New Horizons visits Pluto!

Today’s blog post is from Dr. Michelle Collins, a Hubble Fellow working at Yale.

After 9 years, 3 billion miles, a Jupiter fly by, and some of the most complex route calculations ever implemented, New Horizons reached its destination a couple of weeks ago on July 14th. This NASA probe went whizzing by our distant, dwarf planet neighbour Pluto at a dizzying speed of 31,000 mph, and has already provided us with a wealth of spectacular images, data and science. It will continue to spew out incredible discoveries about Pluto over the coming 16 months or so, as the flyby data trickles back to us.

To say that this space probe has revolutionized our view of this failed planet is a giant understatement. Pluto has long been an elusive, poorly understood system, hovering on the periphery of our solar system. It was discovered back in 1930 by Clyde Tombaugh, an American working at the Lowell Observatory in Flagstaff Arizona. Due to some miscalculations of the mass of Neptune, it was initially believed that Pluto was a massive planet, at least as big as the Earth, and possibly up to 4 times the size of our home planet. So naturally, it was classified as a planet. However, as the decades wore on, the mass of Pluto was revised downwards, finally lurching to a halt at a mass of only ~0.2% the mass of the Earth in 1978, much lighter than originally thought. With this extreme weight loss, and the discovery of similar size – and even more massive – dwarf planets in the solar system (particularly Eris, discovered in 2005), Pluto’s status as a planet was starting to raise some eyebrows. And so, in 2006, when the International Astronomical Union met to decide what the lower bound on a planet should be defined as, Pluto didn’t make the cut, and was relegated to a dwarf planet.

But, aside from it’s low mass, and controversial status as the only de-throned planet in the Solar System, what else did we know about Pluto, pre-New Horizons? Well, not very much, really. Given it’s huge distance (it’s orbit takes it anywhere between 2.7-4.8 billions miles from the Earth during a single Pluto year), it was hard for us to study Pluto in detail, or take a decent image of it, even with the Hubble Space Telescope. We knew it was an icy world, probably with a rocky core, and maybe underground oceans. It is mostly composed of Nitrogen, with some methane and carbon monoxide. It has an extended, tenuous atmosphere and 5 moons – Charon, Nix, Hydra, Kerberos and Styx. It is locked in a binary orbit with the largest of these, Charon. But the other, smaller moons appeared to us a little more than points of light in Hubble images. If we wanted to learn more about their composition, and that of Pluto itself, we’d need to get A LOT closer to Pluto. And so, New Horizons was constructed and launched on a mammoth journey on 19th January 2006 to our favorite minor planet to get a better look. It was the fastest spacecraft ever launched from Earth, and even managed to image Jupiter and its moons as a bonus science project on its way out to Pluto.

Much of New Horizons journey was spent in hibernation (roughly 7 years), and it was finally awoken on December 6th 2014. From then on, it began imaging Pluto with its onboard cameras, LORRI (a high resolution reflection imager) and Ralph (a multi-filter, lower resolution camera and spectrograph). The combination of these two instruments provided us with incredibly detailed, color images of the surfaces of Pluto and Charon, that got clearer and clearer the closer they got to Pluto. In the weeks before the flyby, we could see that Pluto is a red world, with complex geology. A huge, heart shaped ice plain could be seen on its surface (informally named Tombaugh Reggio after the man who discovered Pluto), and evenly spaced dark spots located on the opposite side of Pluto, which are the size of Missouri, surprised astronomers. Huge craters could also be seen, and regions that seemed surprisingly crater-free too. We also learned that Pluto is a little bigger than we thought, with a radius of 1473 miles, making it larger (though still less massive) than Eris. The sheer variety of surface features, not only on Pluto, but on Charon also, increased the anticipation of the New Horizons team as their target drew nearer, as it was clear that the high resolution flyby would provide them with a treasure trove of answers to the questions already forming.

Tensions were probably pretty high on the day of the flyby itself. After traveling 3 billion miles over 9 years, New Horizons needed to hit a window in space that was only 60×90 miles in size within 100 seconds of its predicted arrival time, otherwise it would miss Pluto. But the orbital calculations were bang on, and New Horizons was able to complete its full range of observations of Pluto and Charon, as well as taking detailed images of Pluto’s 4 other moons. Over the course of a few hours, New Horizons made high resolution maps of segments of both Pluto and Charon, with a maximum resolution of 60 meters per pixel. With that level of detail, you’d be able to count the ponds in central park! In addition to these maps, New Horizons also used several instruments – Alice, REX, PEPSSI and SWAP – to study the atmosphere of Pluto.

So, what else do we know about Pluto now? TONS! For example, the high resolution mapping of Pluto has shown us ice flows on the surface, and evidence for recent geological activity, such as cryovolcanism, which is completely unexpected for such a low mass object that isn’t orbiting a more massive planet. It also has huge mountains ranges, that tower up to 11,000 ft above the surrounding plains. These are most likely composed of water ice.

We also know more about Pluto’s atmosphere. For one thing, the solar wind appears to be stripping it away from Pluto, resulting in a cometary tail-like feature. It also has a hazy quality, where gaseous methane molecules are irradiated by UV light, causing them to condense into complex hydrocarbon molecules known as tholins, which are responsible for the reddish color of Pluto. Its atmosphere also seems to have a lower pressure than previously measured, and could imply that half of it is freezing out and condensing back onto the surface as Pluto segues into its colder season.

We also have high resolution maps of Charon, Pluto’s binary companion. It too, has a geologically young surface, which is totally unexpected for such a small moon. It has a complex set of cliffs, troughs and canyons whose sizes eclipse the Grand Canyon here on Earth. These are thought to be signs of fractured crust on the moon, caused by internal processes. It also has an extended, diffuse dark spot at its pole, informally named ‘Mordor’.

Speaking of moons, we’ve also received the most detailed images of Nix and Hydra from New Horizons. Nix is jelly bean-shaped, approximately 22×26 miles in size, and seems to have a large red spot on one of its faces which may be a crater. Hydra has an irregular shape, that has been compared to the state of Michigan and is about 34 miles in length. It too shows signs of cratering.

And this is only the beginning. There’s much more to come over the next year, and we’re highly anticipating the first ever images of the other 2 Pluto moons, Styx and Kerberos, which should be downloaded in October. There’s more to learn about the surfaces of both Pluto and Charon, with detailed spectroscopy coming in from the Ralph instrument, and more to come on the atmosphere too. So stay tuned to NASA for updates. New Horizons and Pluto have plenty more surprises in store for us, as we learn just how complex and awesome dwarf planets can be.