Friday, February 6, 2015

Pluto: not a planet, still exciting

I was ten years old when Voyager 2 reached Neptune. It was a really big deal for me at the time. It was so cool to me that this was a simple, clear example of a small piece of human knowledge and understanding being expanded. Science can explore very complex and arcane ideas, but it can also seem as simple as "we didn't really know what this thing looked like; we threw a camera at it from a few billion miles away, and now we do." That there were new, easy-to-understand discoveries in things like planetary astronomy and dinosaur paleontology while I was a kid, hooked me completely.

The best photo of Neptune, and its moon Triton, as of 1988... (JPL/NASA)
...and what Voyager 2 showed us in August, 1989. (NASA)
And so, I am very excited that this year, on my birthday, the New Horizons probe will reach Pluto. Unlike some in my generation and the preceding one, I don't care that Pluto's not a planet. It just isn't. Calling Pluto a planet means calling a bunch of other little things that aren't planets, planets (basically, we can have eight planets, or 16-20, but 9 doesn't work). But that doesn't make the New Horizons mission any less exciting. Pluto is still major enough to have been called a planet for decades. It's still the tenth most massive thing orbiting the sun. And I am really curious what the thing looks like.

NASA's keeping the hype going with these images that New Horizons sent us the other day, showing Pluto and its largest moon, Charon. When these were taken, NH was further from Pluto (~120M mi) than the Earth is from the sun (93M mi). And it's going to close that gap in 6 1/2 months.

Charon orbiting Pluto, last week.
(NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

About the best photos we had of Pluto prior to New Horizons.
Pluto, of course, is not the only game in town. Nor is it the next biggest thing in the solar system after the planets and the seven largest moons. That honor goes to Eris, another dwarf planet, discovered in 2005 and the impetus (not Iapetus) for the Pluto-reclassification arglebargle. It's bigger than Pluto, but, like Pluto, has a big elliptical orbit that crosses Neptune's (in fact, it's much larger and more askew).

Caused less violence but more grade-schooler sadness than the apple its namesake tossed at the Greek goddesses.
Pluto also isn't the only dwarf planet in the press this week. While New Horizons zips along, the Dawn probe approaches Ceres, the largest object in the asteroid belt. It's the only dwarf planet in the belt, and — get this — it was discovered in 1801 and considered a planet for about 40 years, along with asteroid neighbors Vesta, Juno, and Pallas. Anyway, NASA released some new photos of Ceres that Dawn took on Wednesday, and while Ceres maybe doesn't capture my imagination like the more recent planetary reject, it is pretty cool. Between these missions and the Mars rovers, it's a pretty exciting time for planetary astronomy.

Ceres. Pluto's mother-in-law, if you're into mythology. Gotta feel bad for Juno that the asteroid that achieved hydrostatic equilibrium and therefore dwarf planet status was the one named after her little sister. (NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI)
What will Dawn find on Ceres?