After ten years in space, and a journey of almost four billion kilometers, the European Space Agency's Rosetta space craft finally arrived at its target, the comet 67-P Churyumov-Gerasimenko, last week. I don't know why they didn't go with an easier to pronounce comet, but that's the one they went with.

At the same time, astronomers realized just how hardcore Jupiter's moon, Io, really is. 

I'm Hank Green. Welcome to SciShow Space News.

For the first time in history a man-made spacecraft is orbiting a comet. And as its name Rosetta suggests, it's a chance to decode a few ancient mysteries.

When the solar system formed 4.5 billion years ago much of the material conglomerated into planets. But in the outskirts of the disk of cosmic dust beyond the orbit of Neptune, balls of ice formed.

Every once in a while some gravitational oddity will throw one of these ice balls into the inner solar system, which we then get to observe as the energy from the sun vaporizes the water and pushes it away in a long tail of a comet.

These cosmic leftovers might offer clues about what the solar system looked like at its birth, and help answer questions like: where did all of the Earth's wonderful water come from? 

Of course their distant origins and massively elliptical orbits have them spending a lot of their time way out beyond where we can observe them. So it's pretty cost prohibitive to have engines big enough to sling a big heavy probe out to rendezvous with a comet. 

So, in order to reach comet 67-P, Rosetta used gravitational assists from three earth fly-bys and one mars fly-by to reach the speed necessary to swing out to just inside Jupiter's orbit.
It used these gravitational assists to get up to nearly 60,000 kilometers per hour.

It took ten years and a deeply inspiring amount of balletic mathematics. But it worked.

Now Rosetta will accompany 67-P as it approaches its perihelion. Its closest point to the sun, which lies between the orbit's of Mars and Earth. 

Before it settles into a stable orbit 30 kilometers from the comet's surface, Rosetta will be pushing itself through a bizarre series of orbital operations.The comet's mass of course is so small that every turn of this maneuver only requires tiny amounts of fuel.

Once there it will begin to survey the comet and prepare to drop its PHILAE lander onto the surface of the comet itself in November. 

PHILAE will take the first ever photos from the surface of a comet, directly sample the comet, and, with Rosetta use a barrage of instruments to investigate 67-P's composition which might help us determine whether Earth's water has cometary origins.

Also last week astronomers from UC Berkeley published some findings that suggest Jupiter's moon Io might be even more hardcore than we thought.

About 3,500 kilometers in diameter, Io is only about the size of our moon, but it packs a surprising punch. It's the only body in our solar system other then Earth known to have active volcanoes that erupt molten rock, not just volitiles like water or methane.

These volcanoes may be responsible for a cloud of ionized gas floating around Jupiter roughly in Io's orbit. 

But we've only seen Io erupt in these massive volcanoes 13 times since 1979. Until now, when the Berkeley team focused two near infrared cameras in a near infrared spectrometer on Io for just two weeks last August.

They saw three of them! Which suggests these massive volcanoes might be happening all the time.

The images can't quantify the thermal emissions of these volcanoes, but their size and brightness suggests that they were extreme. One blast produced a lava flow of about 200 square kilometers while another had a bright thermal source described by NASA astronomers as a curtain of fire with an area of 52 square kilometers.

This is all because of a gravitational tug-of-war. Jupiter and its other moons pull Io in various directions throughout it's orbit causing a strong 100 meter tide, like our oceans, but in rock. This creates an enormous amount of friction and pressure inside the little moon keeping its core hot and molten. 

And the same thing, might be happening, to our moon. This month, astronomers from the Planetary Science Institute in China took a close look at the moon's tidal deformations.

Just like Jupiter, Earth exerts a pull on the moon that changes because the moon's orbit is slightly elliptical. This change in gravitational pull actually slightly deforms the moon though not nearly as much as Io.

Well it seemed like the moon was contorting a lot for something made of solid rock and metal. So the team made a new model of the moon that included a more malleable interior, one that's not quite molten, but the rock is hot enough to be kind of soft that better explains its tidal deformations.

It seems the Earth may keep the moon pretty hot too, though luckily, not super volcanically hot.

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