Space Engineers 3D printed model. (WIP) HELP PLEASE

I'm attempting to 3D print this ship that I made in Space Engineers. I've got the .obj file and I've ran into a problem......

Does anybody know how to reduce the poly count significantly enough to make this model run in SketchUp and keep the object solid, so that it is still printable, without sacrificing quality....

I'd prefer it if the poly count goes from 100,000+ to less than 1000.... Or in other words, 5 Mega Bytes to less than 1 Mega Byte.

The softwares needs to be free! I can't afford nothin'!

Other than that, I'll be all good for printing!

Thanks
yshouldicar3