What is a 'second moon' and when will one be in range of Earth? Will people be able to see it? Here's what to know about what's happening.

Starting next week, the moon, Earth’s closest celestial body, will be joined by a new neighbor: a second moon. From Sept. 29 until Nov. 25, astronomers calculate that 2024 PT5— which is what scientists think is an asteroid but have dubbed a “mini-moon”—will be looping around Earth.

PT5 has already been near Earth since July. During the event, the asteroid's geocentric energy will become negative and remain that way for 56.6 days. In Dunn's simulation, the orbit is shown as the red line,