Barring space junk and little green men America will celebrate its 221st Independence Day this Friday, with a tiny visitor, slightly larger than a beach ball, will literally bounce down onto the surface of Mars and begin a new era of low cost exploration of the Red Planet. The NASA Mars Pathfinder spacecraft, a three foot tall landing craft nestled inside a disc-shaped entry shield, will conclude a seven month cruise of the inner Solar System and streak across a darkened Mars atmosphere.