What would people, standing on Earth, see, hear and smell, if a short period comet, with a two mile diameter nucleus, came by at 100,000 miles above us?
Hear and smell: Nothing. The comet and its coma are well outside the atmosphere, no sound or scent can travel through space.
The coma of the comet could be 30000km across, bigger than the moon, and the comet is half the distance to the moon, so the coma could appear about 20 times larger than the full moon. However, it would be very dim. Although its total brightness might be quite high, the surface brightness would be still very low. It is likely that only the central part of the coma would be visible, as a hazy blur, with no clearly defined edge. Some jets might be visible towards the nucleus. In a dark sky it would look quite dramatic.
The nucleus would be only about 4 arcseconds across, however it is close enough to be visible as a star-like body in the centre of the fuzzy nucleus. Telescopes would reveal the shape of the body, but telescopes on Earth would show few details. The comet would slowly track across the sky, clearly moving relative to the stars if you observe it for any length of time. The exact speed would depend on its orbit, but it wouldn't be moving like a meteor, the encounter would take a few hours.
The tail, which might be visible in dark sky locations, would be pointing away from the sun, rather than behind the direction of travel.