Around 65 million years ago, a huge asteroid hit Earth, wiping out the dinosaurs and up to 30% of all life on the planet. But this was not a one-time occurrence.

Celestial bodies have hit Earth before and are bound to do so again. The billion dollar question is: Could the impact trigger another mass extinction?

To answer this question properly, we need to understand the origin of objects set on a collision course with our planet. Three best sources of Earth-bound celestial material are the asteroid belt between Mars and Jupiter, the Oort cloud and Kuiper belt. The Oort cloud is believed to be a thick bubble of icy debris that surrounds our solar system, and a probable origin of most of the long-period comets that have been observed. The Kuiper belt is a disc-shaped region populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) in diameter, and an estimated one trillion or more comets beyond the orbit of Neptune.

These two regions, while both rich in gigantic space debris, contribute to the extinction scenarios in vastly different ways. While the Oort cloud is quite dense, consisting of billions of comets, its very unlikely that many of them jeopardized Earth in the past. In fact, simulations run by scientists confirm that in the past 500 million years, just two or three comets may have struck Earth, causing powerful meteor showers.

The Kuiper belt is next on our list of suspects. Its disc-like structure makes it easier for the comets to find their way to our solar system, but more often than not, they get pulled in by the Sun and evaporate.

Still, movements of both Oort cloud and Kuiper belt objects are complex, and their interactions with other celestial bodies are far from predictable and periodic. In fact, many of them can get pulled from their initial state into orbits of planets and various other celestial bodies, where they can be further slung toward our solar system. The majority of them leave the solar system, collide with other planets and moons, or simply evaporate, passing too close to our Sun. Those that do hit Earth either burn out in the atmosphere or lose a significant amount of their mass before actually hitting planets surface.

What about the asteroid belt between Mars and Jupiter?

Out of 50,000 meteorites found on Earth to date, 99.8% are believed to have come from this region including the dinosaur-killer. But because the asteroids that leave the belt dont get replenished, as time passes, so does the danger. The proof of this theory can be found on celestial bodies such as Earths Moon or the planet Mercury, where older craters (created 3 -3.5 billion years ago) are the most numerous, with considerable fallout in frequency as we move further up the timeline.

Our solar system is also constantly moving along with our Sun, which revolves around the center of the galaxy, passing through various regions of the Milky Way on its 225 million-years long journey. During that period, also known as a cosmic year, occurring every 31 million years or so, we pass through the denser areas of the galaxy. Some claim that these periods coincide with mass extinctions in Earth's history, which means these extinctions are likely periodic. Yet this excellent article by astrophysicist Ethan Siegel shows that there is no scientific evidence to support that claim.

Still, dont rest just yet. Complex movements of our solar system, paired with those of the galaxy, could excite asteroids and comets lying in wait in the Oort cloud or the Kuiper belt, sending them on a collision course with Earth. With billions of them out there, theres a good chance a few could end up at our doorstep.

But, is there an increased risk of mass extinction such as the one Earth faced 65 million years ago? At this point, I dont think so. That is not to say that there is no risk. The universe is complex, and the interactions between planets, asteroids, stars and galaxies are intricate and multi-layered. Anything can happen. If you are interested in humans current capabilities to avert such a disaster, click here (Hint: its not pretty).

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Opinion: These visitors from space could kill us - MarketWatch