Endgame - Rosetta's planned crash-landing

The preparations for the controlled crash at the European Space Agency (ESA) have been going on for some time. In the last weeks Rosetta has been circling it's comet 67p/Churyumov-Gerasimenko in a very uneven elliptical orbit. It's flight path brought the space craft very close to the comet - briefly once during every orbital round. The closest it got to the comet was about one kilometer.

With its unusual flight path, it was finally possible for Rosetta to discover the precise location of its lost landing robot Philae in early September. Its cameras spottet the small device during a 2,7 kilometer high fly by. The discovery confirmed what the scientists believed all along: That Philea had tumbled during it's landing on November 12th 2014 and slid into a crevice, where it could not get enough sunlight to recharge it's batteries and maintain proper radio contact to Rosetta. Since then, the contact between both space vehicles was only rare and sporadic.

Up and down

Already last Saturday, the Rosetta started preparing for it's final maneuver: It left the elliptical orbit and entered a flight pass, which will take it much farther away from the comet - to a height of 19 kilometers. Tonight, on Thursday September 29th  at 22:50 local time, satellite control of ESA in Darmstadt will sharply change Rosetta's path and send it onto a direct collision course with comet Chury. That's at 20:50 GMT.

Since the comet very little gravitation, it will take quite some time for Rosetta to fall down onto Chury. For the 19 kilometers the space craft will need almost 14 hours. The mission control expects the landing at 12:40. Then it's 10:40 GMT.

Into the center of gas and dust

The planned landing spot of Rosetta is on the smaller of Chury's two body parts. It is, so to say, on the head of the comet, which many have compared to a duck. Scientists have named the Region Ma'at. It is a place with numerous pits, which are about 100 meters in diameter and up to 60 meters deep. That's where lots of gasses from the inner part of the comet evaporate and where dust storms emerge from within the comet - particles that form the comet's tail, when it gets close to the sun.

Also the edges of these pits are of interest to the researchers: That's where rock formations called "goosebumps" are located. The experts believe that those hold testimony to the early phases of the creation of the solar system.

Rosetta will focus explicitly on those rock structures during it's crash. It is supposed to fall into a 130 meter wide pit called Deir el-Medina. It received the name after an egyptian town, which has similar rock formations in the neighborhood.

Disclosing numerous secrets

With the 11 instruments on Rosetta and 10 on Philae, the mission managed to produce a wealth of data and send it back to earth. In the process it completely changed the picture, researchers had of comets.

Before, they were considered to be merely dusty iceballs. Now the astrophysicists know, that comets are highly complex bodies. Comet Chury, for example, constists of six times more rock and durst than water. 99 percent of the dust is made up of particles smaller than a millimeter. The comet's nucleus consists mainly of minerals: silicate and sulfide.

Gas and rock was there, before the solar system

The composition of the comet confirmed, what the scientists had suspected earlier: The comets were formed out of the pre solar nebula - a mass of gas and particles, which was hot in the center and cold at the edges and which later became our solar system.

The minerals on Chury most likely came from the hot center of the nebula, while the icy parts - mostly hydrogen, oxygen, nitrogen and some noble gasses were probably added later in the extremely cold outer regions. In comets that situation was then preserved during the formation of the solar system, while the chemical composition of planets changed over time. Now, one thing could be clarified: Comets are definitely not fragements of collided planets or other celestial objects.

Water on earth not just from one source

One question, Rosetta could not answer is: "How did water get to planet earth?" But now the scientists know to ask that question in a different way: "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?."

And there are other things that surprised the scientists. One of them is the dense and fluffy nature of the dust storms and the role of seasons on the comet. A comet's year lasts 6,5 erth years. The heaviest evaporations and dust storms take place during the perihelion - the closest fly by at the sun. During that period, Rosetta was able to observe the ejection and return to the comet of rocks as large as beach balls.

Work for generations of Astronomers

Even after more than two years of intesive data collection, the researchers are now just at the beginning of their evaluation and analysis.

The wealth of data will be good to keep generations of doctoral students busy. And all the data has travelled incredible distances. Rosetta sent all of them by wireless connection from a tiny dish antenna, not bigger than what many households use to receive their TV signals. In contrast the receiving antennas on earth had to be much bigger.

When Rosetta reched it's target in November 2014, Chury was about 500 Million Kilometers from earth. Now, at the end of the mission, the distance has increased to more than 700 million. Then Chury will blow away for several years - somewhere into the cold and very far reaches of our solar system.