how to keep low-res photos handy, while archiving high-res versions?

I have a laptop with a 256Gb SSD that always seems to be full. A lot of my disk space is used by photos (in Aperture and iPhoto).

What I really want is to have low-res versions of all my photos on the SSD with high-res versions backed up to an external hard drive or Time Capsule. If I'm visiting family and someone spontaneously decides that they want to see Christmas from 4 years ago, I want to at least have low-res versions to show, even if I need to go home to my external hard drive for high-res versions. And I want it to sync, such that if I decide while I'm on the road that some photo is crap and delete it, that it deletes the high-res version when I plug back into my external hard drive.

This seems so logical to me that I'm constantly surprised that this problem doesn't seem to be solved. Aperture "vaults" appear to be just a backup copy of the entire library; they don't let you delete the high-res versions from your main computer.

Am I missing something? How do you keep photos from filling up the limited space on an SSD?

Depending on how your Aperture library is set up, I think it will do this automatically.

If you use a referenced library (see the Aperture manual for more information on managed vs referenced), Aperture won't store the hi-res masters in its own library - it will leave the file management up to you. It does, however, keep a low-res preview of the image in the library. So you could store your images on an external HDD, but still have access to the low-resolution previews when the HDD isn't available.

I haven't tried this myself, so I'm not sure how small the previews are, but I'm guessing they'd be good enough for the purpose you're describing.