Illustration by NASA, ESA - P.Carril

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Asteroid 2004 BL86 (illustration above) passed our planet at about three times the distance of the moon from the Earth.

Illustration by NASA, ESA - P.Carril

Giant Asteroid Sails Past Earth, Raising Safety Questions on Terra Firma

The space rock known as 2004 BL86 has gone by, but it was a squeaker, cosmically speaking.

A mammoth asteroid safely sailed past Earth on Monday, and scientists have started to look at information gleaned from the flyby that may help curb future risks of an impact.

Known as 2004 BL86, the space rock posed no danger to us Earthlings, passing at a distance of 745,000 miles (1.2 million kilometers) at 11:19 a.m. EST. While this may sound like a comfortable distance—it’s about three times the distance of the moon from the Earth—it is considered a cosmic squeaker.

Early radar observations by NASA, which used some of the world’s largest radio telescopes to view the asteroid, have shown some surprises. For instance, the rock measures some 984 feet (300 meters) across, rather than the earlier estimate of 1,600 feet (500 meters).

The early images also show what looks like a companion moon next to the asteroid. And they reveal that the asteroid is tumbling so fast that a day on its surface is no more than 2.6 hours long.

Backyard sky-watchers will have a chance to see the giant rock on Monday night when, for several hours, BL86 will reach a visual brightness of magnitude 9. That means small telescopes and possibly even large binoculars will reveal the asteroid—as long as you know where to look. (Learn more about how to see the asteroid.)

Monday’s pass is considered the closest that an asteroid this large will get to Earth until the year 2027, when the space rock known as 1999 AN10 whips by our planet within the moon’s orbit.

That’s a pretty close call. So should we be paying more attention to the potential threat of an asteroid hitting our planet? Here are a few basic questions and answers.

What are scientists hoping to learn from Monday’s close encounter?

NASA scientists are using the radio telescopes to bounce radar signals off the asteroid’s surface so they can map it out in detail. They want to get a much clearer idea of its shape and size and how it is tumbling through space.

The information will be critical for the future asteroid retrieval mission that NASA has planned in the next decade, when scientists will get their hands on their first samples from these ancient cosmic rocks.

This week’s flyby offers a unique opportunity for space agency scientists and engineers to better design these future missions.

Do we have a sense of the probabilities for getting hit by destructive meteors?

It turns out that really massive objects, like the six-mile-wide (ten-kilometer) asteroid that might have wiped out the dinosaurs, are quite rare. Astronomers estimate they have already found least 94 percent of them, which they are tracking.

Those that are a few hundred meters across, like 2004 BL86, may impact our planet every half million years or so. They would cause regional devastation, enough to wipe out entire countries.

The more immediate concern appears to come from rocks that are between a few dozen meters wide to a couple of hundred meters wide. Asteroids of this size could easily flatten an entire city.

And there is precedence for such impacts in recent human history, including the Tunguska event in 1908, which leveled a 770-square-mile (2,000-square-kilometer) swath of forest in Siberia.

Coincidentally, the same region of the globe got hit again in February 2013, when a 60-foot-wide (18-meter) rock exploded in the atmosphere above the Russian town of Chelyabinsk, causing over a thousand injuries. There could be upwards of a million of these smaller objects buzzing around Earth’s orbit, making them the most likely to hit our planet. And we know the locations for less than one percent of them.

Are we prepared for the threat of such impacts in highly populated areas?

While there are no definite figures, astronomers believe that bus- or house-size asteroids like the one that hit Russia occur once or twice a century and mostly fall over the ocean or unpopulated areas.

But the ongoing threat has some scientists arguing that the mission of discovering and tracking all near-Earth objects, as well as setting up contingency plans for deflecting them on short notice, should get more resources.

We continue to discover these potentially hazardous asteroids, sometimes only days before they make their close approaches to Earth.

A non-profit organization called B612, led by former NASA astronaut and engineer Edward Lu, has been working to put an infrared-based telescope into space that can see small, faint asteroids. The group wants to launch the telescope by 2018.

Follow Andrew Fazekas, the Night Sky Guy, on Twitter, Facebook, and his website.