Sentence:  The radial velocity varies inversely as the particle size - a 1000 - | m diameter particle near the orbit of <WSD>Mars</WSD> would reach the sun in about 60 million years . 
Meaning of the word marked with <WSD> tags:  a small reddish planet that is the 4th from the sun and is periodically visible to the naked eye; minerals rich in iron cover its surface and are responsible for its characteristic color