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SYFY WIRE Bad Astronomy

The Complicated Trajectory to Understand a Comet

By Phil Plait

In the summer of 2014 the Rosetta spacecraft approached and entered orbit around the comet 67P/Churyumov-Gerasimenko. No mission had ever done that before; humans had sent missions flying past comets, and even one to smash into a comet, but no long-term stay had ever been attempted.

Comets have very little gravity, so the term “orbit” is a bit loose here. Rosetta shifted around the comet, constantly changing its trajectory to accomplish the science needed to better understand the bizarre little wordlet. The European Space Agency released a short video outlining the spacecraft’s path as it moved around, and it’s worth watching:

I like how they highlighted important mission milestones, like when orbit was first achieved, and when the Philae lander was deployed (and when it was last contacted). Part of the mission objective is to sample the environment around the comet; as the ice on the comet is warmed by the Sun, it turns into gas, dislodging dust and small debris from the surface. Observing how this process works will help scientists understand what comets are made of and how they’re put together, and you can see how it zipped around the comet both close and far, exploring these different volumes of space around it.

One big day was Aug. 13, 2015, when the comet achieved perihelion, its closest approach to the Sun (at about 1:50 in the video). The amount of light and heat received was at a maximum, and after that started to dip down. Rosetta kept its distance, in part to get an overview of the comet, but also to keep it far from any bigger (or even eruptive) outgassing events.

In 2016 Rosetta dipped low over the comet, then moved very far away to put the comet between the spacecraft and the Sun, so that the space around it was backlit, the better to see the material ejected. The images it sent to us were dramatic, to say the least.

It also shows the final orbits of the mission, leading up to the biggest day of all: Sept. 30, 2016, when the spacecraft will set itself down on the surface, ending this grand adventure.

But not the grand adventure. Rosetta may have been the first spacecraft to orbit a comet, but it won’t be the last. We’ve learned so much! And part of being a pathfinder is learning how to do such a mission in the first place. It’s not easy, which is a big part of why I’m showing you this video; look how complicated the path was, and imagine how difficult planning it was. But that’s what we humans do, when we so desire: We take the hard way, accept the challenges and risks, and begin the journey.

May this one go on as long as humans look up and wonder.

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