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Today’s the Day We Reach Pluto

UPDATE: After nine and a half years of flying towards , is now flying away from Pluto. And here is the best image yet taken of the icy dwarf planet. Amazing!


It’s taken the New Horizons spacecraft 3462 days (nine-and-a-half years) to fly the 3 billion miles to Pluto in the outer reaches of our solar system. Today at 1250 BST it will make its closest approach, zipping past Pluto at 30,000 miles per hour, gathering data as it does so.


Everything has been building towards this moment for the thousands of scientists and engineers anxiously waiting for images and information about the tiny ice world. But for now it’s all in the hands of the automatic systems aboard New Horizons. It has turned its antenna away from Earth so that it can focus its attention on Pluto and its moons (Pluto has five known moons, Charon, Styx, Kerberos, Nix, and Hydra). This means that we currently don’t have any way of communicating with or receiving data from New Horizons. It’s on its own until the pre-programmed sequence turns its antenna back towards Earth and begins transmitting back to us. We should begin to receive signals again around 0200 tomorrow (Wednesday) morning.


And what do we hope to see? It’s almost impossible to predict what new imformation this flyby will reveal, but one thing’s for certain: the images will get a whole lot better. The picture above was taken on Sunday from a distance of 2.5 million km. That’s 100 times further than today’s closest approach. The best resolution images we’ll take of Pluto today will allow us to resolve down to 100m per pixel, far better than anything we have seen so far. The above image has a resolution of several km per pixel for example.

So will we see anything at 1250 today? While we won’t start to receive the hi-res images until tomorrow, NASA has held back the final image of Pluto taken by New Horizons before its antenna swung away from us. This is a failsafe image, just in case we don’t hear from New Horizons again*. This image will be released today at the moment of flyby, so stay tuned.

Pluto: The Largest Dwarf Planet

When Pluto was discovered in 1930 it was named the ninth planet in our solar system, but then in 2005 astronomers discovered another object out beyond Pluto, which we called Eris. That name – after the Greek goddess of discord – is apt, as it threw the definition of a planet into chaos. Eris, which at the time was thought to be a little larger than Pluto, must surely be a planet too. But what happens when we discover more such objects out beyond Neptune?

This part of our solar system is known as the Kuiper Belt, and is a little like the asteroid belt only icier. There could well be hundreds of these so-called “Plutoids” or TNOs (Trans Neptunian Objects) out there. To avoid the problems of hundreds of new planets, the International Astronomy Union created a definition of a planet in 2006 that deliberately excludes Pluto and all the other Plutoids.

So Pluto went from being the smallest planet to the second largest dwarf planet (after Eris). But recent measurements made by New Horizons have allowed us to recalculate Pluto’s size and it turns out to be larger than Eris, by a whisker.

Eris is 2326km across (give or take a few km). Measuring Pluto is tricky because of its thin atmosphere, which makes the edges of the dwarf planet fuzzy. However New Horizons is close enough that it can make better measurements than we have had before, which put Pluto’s diameter at 2370km. Pluto is now the king of the dwarf planets!

* Flying through space isn’t risk-free. There are lots of tiny pieces of dust and rock floating out there. Due to its incredible speed even a small particle could wipe out New Horizons if it impacts. As we approach Pluto the number of these particles increases, but it’s still highly unlikely that we’ll experience a catastrophic impact. We’ll know for sure when New Horizons re-establishes contact at around 0200 on Wednesday 15 July

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