What if an asteroid strikes near you? Here's what happens...

World Asteroid Day is this Friday.

If there's one thing scientists agree on, it's that there is a cosmic alarm clock ticking somewhere, and when it goes off, billions of people could die.

LOS ANGELES, CA (California Network) -- It's the most ordinary of days. You're powered through most of the workday, and you're looking forward to relaxing at home when everything ends. Although you are indoors, the flash outside catches your attention. Everyone gasps at how intense the light is, there are no shadows. The heat follows, and it sears your skin. You cannot hide from it. A shockwave ends your life so quickly; you don't even hear it. You didn't even suffer, or have time to consider what just happened.

What has happened is something that has happened many times before, with equally dramatic result. A large meteor has hit the Earth, instantly killing millions of people within its blast radius. Millions more die in the shockwave traveling faster than the speed of sound. Raining debris kills millions more. Smoke and fire take even more. In the days to come, the planet's temperature plunges.

The sun shines weakly through the cloud of ash and dust that envelops the planet. Plants begin to die. Animals begin to die off within days. The carcasses of bloated, dead fish wash ashore in vast numbers. The smell of death permeates everything, everywhere.

After a few weeks, the ash begins to settle, and the sun shines a little more brightly, but it will be years, possibly decades before the planet becomes warm again. It will take millions of years for life to recover. The humans that remain will be left to rebuild civilization with a fraction of their knowledge and resources.

This scenario is a promise. It is guaranteed to happen. The only question is when.

Astronomers know there are thousands of asteroids in space which pose a threat to Earth. Each time they find a new one, they track it. They can pinpoint these space rocks with accuracy at any moment. And they can predict where they will be at any moment in the near-future.

But there are two things scientists can't do much about. The first is that over time, small disturbances can cause a known, harmless object to change its courts, and become a dangerous, Earth-bound asteroid. The second is that there are some comets and asteroids that can come in from the deep solar system, even from the direction of the sun. Such threats cannot be reliably detected.

The danger faced by Earth is so extreme that NASA and other agencies have banded together and cooperate to search for these objects. There are ideas, but no solid plans, for what to do should one be detected on a collision course with Earth.

Friday is Asteroid Day; it is a worldwide event designed to raise awareness of the threat posed to Earth. It is impossible to say when something like this could happen. It could be today, tomorrow, or even millions of years into the future. But it merits attention because when it does happen, it will be devastating. It's an existential threat. It's inevitable, but it may also be preventable.

What happens next is the product of popular opinion. If the people agree that asteroids are a threat and we should hunt for them, then funding for programs that do this should continue. If the public disagrees, and the funding stops, then the likelihood of detecting a threat early also falls to nearly zero. The choice is ours, but the time and place are not.