Look into the smart previews feature. There's no equivalent in Aperture, but these are highly compressed versions of your originals and provide significant offline editing capability.
Thanks John, I will look into smart preview sizes now. I was hoping to avoid them as I'm sure the file size will have to go up and therefore I probably won't be able to store the whole library locally. I will certainly look into it though, and hopefully that will solve the problem. Perhaps I will have to use smart previews for part of the library, and regular previews for the rest.
It is strange though, don't you think? The preview file is obviously there as you can view it and zoom into it, but there is no export option.
The size goes up, I seem to remember they're 1 mb per photo so no bigger than regular previews, but the benefits are big.
When I travel I now take my complete catalogue with smart previews for tens of thousands of images (based on a smart collection finding starred images and those from the region/subject I'm revisiting). I don't bother with regular previews as they will be generated from the smart previews. So when I am offline I can copy metadata from previous visits, remind myself of locations, and also meet some needs to play with or output existing pictures. I could even make rough adjustments to images where the originals are back home. At the end I just copy this catalogue back to my main computer.