Sentence:  De Jager ( 1955 ) has calculated the times required for these particles to reach the atmosphere under the influence of the Poynting-Robertson effect , which in this case causes the <WSD>orbits</WSD> to become more and more eccentric without changing the semi major axis . 
Meaning of the word marked with <WSD> tags:  the (usually elliptical) path described by one celestial body in its revolution about another