 in 10 for the next century.
Calling them "the only major natural hazard that we can effectively protect ourselves against,"
Direct nuclear explosions. As immortalized in the movie Armageddon, we could blow an asteroid out of the sky like so much interplanetary skeet. But there’s a catch: The pieces could still hit Earth, and we might not have enough firepower to do serious damage. It’s a last-minute, last-ditch option.
Nearby nuclear explosions. Rather than blowing it up, a nuclear explosion could reroute a space rock’s trajectory clear of Earth.
This would need to be done decades before the asteroid reached us. They are, after all, not easy to steer. But models suggest it could work.
Laser sublimation. Several spacecraft could use machines that direct beams of concentrated sunlight to the surface of an appropriately icy asteroid. As it heats up, it’ll spew a plume of debris and change course. Of course, anyone who’s ever tried to set a ball of paper aflame with a magnifying glass knows it’s not easy. Now imagine that the ball was rotating and traveling faster than sound.
Electric propulsion. To adjust course, land a spacecraft on an asteroid, fire up some rockets and push off. It’s a potentially powerful approach, but controlling that push on a spinning rock will be difficult.
Gravity tractor (pictured above). Every object exerts a gravitational pull, including a single spacecraft. Merely by hovering above the asteroid, it could pull the rock off course. The approach could even be tried with the asteroid belt-exploring Dawn spacecraft, scheduled to finish its tasks by 2015. This is potentially much easier than electric propulsion, but not quite as powerful.
Solar sails. Installing a photon-catching sail on an asteroid would be even harder than landing a ship, but it would certainly be prettier.
None of these approaches will work unless people see the asteroid in time to plan for it.
Under NASA’s Near Earth Objects program, six U.S. observatories
"search every clear night for these kinds of objects. They are tracked, cataloged and stored," said Steve Chesley, an astronomer at NASA’s Jet
Propulsion Laboratory. "NASA’s goal is to find 90 percent of those that are one kilometer across and larger. We’re at 82 percent right now, and we’ve only been aggressively searching at current levels for eight to 10
years. Those ones just haven’t flown into view."
NASA has suggested two possible methods of protecting the Earth from an asteroid or comet determined to be on a collision course.
Destroying the object before it hits the Earth
Deflecting the object from its orbit before it his the Earth
To destroy the Earth-approaching object, astronauts would land a spacecraft on the surface of the object and use drills to bury nuclear bombs deep below its surface. Once the astronauts were a safe distance away, the bomb would be detonated, blowing the object to pieces. Drawbacks to this approach include the difficulty and danger of the mission itself, and the fact that many of the resulting asteroid fragments might still hit the Earth, resulting in massive damage and loss of life.
In the deflection approach, powerful nuclear bombs would be exploded up to half a mile away from the object. The radiation created by the blast would cause a thin layer of object on the side nearest the explosion to vaporize and fly into space. The force of this material blasting into space would "nudge" or recoil the object in the opposite direction just enough to alter its orbit, causing it to miss the Earth. The nuclear weapons needed for the deflection method could be launched into position well in advance of the object's projected Earth impact.
But most important is its exact timing and our preparation to tackle such a disaster.
While these and other methods of protection have been considered, no definite plans have been fully developed. Scientists of the Asteroid and Comet Impact division of NASA's Ames Research Center warn that at least ten years will be needed to send a spacecraft to intercept an incoming object and deflect or destroy it. To that end, say scientists, NEO's mission of detecting threatening objects is critical to survival.
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