Assuming by "space" the OP just means low Earth orbit, a nuclear explosion is orders of magnitude larger than what is necessary to see with the unaided eye. From LEO, you can see a large campfire! Or consider it from the other direction: you can see most satellites with the naked eye from the ground. They are much, much dimmer than a nuclear explosion as they are merely reflecting light from the sun. Then, certain satellites with flat, reflective surfaces will "flare", reflecting direct sunlight to a small area on earth. These are spectaclarly bright to the naked eye - much brighter than any star or planet.
http://www.satobs.org/iridium.html