Within the past two weeks, astronomers used the Hubble Space Telescope to identify a fifth moon circling Pluto.
It's hard to believe, but the arrival of NASA's New Horizons spacecraft at Pluto is just three years away. The logistics of the high-speed flyby, already challenging, just got more complicated: Pluto turns out to have a fifth moon.
Although for now its official designation is S/2012 (134340) 1 — "134340" being the minor-planet number assigned to Pluto — the new find has been nicknamed "P5". (Easier to remember, don't you agree?) Its existence was announced last night by the IAU's Central Bureau for Astronomical Telegrams.
Long-time Pluto-watcher Mark Showalter (SETI Institute) led the nine-member discovery team. They took advantage of Pluto's opposition on June 29th, when this little world was a mere 31¼ astronomical units (2.9 billion miles) away, to image the system 14 times from June 26th to July 9th with the Hubble Space Telescope and its Wide Field Camera 3.
"Here's an interesting stat," Showalter notes. "P5 is 1 arcsecond from Pluto and fainter by a factor of 100,000. I continue to be amazed at what Hubble can do with fine-tuned observations."
Yet even with HST's powerful optics, P5 shows up as barely a blip. Its magnitude is just 27, which puts its diameter somewhere between 6 and 15 miles (10 and 25 km), depending on the reflectivity of its surface. The orbit is still uncertain, though the tiny moonlet appears to be circling in the same plane as Pluto's other satellites and roughly 26,000 miles (42,000 km) out. That puts P5 nearer to Pluto than Nix, Hydra, and the not-yet-named P4 (discovered last year) though not nearly as close as Charon.
It's no coincidence that all these moons orbit in the same plane as Pluto's equator. Most likely they formed from debris tossed out when a renegade object struck Pluto long ago. Collisions in this distant region of the solar system are typically so slow that most of the resulting fragments couldn't have reached escape velocity, which is a bit under 1 mile per second for Pluto. So most of it would have stuck around.
But that doesn't automatically lead to satellite formation. Ballistically speaking, any stuff that lingered should just have just fallen back onto Pluto itself. However, tidal interactions among the most massive chunks could have allowed enough of them to remain in orbit to form Charon and the other moons.
What's driving the search for Pluto's extended family is the possibility that such small objects or even rings might pose a danger to New Horizons as it zooms through the system at 32,000 miles per hour (14.3 km per second) on July 14, 2015. Plans now call for the spacecraft to pass well inside Charon's, at a point about 6,000 miles from Pluto.
"We have been searching for hazards because this is the last chance to observe Pluto before the team has to settle on the backup flyby trajectory," explains Showalter. "Searching for new, interesting targets is just a fringe benefit."
Here's the NASA/STScI press release describing the discovery of P5.