There are no asteroids that we know of currently on a collision course with Earth. NASA keeps a running list of the biggest threats, and nothing has ever caused alarm. You can see a list of some potentially hazardous asteroids (PHAs) that will pass close to Earth by 2200 here, and some with a risk of impact probability here.
NASA uses a scale to judge the threat of asteroids, called the Torino scale. This ranges from zero, or no chance of hitting us, to 10, which means a collision is certain. There are no known asteroids that measure above zero at the moment.
But if there was one on a collision course, there are a number of solutions to stopping it. There is the Armageddon approach, although this would involve using a nuclear bomb to change the trajectory of an asteroid, rather than blowing it up. Another involves using a large spacecraft as a gravitational tug, to pull the asteroid off our path. A few other methods involve similar techniques.
All, however, need us to find any PHAs far in advance. These methods rely on us spending years to slowly change the trajectory of an asteroid; there’s no solution if an asteroid is going to hit within a year or so. Thus, we need to get better at tracking them – which is what Asteroid Day is all about.
“Based on what’s known about the NEO population and Earth’s impact history, scientists predict that Earth will experience another large-scale impact someday in the future – they just don’t know exactly when,” Asteroid Day notes in its mission statement. “Our goal is to raise public awareness about asteroid science and plans for planetary defense.”
We’ve got a few interesting missions in the works to teach us about asteroids. NASA is currently flying a spacecraft called OSIRIS-REx to an asteroid, to return a sample to Earth. And the upcoming Large Synoptic Survey Telescope (LSST), will help us better track them when it comes online in 2019.
At times, the threat of an asteroid can seem a bit far-fetched. But you only have to look back to 2013, when an unseen asteroid exploded above Chelyabinsk in Russia and injured hundreds, to see the threat they can cause.
The dinosaurs didn’t have a space program (that we know of) to avoid their own extinction. But as we do, it probably makes sense to prepare for such an eventuality.