Pluto in Retrograde

Image Credit: Adam Brown

Image Credit: Adam Brown

I created the animation above showing the dwarf planet Pluto moving against the background of stars. Can you spot the tiny moving dot? This animation is similar to the way Pluto was discovered in the first place. Taking pictures of the sky on subsequent nights, then comparing the photos, astronomers at the Lowell observatory noticed the motion of the tiny dot. Further investigation confirmed the motion and Pluto was discovered.

You'll notice Pluto moving from right to left, then making a U-turn and going the other direction. This apparent change in motion is due to Earth passing Pluto in their orbits around the sun. Because we're orbiting faster than Pluto, we're leaving it in our rear-view mirror. This backwards motion is called retrograde.

Pluto orbits the sun at about 5.9 billion km, which is 39 times farther from the sun than Earth. It's one of several similar objects in the region known as the Kuiper Belt. We don't know much about these distant, tiny worlds, but we're about to learn more. After nearly a decade of traveling the vast solar system, the New Horizons spacecraft will fly by Pluto with it's closest approach on July 14, 2015. We already know that Pluto has several moons, one of which is almost as big as Pluto itself. This places the barycenter, or the gravitational balance point between the two, outside of Pluto. So, Pluto and its largest moon, Charon, both orbit a point between them, making a wobble as you can see in this animation captured by the New Horizons spacecraft in April.

This image is in the public domain.

This image is in the public domain.

New Horizons was about 110 million km from Pluto when these images were taken, so the resolution is not very good. Still, we can start to see some interesting features on Pluto. Those dark and light regions will be interesting to investigate as New Horizons gets closer. NASA scientist believe that Pluto has at least 5 moons, maybe more.

Since New Horizons launched, NASA scientists have been using the Hubble Space Telescope to search for other possible targets in the Kuiper Belt for New Horizons to visit after the Pluto fly-by. They have identified three possible targets. The most likely has been temporarily named PT1, which orbits the sun at 43 times the distance Earth does.

This is an exciting time for outer solar system exploration. It's likely that there are many objects like Pluto in the dark, cold, Kuiper Belt, and learning about them will help us better understand how our solar system formed and how bodies in space interact with each other. Stay tuned for more news as closest approach nears. For more information about the New Horizons mission, visit their website.

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Works on this page credited to Adam Brown are licensed under a Creative Commons Attribution 4.0 International License.