What is the closest orbit to Earth?

Excuse me, I was wondering if you could elaborate on the concept of the closest orbit to Earth. I understand that the Moon has an orbit around our planet, but is there another celestial body that orbits even closer? Could you possibly explain the factors that determine the closeness of an orbit and how it relates to Earth's gravitational pull? Additionally, are there any implications for our planet or its inhabitants if a celestial body were to enter an orbit even closer than the Moon's? Thank you for your time and expertise in this matter.