Sentence:  The resolution of the images obtained by this American probe [Messenger] will depend on its altitude [above Mercury] at any one time: about ten meters at <WSD>perigee</WSD> (200km altitude), but only one 1 km at apogee (15000km). 
Meaning of the word marked with <WSD> tags:  The point, in an orbit about any planet, that is closest to the planet: the periapsis of any satellite.