Here is an up-to-date map of the inner solar system. It shows Mercury, Venus, Earth and Mars cyan or white squares, and their orbits are represented by the blue ellipses around the sun (the yellow dot at the center). The Earth is highlighted because of its special importance to us. Small green points mark the location of asteroids which do not approach close to the Earth right now. Yellow objects (with the exception of sun) are Earth-approaching asteroids which are called Amors after the first one discovered. Amors have orbits which come close to the Earth but they don’t cross the Earth’s orbit, yet.
Finally the red boxes mark the location of the Apollo and Aten asteroids. These cross the Earth’s orbit and are the most directly identifiable astronomical threat for the near future.
It is estimated that there are perhaps 100,000 to 1,000,000 undiscovered asteroids on similar Earth crossing orbits.
Why does it seem as if there are so many more asteroids passing us now than ever before? Simply because astronomers’ tools for finding asteroids have become more powerful, and because more astronomers are looking for asteroids now than ever before.
Is this map scary? Yes, and no. One thing to remember here is that the map is not to scale, with respect to the sizes of the dots representing the asteroids, and the space in between them. Space, even the near space just outside our little Earth-moon system, is almost inconceivably vast.
Also, today we know that fairly large asteroids strike our atmosphere several times each year. Recently released data from the Nuclear Test Ban Treaty Organization, which operates a network of sensors that monitors Earth continuously for the infrasound signature of nuclear detonations, shows that there have been 26 atom-bomb scale impacts to our atmosphere since the year 2000. Nearly all of those passed unnoticed because our atmosphere did its job … and kept us safe.
And, yet, asteroid impacts to Earth’s surface do occur, also. Astronomers are confident they know all the potentially Earth-destroying-sized asteroids, and they know none are headed our way for the foreseeable future. It’s the city-damaging-sized asteroids they’re still not sure about.