Earth could capture an asteroid, but only under certain conditions. The asteroid would have to be a certain size, traveling at just the right speed, and grazing by Earth at just the right angle. For example, a bus-sized asteroid grazing Earth’s atmosphere might be captured by our planet’s gravity. Afterward, moon’s gravity might pull it into a stable orbit above Earth – to give Earth a second moon.
Planetary scientists believe that asteroid capture was common billions of years ago. The planets are thought to have formed by a process of “accretion” – where small chunks of debris came together to form larger chunks. So there were lots more chunks – what we now call “asteroids” – moving through the solar system back then.
Also, dense gas and dust surrounded the forming planets. When an asteroid passed through this material, drag slowed it down – making capture by a planet easier. Mars’ two small moons were probably captured in this way, as were various satellites of Jupiter, Saturn, Uranus, and Neptune. Now asteroid capture is less common – but it is possible if the right asteroid comes along.