NASA has released this graphic showing the orbits of all known Potentially Hazardous Asteroids (PHAs), that is, asteroids that are considered hazardous because they are fairly large (at least 460 feet or 140 meters in size), and because they follow orbits that pass close to Earth’s orbit (within 4.7 million miles or 7.5 million kilometers).
Astronomers have categorized more 1,400 PHAs as of early 2013.
Being classified as a PHA does not, of course, mean that an asteroid will impact the Earth. NASA is quick to point out:
None of these PHAs is a worrisome threat over the next hundred years.
Astronomers with NASA and other organizations do track these asteroids, continually refining what we know about their orbits so that more precise predictions can be made about their future close approaches and impact probabilities.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.