Are asteroids a threat to earth?

Yes. Asteroid impacts have been responsible for mass extinctions before; for example, al large animals on Earth were killed off when an asteroid struck the Earth about 65 million years ago. About 15 thousand years ago, an asteroid or comet probably struck northern Canada, touching off the "Younger Dryas" mini-ice age, which killed off the wooly mammoths and saber-toothed tigers among others, and probably most large animals - and humans - on North America.

Another asteroid strike WILL CERTAINLY HAPPEN. We just don't know when. Just one year ago, a small asteroid struck Russia and exploded over the city of Chelyabinsk, breaking every window in town and causing thousands of injuries. Several years ago, a smaller asteroid exploded above Sumatra, causing no damage or injuries.