A Phil Space article for BBC online ('Eternal planes' to watch over us) contains this picture of Helios, a NASA prototype that crashed in 2003.
Ignoring for a minute the uses to which such aircraft could be put, the Helios is a splendid contraption, reminiscent of something from the earliest days of powered flight.
Not surprising it crashed, somehow.
Could solar powered aeroplanes ever be feasible for human transport, and if so how long could it take? Could the current rate of advances in PV, battery and other technologies mean this is not altogether implausible?
Coming back down to earth, George Monbiot (we spent a useful hour or two yesterday clearing brambles yesterday to prepare new ground for unusual apple varieties) says he's read an analysis which shows that the Sustainable Aviation Group's stated goal of a 50% increase in fuel efficiency per passenger kilometre by 2020 - see this post from 25 July - is completely impossible.
Following a meeting with Philip Stewart - see this post from 22 July - I calculated that, thanks to getting on for 100 hours flying last year, my ecological footprint would be about 6.9 planets (if everyone behaved like that).