Circular orbit speed

At any given distance there is exactly one speed that produces a perfectly circular orbit. Go slower and gravity wins on part of the loop, pulling you into an ellipse that dips closer. Go faster and you swing out farther. That special value is the circular orbital speed:

v = √(μ / r)

where μ = G·M is the central body's gravitational parameter and r is your distance from its centre.

Notice what the formula says: the closer you orbit, the faster you must go. A satellite skimming low over the Earth races around in about 90 minutes; the Moon, far away, takes 27 days. Being close means gravity is stronger, so you need more sideways speed to balance it.