However, even though using solar power in space is no more or less green than using a nuclear reactor as most probes do, the R&D invested in making solar panels light and efficient will create more applications for them on earth. This is the immediate green upside of space exploration.

You should note too that these are not solar sails. Solar sails are extremely large, very thin membranes attached to spacecraft which use the pressure of solar radiation to propel the craft away from the sun. They have never been deployed except in small scale experiments and scifi novels. These are just solar panels which collect sunlight to turn it into electricity. All of the craft’s propulsion comes from chemical rockets which put it on course and pressurized gas to make adjustments later on.