Sentence:  De Jager ( 1955 ) has calculated the <WSD>times</WSD> required for these particles to reach the atmosphere under the influence of the Poynting-Robertson effect , which in this case causes the orbits to become more and more eccentric without changing the semi major axis . 
Meaning of the word marked with <WSD> tags:  a period of time considered as a resource under your control and sufficient to accomplish something