NASA has been mapping asteroids that might be a threat to Earth for a few years now. In 2013 NASA released a map of potentially hazardous asteroids. Astronomers think that there are around 1 million potentially dangerous "rocks" out there, but only thousands of them have been identified so far.

Rocks with a size of only around 1km wide can potentially end human civilization. Most of these big ones cruising through Earth's neighborhood have fortunately been identified (well, as far as we know), and most of them don't pose any threat to us in the foreseeable future. But also just 20-meter-wide asteroids have been known to cause problems, and there are a lot more of them, and mapping all of these is more problematic. Possibly only around 30 percent of rocks with a size of 100 m have been mapped, and these can already destroy areas the size of countries. And when it comes to even smaller asteroids like the one that came down down in 1908 in Tunguska that flattened 2,137 square km forest, we only know about one percent of them.

Right now the biggest known threat to us from outer space is an asteroid that has been named Apophis (99932 Apophis) which will be flying by Earth in 2029, April, Friday the 13th.

How big is Apophis?

Apophis is roughly 340 meters in diameter and with assumed mass of around 6.1*1010 kg.

Will Apophis hit Earth?

The odds of hitting Earth on April 13, 2036, have been completely eliminated. The odds of it hitting us on April 12, 2068 are estimated to be around 1 in 150,000. When it comes to Apophis hitting Earth on 2029, the calculated likeliness of it happening was about 2.7 percent when it was first discovered in 2004. Now the new calculations show that the odds are less than 1-in-100,000.

How close to Earth will Apophis come?

It will pass Earth at around 31,000 kilometers (19 000 miles), so in relative terms, it will be very close to us, and visible with a naked eye to about 2 billion people. Scientists are less worried and more excited by the occasion. "The excitement is that an object this large comes this close about once per thousand years, so it's all about, What's the opportunity?" said Richard Binzel, a planetary scientist at MIT.

What would happen if Apophis hit Earth?

If it did hit Earth (which, at this point, is very unlikely), it could do significant damage and could kill more than 10 million people, depending on where it would hit. Although it wouldn't be a civilization ending event, the impact would release energy equivalent of 1,125 Megatons. In comparison, the bomb dropped on Hiroshima had a force of only 0.015 Mt (or 15 Kiloton).