Sentence:  If particles that originally occupy a box which is 20RE long in the earth-<WSD>antiearth</WSD> direction and 10Rf wide perpendicular to the neutral sheet leak out of a slit of 4Rf wide with a speed of 400 km/s, it takes 800s before all the particles are lost. 
Meaning of the word marked with <WSD> tags:  Facing or moving away from earth.