An asteroid's near miss

A cosmic heavyweight just paid earth a visit: Asteroid "2004 BL86" passed our planet by at a distance of about 1.2 million kilometers (about 745,000 miles), or more than twice the distance to the moon.

"The asteroid poses no danger, but it reminds us that earth, too, can be hit by such an object," says Alan Harris, a senior researcher at the Institute of Planetary Research of the German Aerospace Center (DLR).

Researchers have known since 2004 that the asteroid was approaching. With high resolution telescopes, they're constantly searching the skies for similar near-earth-objects (NEO).

Alan Harris' international "NEOShield" team is tasked with detecting threats from asteroids -and possibly even with defending against them.

"A hit by such an object would result in a natural disaster that our civilization has never seen before," he says.

While a rock like 2004 BL86 could wipe out an entire country, something not much larger - with a diameter of one kilometer - could cause damage all over the globe.

Baggage from the past

Asteroid "2004 B86" is one of countless large rocks racing through space, all of them remnants from the period when our planetary system was formed about 4.5 billion years ago.

Most of the asteroids have a predictable orbit around the sun. "But sometimes the strong gravity from Jupiter can reroute asteroids. That's how they get close to Earth's orbit," Harris explains. Such is the case with "2004 BL86".

An asteroid that came down in the Siberian town of Chelyabinsk in February 2013 recently showed what kind of damage even very small asteroids can cause: The shockwave destroyed windows and took off entire roofs. About 1,600 people were injured. That asteroid, however, was only 20 meters in diameter - almost nothing compared to "2004 BL86".

That damage could have been even worse, but fortunately the asteroid of Chelyabinsk consisted mostly of rock-like material but didn't have a metallic core; scientists differentiate between metal and stone asteroids. Those that consist of metal are more dangerous, because they do not explode or break apart as easily when entering the Earths' atmosphere.

They are therefore larger when they slam into the ground and cause far more damage.

Fortunately, though, "Those metal-objects are rather rare," Harris says.

No earthly answer - yet

Tuesday's asteroid offered some spectacular views for amateur astronomers. "2004 BL86" will sped by at 15 kilometers per second.

Harris and his colleagues at NEOShield are working on what to do if an asteroid actually is on a collision course with earth. So far, the planning has not gotten beyond the phase of collecting ideas.

"We would send a spacecraft to crash into the asteroid with the intention of changing its course very slightly. This could be enough to prevent it from hitting earth."

But such an approach would take time. "Five years would be good - ten years would be better," Harris says.

To prepare, Harris and his team would like to conduct test trials with smaller asteroids. So far, however, they have not received financial backing for such a project.

"At this point we can not say with certainty how such an asteroid defense system would work."

Next visitor: Apophis

On April 13, 2029, the next huge asteroid will approach earth, albeit this time much closer: Aphophis will pass us at only about 30,000 kilometers' distance. This is less than the distance of geostationary TV satellites. The object has a diameter of about 300 meters (0.19 miles), slightly smaller than 2004 BL86.

"Apophis will not only get closer, but will also fly by earth more often," Harris explains. NASA expects the asteroid to buzz the Earth twelve times between 2060 and 2105.

And since the later trajectories of Apophis could be influenced by the gravity of earth during the near-misses, "We will have to watch it very carefully," he says.

However, the researcher is not worried about planet earth in the near future.

"We know that nothing really dangerous is going to approach us within the next 100 years."