[erre]

Rosetta wouldn't have some mirrors, or even reflective surfaces, would it? It could position itself to reflect sunlight onto Philae.

I mean, I'm sure Rosetta has neither (in useful conditions), but there's a thought for next time :/

[sylvinus]

This was asked in a French Q&A session with the ESA and they dismissed it as "science-fiction" :)

http://www.lemonde.fr/sciences/article/2014/11/13/philae-peu...

[erre]

Well, I meant it more as "hopeful pondering", but point taken :)

[Ono-Sendai]

That would be possible in theory, a few problems though:

You would need a flat surface to reflect light towards Philae, so the only appropriate surface would be the solar panels. The solar panels are not mirrors however, they would probably only reflect about 5%-10% of the sunlight towards Philae. This is even assuming the solar panels are sufficiently flat.

Also the location of Philae is unknown (AFAIK), so there is the problem of where to aim the reflected light.

Finally the relative direction from Rosetta would presumably change as the comet rotates, which means the solar panels or Rosetta as a whole would need to be continuously aimed at Philae, using propellant and/or other energy.

[thyrsus]

Apollo retroreflectors.

https://www.xkcd.com/1441/