SetInterval Experiment

I need to have something write to the page while the rest of it runs in the background... Click in the canvas below to 'place' a rock. Not really, but the canvas will display the initial operating area for your mining outfit. This will be a 5.2 m. km square box (+/- 15 m. km?). This represents the same orbital plane as Earth and should have over a hundred interesting 'targets'. Currently the page is updating my 'rocks' every 500 ms using setInterval and changing a property of the DOM in my page. This leaves my canvas window alone, which is kind... kind of writing a novel here! Anyways, what I am trying to do is have the parts I don't need right away chugging along in the background while you do more interesting things.