Goal of the planned mission MIRI is "to explore the Kupier Belt".
Explanation:
MIRI, Mid-infrared Instrument, was designed to play an important role in NASA's "James Webb Space Telescope"(JWST).This is a joint project involving the Space Agency Europe and Canada. This telescope is designed to bring the images of stars and galaxies in infrared light. This grasps a wavelength of 5 to 28 microns. It also has a sensitive dectector allowing to see the red shifted light, stars that are newly formed and the comet that are blur.
Also, to explore the Kuiper belt that is located in the outer solar system that extends from the Neptune to sun. It is believed to be similar to asteroid belt but 20 times broad and 200 times massive than it.
So in one hour half the amount remains (that's what half life means). In two hours 1/4 (or half of half) would remain and in three hours 1/8 would remain. So the answer is 1/8
