After ten years in interplanetary space, the New Horizons probe is on final approach for its fly-by of Pluto, the outermost planet in our solar system. And yes, after some debate, it has been re-reclassified as a true planet, to the delight of astronomy geeks everywhere. What’s even better is how Pluto looks. It strongly resembles something you’d see on a 1950’s pulp sci-fi magazine cover.
Pluto turns out to be 1,473 miles (2,370 kilometers) in diameter, somewhat larger than many prior estimates. Images acquired with the Long Range Reconnaissance Imager (LORRI) were used to make this determination. This makes Pluto larger than all other known solar system objects beyond the orbit of Neptune.
“The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest,” said mission scientist Bill McKinnon, Washington University, St. Louis.
The size estimate means Pluto is a bit less dense than we thought, and that means the ice content in its interior is a bit higher. Pluto does have an atmosphere, as it turns out, but this atmosphere is a little more shallow than we thought. It was the difference in atmospheric depth that may have created the illusion that Pluto was smaller in diameter than it is.
Measuring Pluto’s size has been a decades-long challenge due to complicating factors from its atmosphere. Its largest moon Charon lacks a substantial atmosphere, and its diameter was easier to determine using ground-based telescopes. New Horizons observations of Charon confirm previous estimates of 751 miles (1208 km) kilometers) across. LORRI, the Long Range Reconnaissance Imager aboard the New Horizons spacecraft, has also zoomed in on two of Pluto’s smaller moons, Nix and Hydra. Though they’re about the diameter of the city of Denver, Colorado, they’re expected to be irregular and not spherical, since both will be well under the 500 mile diameter necessary for gravitation to pull either body into a spherical shape. Nix is about 20 miles across, and Hydra is about 30 miles across, so there’s a strong likelihood that each will look like deep frozen potatoes. There are two more moons, Kerberos and Styx that are even smaller.
The closest approach to Pluto will happen in about seventeen hours, and then New Horizons will slingshot past it into deep space. Why can’t it drop into orbit? The answer has to do with physics; to put New Horizons in orbit around Pluto would require thrust about equivalent to a Saturn 5 booster, and there was just no way to push that much fuel out there along with it to accomplish something like that. A flyby it is, and we’ll have to be satisfied with that until we can send something out there that uses an ion drive and a much slower approach.
There will still be enough time to get a pretty good mapping pass of Pluto, and this time the geographical features will be named for familiar place names and characters from science fiction! The International Astronomical Union was founded in 1919, and one of the first things they did was establish a committee to handle the naming of astronomical bodies and features on them. The problem they’re having now is that there are just too darned many objects to name, so they had to fall back on a boring formulaic method for naming things.
The New Horizons team have coordinated a crowd sourced contest for naming features on Pluto. They came up with an extensive list of names on Pluto’s theme – exploration and the underworld – for the public to vote on. (And yes, Cthulhu is on the ballot).
They’re also accepting nominations for names not on their list. Voting closes on April 24th and the resulting shortlist will be sent to the IAU for the final decision.
These naming campaigns help make the Earth-bound feel included in space exploration.
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