Hm. Interesting question...a feather weighing half a gramm would, at 2.5 km/s relative to you, carry about 1,5 kJ of kinetic energy. Wikipedia tells me that's equivalent to 2 Magnum revolver rounds. If it hit you with the pointy end, you would probably end up with a quill-shaped hole in your body. No idea what would happen to the rest of the feather, though. If it hit with its side, there's always the possibility of blunt trauma even if no penetration occurs. (assuming we ignore spacesuits and similar contraptions, instead exposing our brave volunteer test dummy to the elements) Edit: regarding solar sails. 10 g/m^2 is a good approximation of their current area density. Let's say your body has a cross-section of 1m^2, slightly more in a spacesuit. If the impact tears, oh, let's say 2m^2 of sail area out, 50 Ns get imparted on you. For a 100kg astronaut, the resulting velocity change will be 0.5m/s^2. I frankly have no idea what to make of this. Chances are better if the material is tensile because instant velocity changes are bad for humans, and you would want to minimise your body area to, well- rip through the material like a bullet. If the sail doesn't tear, you get accelerated rather quickly and probably get your internal organs reduced to a mush that makes the recovery team vomit in the airlock. So there's that.