"The propellant required to inject the spacecraft from Earth orbit to the Mars transit trajectory is then 51.7 tonnes for the ISPP case and 130.3 tonnes for the non-ISPP option. In terms of total payload mass launched from Earth, the ISPP option would result in a 60-percent lighter vehicle -- just 86.8 tonnes, compared to 218.5 tonnes for a vehicle carrying its own fuel for the return trip. With current launch costs as high as $4.5 million per tonne ($2,000 per pound), the use of ISPP can save nearly $600 million."