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"Solar System debris rains down on Earth in vast quantities - more than a hundred tons of it a day. Most of it vaporizes in our atmosphere, leaving stunning trails of light we call shooting stars. More hazardous are the billions, likely trillions, of leftover rocks - comets and asteroids - that wander interplanetary space in search of targets. Most asteroids are made of rock. The rest are metal, mostly iron. Some are rubble piles - gravitationally bound collections of bits and pieces. Most live between the orbits of Mars and Jupiter and will never come near Earth. But some do. Some will. More than a thousand known asteroids are classed as "potentially hazardous," based on size and trajectory. Currently, it looks doable to develop an early-warning and defense system that could protect the human species from impactors larger than a kilometer wide. Smaller ones, which reflect much less light and are therefore much harder to detect at great distances, carry enough energy to incinerate entire nations, but they don't put the human species at risk of extinction."

This got us thinking: What happens when smaller (and bigger) objects fly into our atmosphere? We used Purdue University's "Impact: Earth!" simulator to find our answers. Our own Bob Al-Greene illustrated the results, as seen in the gallery above.

Some highlights: Rocks the size of basketballs enter our planet about once a month; most burn apart in the atmosphere before they reach the surface. Objects as long as standard school buses (roughly 12 meters around) only sneak into the Earth every 20 years or so - but, as seen in Chelyabinsk, the damage can be much greater.

All results assume the object is traveling at a 45-degree angle, with a density level of 3000 kg/m^3 and a velocity of 11 km/s. Everything is assumed to be seen from 100 kilometers away from the direct impact zone.

Click "Show As List" on the bottom-left of the gallery to view larger images. And check out our Google Hangout with asteroid experts to learn more about what's being done to fend off space rocks, from basketball-sized to London-sized.

"Enter the planetary defenders, a group of astronomers, physicists and aerospace engineers who have since the early 1990s been locating flying space rocks, painstakingly plotting their orbits, and thinking of ingenious schemes to drag them off course or blow them up should they be on a trajectory toward us. Finally, they have been imagining how the fractious family of man might come together with a contingency plan to literally save the planet, like Bruce Willis and Ben Affleck in Hollywood's Armageddon. NASA has identified 94% of the huge, potentially civilization-ending asteroids nearby (none of which is on an earth-trajectory for now). But only about 1 percent of the 500,000 Near Earth Objects around the size of 2012 DA14 orbiting near earth's orbit have been tracked. The space agency's global Spaceguard program connects professional and amateur telescopes looking for smaller NEOs. A telescope in Spain picked 2012 DA14 when it was 2.7 million miles away, and reported it to the Minor Planet Center in Cambridge. Later,scientists calculated its trajectory, based on a few plot points of its movement."

A 2013 meteor explosion above Chelyabinsk, Russia injured more than 1,700 people. It was completely unexpected -- and it could happen again. Here's what NASA's doing to make sure we know how to act when (not if) the next one hits.

Later this year, Europe's Rosetta probe will orbit and land on comet 67P/Churyumov--Gerasimenko. New images of the comet show that it will be a lively place when Rosetta arrives.

A new documentary from NASA tells of the difficulties of sending an astronaut into cold, deadly space, plus how far we've come since Gemini 4. Since the days of the first "extra-vehicular activities", spacewalks have allowed mankind to walk on the Moon, build the International Space Station, and save the Hubble telescope. And we're not done yet. Future spacewalks will take us even farther-to asteroids orbiting the Moon, and eventually, Mars. Check out the documentary to learn more:

"Alright people, let's move on. Nothing to see here. You know that asteroid of death that whizzed by Earth today at an altitude that's actually below some of our satellites? You know that meteor that exploded in the skies over Russia today, injuring nearly 1,000 people? And you know all that speculation that they're somehow connected-that the Earth has stumbled into some kind of storm-front of space rocks, any one of which will annihilate us eventually? Forget it. The two incidents have absolutely nothing to do with each other, and neither one should cause us all that much worry. Yet."

This year will be another exciting one for space exploration. While 2014 will be remembered as the year we landed on a comet(!), 2015 may be known as the year of Pluto (and other dwarf planets). The New Horizons spacecraft begins its approach to Pluto this month, and will get closest to the dwarf planet in July, taking in the best view ever of the icy, remote world-possibly revealing a dramatic landscape with mountains, volcanoes, and geysers. In March, the Dawn spacecraft will arrive at Ceres, a dwarf planet in the asteroid belt between Mars and Jupiter. Ceres is another icy world, possibly with liquid water under a frozen surface, making it potentially habitable for life.

Since its inception in 1958, NASA has accomplished some pretty spectacular feats of science. Our country has landed humans on the Moon six times. We've successfully put laboratories onto the surface of Mars, and we've flown by every single planet in our solar system, including the recently promoted asteroid-turned-dwarf planet, Ceres.
Despite decades of scientific and technological achievements, some people still think that funding NASA is a waste of money. However, when you do the calculations, it turns out we are actually getting a great value from this government-run agency.