What is perigee?

Perigee and apogee are a pair: the near point and the far point of the same oval. A perfectly circular orbit has neither, which is why the space station — whose orbit is very nearly circular at about 420 km — is usually quoted as a single altitude instead.

Perigee matters because it decides how long an orbit lasts. Even hundreds of kilometres up there is enough gas to cause drag, and the density climbs steeply as you descend, so a low perigee dips the spacecraft into noticeably thicker air on every single lap.

The effect is self-reinforcing, which is what makes it dangerous. Drag at perigee removes energy, which lowers apogee, which makes the whole orbit smaller and rounder and pushes the spacecraft into thicker air more of the time. Left alone the process accelerates until re-entry.

That is why the space station needs regular reboosts. It circles the Earth roughly every 93 minutes, about 15.5 times a day, so it meets that thin drag fifteen times daily and would eventually spiral down without a periodic push from a visiting spacecraft.

The same physics is used deliberately. Firing an engine at perigee is the most efficient place to raise the far side of an orbit, and deliberately lowering perigee into the atmosphere is how a spacecraft is brought home or a satellite disposed of at the end of its life.