Astronomers using the Hubble Space Telescope have discovered another moon around Pluto, bringing the total number of moons to five. The announcement was made on July 11, 2012 and is causing space experts to review their perceptions on Pluto and other Kuiper Belt Objects.
NASA reports that the discovery was made by a team of astronomers led by Mark Showalter of the SETI Institute and including Harold Weaver of the Applied Physics Laboratory of Johns Hopkins University, and S. Alan Stern, Andrew Steffi, and Marc Buie of the Southwest Research Institute.
According to NASA, the group used the Hubble Space Telescope to make close observations of the Pluto system.
The new moon had been unofficially designated P5 and officially designated S/2012 (134340) 1.
According to the abstract of IAUC 9253, the official International Astronomical Union announcement of the moon’s discovery, the moon’s diameter is between 10 and 25 km, its orbital radius is about 42,000 km (+/- 2000 km) and its orbital period is about 20 days. It orbits between Nix and Charon, making it the second moon in order of increasing distance from Pluto.
The team says that P5 also orbits in roughly the same plane as Pluto’s other moons, Charon, Nix, Hydra, and the as-yet unnamed P4.
NASA says that P5 was directly imaged by the Hubble Space Telescope on June 26, 27, 29, and on July 7, and 9.
Teams using the Hubble Space Telescope have previously discovered three other moons of Pluto, Nix, Hydra, and P4. Nix and Hydra were discovered in 2005, while P4 was discovered in July 2011, almost exactly one year ago.
Charon, Pluto’s largest moon by far, was discovered by the U.S. Naval Observatory in 1978.
Scientists currently theorize that the moons were formed from the debris from a collision between Pluto and another Kuiper Belt Object in the distant past.
Hubble has been observing the Pluto system closely for some time in support of the New Horizons mission, scheduled to fly close to Pluto in July of 2015. Since New Horizons is moving extremely fast, about 23 km/s, a collision with even a tiny particle could destroy the spacecraft.
Unfortunately, according to team member Harold Weaver, “The discovery of so many small moons indirectly tells us that there must be lots of small particles lurking unseen in the Pluto system.”
This may make New Horizons’ journey through the system dangerous. But NASA and the New Horizons team will continue to use Hubble observations to plot the safest course possible for the spacecraft.
According to an article published by New Horizons Principle Investigator and Pluto team member S. Alan Stern in Astronomy in June 2006, he and his colleagues began to worry about what the then-upcoming New Horizons mission might encounter at Pluto. He had previously been interested in finding small moons around Pluto further out than Charon, and had begun searching in 1988, reviewing telescope data from 1987, 1988, and then 1990, and then Hubble data from 1991 to 1994. All those reviews failed to uncover any other satellites.
But with the New Horizons mission moving forward, the search intensified. Instead of reviewing existing Hubble data, the team asked for new observing time in 2003 and 2004, but were rejected. They finally received their chance in 2005. Close review of the data revealed two satellites, now named Nix and Hydra.
P4 was discovered much later, in 2011, but the impetus for its discovery was the same as for Nix and Hydra, and is still the same for P5.
The New Horizons spacecraft, launched in 2006, is still waiting for refinements to its trajectory for its upcoming encounter with Pluto. Its closest approach is set for July 14, 2015. After that, it will fly on deeper in the Kuiper Belt. Mission managers hope that it will pass near another KBO, allowing additional observations, but no suitable objects have been found. Eventually, New Horizons will exit the Kuiper Belt and speed out of the Solar System altogether.