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The image at your right is currently the best and clearest one we have of the former planet Pluto, now correctly classified as a dwarf planet.* If it looks unimpressive to you, you’re not alone. Based on a series of observations from the Hubble Space Telescope this fuzzy blur is the best we can do with the technology currently in place.
One of the great things about modern astronomy, however, is that our technology can move at incredible velocity. Literally. In the case of the New Horizons probe, our technology is moving at about 31,500 mph. On July 15, 2015 — one year from yesterday — that will put it within just 17,000 miles of this distant little ice world. For the first time, we’ll see exactly what Pluto looks like, and in HD. You’ll likely never see this picture again outside of a history book.
This will be the first time in my lifetime — all 33 years of it — that we’ve seen something this famous and this unknown up so close. By the time I was born, we had sent probes to Mercury, Venus, Mars, Jupiter, and Saturn, and Voyager II swept by Uranus and Neptune before I was old enough to appreciate them; all of them were — to a fair extent — known quantities. This isn’t to say that our discoveries and explorations within our own solar system since then haven’t been amazing: we’ve explored miles of the martian surface, discovered the lakes and rivers of methane on Titan, geysers on Enceladus, orbited major asteroids, and impacted comets head on.
Amazing, yes, but none of these captured the public’s imagination for more than a few moments. New Horizons should be different: everyone knows what Pluto is, and — as the International Astronomical Union discovered when they demoted Pluto from its status as a planet — people have strong emotions about it. For at least another generation, Pluto will still have a special status, if only as the only ex-planet.
Think of it this way: almost no one outside of astronomy nerd circles can name any of the other bodies that share Pluto’s designation. Orcus, discovered in 2004, has an orbit that’s almost a perfect inversion of Pluto’s, though it’s significantly smaller. Eris, discovered in 2005, is roughly the same size as Pluto, though also further out. Even maps of the solar system that correctly list Pluto as a dwarf planet still include it, but none of these other bodies. For whatever combination of reasons, Pluto continues to hit the public imagination in a way none of these other worlds has (they certainly don’t make it past spell-check unnoticed).
So what do we currently know about Pluto? A few interesting facts:
- It’s very small; smaller than at least seven moons in our solar system, including our own.
- It’s largest moon, Charon, is so relatively large and close to Pluto, that it probably makes sense to count the two of them together as a binary planet.
- In just the last few years, four additional, very small moons have been discovered around Pluto.
- It has a twin, Orcus. The two both have unusually large moons and have orbits of (very nearly) the same size and at (very nearly) the same inclination to the rest of the solar system. Orcus, however, is much smaller.
- Almost every important aspect of its discovery and exploration has been by Americans: Clyde Tombaugh discovered it; James Christy discovered Charon; Mike Brown, Chad Trujillo, and David Rabinowitz started the path that led to its demotion by finding similar objects; and New Horizons is a NASA program.
What are we hoping to find out?
- Is its atmosphere permanent, or does it only stick around while Pluto is relatively close to the Sun? The former had been suspected, but discoveries in just the last year suggest that the latter may be true.
- Did Pluto and Charon ever have liquid oceans beneath their surfaces?
- How heavily has Pluto been impacted by meteors?
- How similar is it to Neptune’s moon, Triton, which has long been suspected to be a Pluto-like object that wandered into the inner solar system?
There will likely be surprises — there almost always are when we do this sort of thing — and that’s the wonder and the joy of it. I can’t wait to see what we find.
* What defines a “planet” had been a source of contention for some time, and remains so. The difficulty the IAU found itself in in 2006 was that there was no set of objective criteria that could be found that included all nine of the “traditional” planets without including dozens more besides Pluto, which nobody wanted to do (that darn sentimentality again). Their solution was to adopt a definition that excluded Pluto while creating a new category for dwarf planets — which are not planets! — that would include Pluto and another dozen or so objects.
Under the new definition, a planet is a body that 1) orbits directly around a star, 2) is massive enough to form itself into a ball, and 3) has swept its orbit clear of most other objects. A dwarf planet meets the former two criteria, but not the last.Published in