Yeah, a few asteroids at first, then more and more... you could have some asteroids come one by one too, these could be faster to test your reflexes.
Progressively, why not. I'll have a think about it! However, you didn't mention the fun part of this stage. Do you know what I'm talking about?
You could start by just duplicating the player ship to use as enemies (if I understood correctly, you don't have sprites for enemies?) After, you can make actual enemy pirate ships. (It's easy to do actually... how did you implement the player ship? did you make it yourself?)
I've been talked to do so, so the "escape capsule" has now a blaster that was missing in the very first implementation in Java/OpenGL (see webplayer html page for controls). But again, if you have some 3D/2D models/sprites that you think could make sens in this "asteroid field and space pirats hideout" context, please advise