Pluto, at last!!

The last photo of Pluto for a bit, hopefully more soon.
New Horizons' should reestablish contact with earth
Tuesday night (7/14/2015) and begin sending 10 years
worth of data back to earth, a process that will take 16 months.
NASA image.

Congratulations to Alan Stern and the teams on New Horizon, the spacecraft that has spent a decade getting out to Pluto! To emphasize what a feat this is, here's a quote from a NASA press release:

"New Horizons' almost 10-year, three-billion-mile journey to closest approach at Pluto took about one minute less than predicted when the craft was launched in January 2006. The spacecraft threaded the needle through a 36 by 57 mile (60 by 90 kilometers) window in space--the equivalent of a commercial airliner arriving no more off target than the width of a tennis ball."

Pluto was discovered only 85 years ago by Clyde Tombaugh, an astronomer at the Lowell Observatory in Flagstaff. Tombaugh was doing a systematic search for a planet, dubbed "Planet X" at the time, beyond the orbit of Neptune. He would take photographs of the sky several nights apart and compare the images using a "blink comparator," an image that allowed rapid comparison of images. With this technique, astronomers can distinguish between stars, which do not move, and moving objects such as asteroids, comets and, in Tombaugh's case, a planet. It showed up very close to the place that Lowell had predicted.

Kuiper Belt (blue dots). Attribution:
WilyD at English Wikipedia
The yellow dot is the sun.
J,S,U,N are Jupiter, Saturn, Uranus, and Neptune.

Pluto resides in a region of the Solar System known as the Kuiper belt, shown in the image here.  It was believed, until the 1990's, that Pluto was uniquely large and the Kuiper belt objects were unknown. Hence, Pluto was called a planet. The Kuiper belt was discovered in the 1990's, causing some to call Pluto's status as a planet into question, and with the discovery of Eris in 2005, a body 27% more massive than pluto, Pluto's status was sealed. The International Astronomical Union had to define the term "planet" for the first time because there was the possibility of "too many" planets!! To the dismay of some (many?) Pluto was demoted to a "dwarf planet" category.

So, what are the basics known or believed to be known at this point? The size of Pluto had been uncertain, and one result already from New Horizons is a new diameter--2370 km, up from an earlier value of 2302 km. This diameter is only about 2/3 of the diameter of the Moon. It's acceleration of gravity is 0.067 g, escape velocity is 1.23 km/s. The surface temperature varies between 33-55 K, with a mean of 44 K, truly a frigid planet. It's atmosphere consists of nitrogen, methane, and carbon monoxide at a maximum summer pressure of 0.30 Pa. The surface is 98% nitrogen ice. The color varies from black to dark orange to white--being similar to that of Io (the satellite of Jupiter that looks like a pizza).

Is there the possibility that tectonic or "volcanic/geyser" activity will be discovered on Pluto? The interior is believed to have a dense rocky core of approximately 1700 km diameter, and if radioactive heating is still significant today, it's been speculated that there could be a subsurface ocean 100-180 km thick at the core-mantle boundary.  Here's a New Horizon's blurb that summarizes some of the facts and possibilities. We've been surprised before (Io, Triton, Enceladus) so here's hoping for some action!!

Go New Horizons Team, and thank you!!