Farewell to Rosetta

The Rosetta spacecraft's mission has ended, following its impact onto the surface of Comet 67P/Churyumov-Gerasimenko. Now, mission scientists will begin processing Rosetta data to learn more about comets and their place in the history of the Solar System. We look at what the mission has taught us so far.

Relive Rosetta's final hour in this replay of ESA's livestream

On 29 September, the Rosetta spacecraft began its final ‘collision manoeuvre’ to descend onto comet 67P/Churyumov-Gerasimenko, eventually colliding with the comet at 11:19 UT the following morning.

Rosetta spent just over two years studing comet 67P and during that time has brought scientists - and the public - closer than ever before to the workings of comets and their place in the history of our Solar System.

“Rosetta has completely changed our picture of comets,” says Eberhard Grün, who works on the Rosetta mission from the Max Planck Institute for Nuclear Physics in Germany. “Previously, they were pictured as dirty ice balls – or, as some prefer, icy dust balls – but now we know them, or at least this one, to be geologically complex worlds where a myriad of processes are at work creating the incredible surface structure and activity of the comet.”

Data from the Rosetta mission has revealed that comets like 67P could be leftover pieces from the formation of the Solar System, as opposed to young fragments of rock and ice formed by collisions between larger bodies. Outbursts of gas from the comet’s surface have revealed molecular nitrogen and molecular oxygen, and since these gases would require very low temperatures in order to become trapped in the comet’s ice, this suggests they were trapped during its formation. This in turn tells ESA scientists that the comet was born in a cold region far from the Sun. Studies also showed a high ratio of deuterium to hydrogen in the comet, suggesting that comet 67P was formed some distance from the Sun in the young Solar System.

Comet 67P/Churyumov-Gerasimenko experiences a huge outburst as it approaches the Sun on 12 August 2015. Outbursts like this have enabled scientists to learn more about its internal workings and how it was formed.

The Rosetta mission has also attempted to shine light on whether water was brought to Earth via comets or asteroids. Studies of the water in comet 67P found it is of a different chemical makeup to water found on Earth and also water analysed on other comets, suggesting that Earth's oceans were not solely delivered by external bodies.

"The essence of the question has changed,” says Bonnie Burette, NASA’s project scientist on the Rosetta mission. “Now we have to ask: what fraction of the water on Earth came from comets, how much came from asteroids or other bodies, and how much was here to begin with?” Mission scientists will continue to study Rosetta data over the coming years in the hope of finding more answers to this question.

Perhaps the most documented of comet 67P’s features is its ‘rubber duck’ shape, consisting of a smaller lobe connected onto a larger one. The shape is important in terms of what is going on below the comet’s surface, as well as the seasons it experiences during its orbit of the Sun. “We now know that (its activity) is related to the shape of the comet, and the way the release of gas from the two lobes distributes internal stresses between the body and head,” says says Marco Fulle, a Rosetta scientist based at the National Institute of Astrophysics. “This allows the spin axis to remain well fixed in space, which in turn means regular seasons and repetitive behaviour along the comet’s orbit.”

Despite this regularity, the two lobes experience very different seasons from each other during the comet’s 6.5 year orbit around the Sun. The northern hemisphere has long summers of about 5.5 years, while the southern hemisphere experiences long, cold winters. This changes as the comet reaches perihelion - its closest point to the Sun - and the southern hemisphere is plunged into a hot summer. The seasonal changes cause dust to be transferred across the comet, enabling Rosetta to capture it for study. These dust grains could reveal much about the building blocks from which our Solar System was formed.

Data will continue to be analysed over the coming years, as ESA scientists work to piece together the story of Comet 67P/Churyumov-Gerasimenko.

“For me, the big answers will surely come by piecing all of this together and combining observations with new models – which can now be improved thanks to Rosetta’s results – to address how comets work, where they come from and how they fit into Solar System’s history,” says Rosetta project scientist Matt Taylor. “Now that the operational side of the mission is drawing to a close, we’ll have much more time to delve into this treasure trove of data.”

Comet 67P/Churyumov-Gerasimenko’s ‘rubber duck’ shape is apparent in this image captured by Rosetta’s NavCam on 27 March 2016.
Credit: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0

BBC Sky at Night Magazine will continue to follow the progress of the Rosetta mission as new discoveries are made in the aftermath of the spacecraft's final moments. Follow us on Twitter and Facebook, or access print subscriptions and digital issues here.

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