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Why can’t we get a good picture of Pluto?

We have crisp pictures of all eight planets, but for some reason, we just can’t get a good snapshot of Pluto. New Horizons, a NASA spacecraft on approach to Pluto, may change that and has now taken its first picture.


Anybody who has an interest in the solar system will be able to tell you that we’ve taken some beautiful and crisp photos of all the main attractions of the solar system, far away nebulae and even galaxies billions of lightyears away. For some reasons though, there’s one place we just can’t seem to get in focus: The dwarf planet, formerly 9th planet, Pluto is far from the sun, and our probes and telescopes have only, at best, managed to produce blurry, disappointing image.

NASA’s New Horizons spacecraft is going to change that. It’s currently en route to Pluto, and will be arriving in July 2015, where it will make a 12,500 km close encounter and provide us with some amazing new images that will finally lift the veil of blurriness and give us spectacular views of the planetoid. NASA launched the $700 million mission in 2006, and while its still 880 million km away, it has gotten close enough to snap its very first picture of Pluto with it’s largest and closest moon, Charon.


New Horizons’ first picture of Pluto and Charon

While not more than yet another blur at the moment, the photo is still worthy of note: “The image itself might not look very impressive to the untrained eye, but compared to the discovery images of Charon from Earth, these ‘discovery’ images from New Horizons look great!” said Hal Weaver, scientist on the project, “We’re very excited to see Pluto and Charon as separate objects for the first time from New Horizons.”

When on its closest approach, New Horizons will provide almost a million pixels of Charon, and even more of Pluto. It’ll be the first time we get a really good look at it, and thanks to New Horizons’ approach angle, the photos will reveal unique new information about the surface of the dwarf planet and its moon.

Why has it taken this long to get a good look at Pluto though? If we can photograph galaxies several lightyears away, why is it so hard to focus on something in our own solar system? That’s a question Emily Lakdawalla of The Planetary Society gets asked a lot. She explains that it all comes down to relative sizes and distances. The reason Hubble can take sharp pictures of galaxies, but not of Pluto, is because while Galaxies are much further away, they’re also much larger.


This is the best Hubble can do. Not exactly a glamor shot for Pluto

Looking at the relative size to distance between Pluto and galaxy NGC5584, we’ll find that for the galaxy, it’s a ratio of 50,000 light years across / 72 million light years away, which is 0.00069. Meanwhile, for Pluto, it’s 2400km across / 4675 million km, which is 0,00000051. The relative size to distance of the galaxy is thus actually more than a thousand times larger than for Pluto. Then we have to add in the factor of Hubble’s angular resolution. A single pixel on a Hubble image spans 0.04 arcseconds, or 0.00005 degrees of the night sky. All this combined means that Hubble can capture a 3600 pixel wide image of the galaxy. Meanwhile, Pluto will fill up less than 2 pixels.

All of this might still seem hard to grasp, but the truth is that distances in space are really immense. Pluto is pretty small, but even if we put the solar system’s largest planet, Jupiter, in Pluto’s orbit, it would only take up 150 pixels on a Hubble image. Hopefully now, when we finally have a space probe closing in, we’ll get something a little better than 3 pixels.

Via Space.com

David F.
A grad student in experimental physics, David is fascinated by science, space and technology. When not buried in lecture books, he enjoys movies, gaming and mountainbiking

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