In July, the Hubble Space Telescope spotted the fifth moon of Pluto, an icy ball 10 to 25 km across that was just a pinprick of light in the image. Much of the press coverage focused on whether that discovery should make Pluto a full-scale planet. But I was far more interested in Pluto, its moons, and the amazing optical feat of finding something so small and so far away.
My interest in optics grew from a fascination with astronomy. I'm old enough to remember the 1978 discovery of Pluto's largest moon Charon. The discovery images show a small bump on the fuzzy ball of Pluto, recorded on a photographic plate by a ground telescope. Comparison of a series of images showed that the bump moved as the unresolved moon orbited Pluto. In the days before adaptive optics, seeing even that much seemed amazing.
Hubble resolved Pluto and Charon soon after its launch in 1990. It was a badly needed success for Hubble in its troubled early years, but scattered light in the background of the photo clearly shows the spherical aberration of the telescope's primary mirror. Pluto and Charon are both blurry and diffuse, but the photo clearly shows them as separate worlds, with Pluto clearly the larger and Charon roughly half its size. Once NASA added corrective optics to fix the spherical aberration, the Faint Object Camera produced much sharper photos in 1994.
Further upgrades have made Hubble even better. In 2005, it spotted two roughly 100 km moons, later named Nix and Hydra. Last year, astronomer Mark Showalter of the SETI Institute (Mountain View, CA) began a series of Hubble observations to check for other little moons which might scatter dust into the path of the New Horizons spacecraft when it visits Pluto in July 2015. Earlier this month, Showalter downloaded a new batch of Hubble data, and in an hour was on the phone reporting the discovery. A few days later, he told me "I'm still struck by just what an amazing instrument Hubble is. This little object, [called] P5, is fainter than Pluto by a factor of 100,000 and separated by one arc second."
It's amazing and wonderful. And so far Hubble's images show New Horizons is on a good path to avoid any dangerous dust, so we can see close-ups of Pluto three years from now.
