John and Mark and Rocks

Explanation: New Horizons co-investigators Mark Showalter and John Spencer pose against a Pluto-themed backdrop with some rocks at the Johns Hopkins Applied Physics Laboratory in Laurel, MD. Spencer and Showalter were part of an effort to search for moons or rings that could have damaged the New Horizons spacecraft as it passed through the Pluto system. While these rocks are rather large, at 14 km/s even a thumb-nail-sized debris fragment could have destroyed the spacecraft, ending the mission.

Pluto

Image credit: NASA/JHUAPL/SwRI/Henry Throop