Potentially hazardous asteroid
A fairly large asteroid whose orbit comes close enough to Earth's that it needs careful watching.
Astronomers label an asteroid potentially hazardous when two things are true. First, its orbit passes within about 7.5 million kilometres of Earth's orbit, which is roughly 20 times the distance to the Moon. Second, it is bright enough to be at least about 140 metres across. Such an object could cause serious regional damage if it ever hit. The label does not mean an impact is expected; it means the asteroid is worth tracking.
Thousands of these objects are known, and none of them is predicted to hit Earth in the next century. Careful tracking matters because small pushes from planets can change an orbit over time, and early warning is what gives planetary defence missions a chance to work.
The best-known example is Apophis, about 340 metres across. When it was discovered in 2004, early calculations suggested a small chance of an impact in 2029. Better measurements ruled that out. On 13 April 2029 it will pass about 32,000 kilometres above Earth's surface, closer than some satellites, and should be visible to the naked eye from dark places. You cannot tell a hazardous asteroid by looking; it is a label based on orbit and size.
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Level: Explorer Ages 10 and up