It sounds simpler than it is.

The European Space Agency (ESA) in Paris has selected the most likely landing spot for Philae - a lander traveling on the robotic space probe Rosetta - to land on comet 67P/Churyumov-Gerasimenko, and on November 12 it happens.

The site is called Agilkia and is located in a valley on the "head" of the comet, which is shaped roughly like a duck.

The lander lands on a moving comet. Just like that.

It could go horribly wrong.

But the closer we get to the date, the more likely it seems.

Since August 6, Rosetta has been following the comet at close proximity.

They feel they have got so close that scientists and space engineers have even coined a nickname for the comet - "Chury."

Rosetta's camera system OSIRIS has sent ever more detailed pictures of the comet back to Earth.

Some of the most recent have shown rocks and debris - less than one meter in diameter.

A difficult decision

In selecting the landing spots, researchers and engineers had to look at many different aspects and weigh up the pros and cons of each location carefully - a complex task that involved constant rethinking.

The landing area should be as flat as possible to enable Philae to land securely.

It is should also be a place exposed to sunlight for at least six hours of a comet's day, or of one comet revolution around the sun.

This is to ensure that Philae's batteries can always be recharged - and that for at least six months after the landing.

Too much sunlight, however, is not good. The landing robot could overheat.

Then, scientists have to make sure Philae is a position to maintain a constant radio link to the spacecraft Rosetta in order to be able to exchange data and commands.

Besides these technical criteria for selecting a landing zone, scientists want Philae to be able to find out how fast gases evaporate from the surface of the comet and how many dust particles they carry with them.

They are looking for organic compounds, which are more likely to be found in active regions. Philae, after all, is supposed to be searching for the building blocks of life.

The third dimension

By August 25, researchers and engineers had narrowed down their search to a total of five possible landing spots.

Three were located on the "head" of the duck-shaped comet.

The remaining two were located on Chury's main body.

In the time since, teams from all over Europe have looked deeper and deeper into these spots to evaluate which are the best ones.

Researchers from the DLR Institute of Planetary Research in Berlin Adlershof created a three-dimensional computer model of the comet, using a method called "stereo-photogrammetry."

It is based on a complex computer program which the Berlin researchers developed in the 1990s and have continued to improve.

They have used it to create models of celestial bodies throughout the solar system. They have also modeled third dimension for the surfaces of Mars, Mercury and the Earth moon, as well as moons of Jupiter and Saturn, and models of asteroids.

The unpredictable comet

"This duck-shaped comet has such an irregular form that the coordinates we use simply do not work to localize one spot on the surface," says DLR comet researcher Ekkehard Kührt.

This is the problem: "The North Pole is located at Chury's neck, between the head and its body, inside a valley that is about one kilometer deep. So we have given up thinking in the usual [terms of] latitude and longitude."

Instead, the researchers are using a system of three Cartesian coordinate axes to create a 3D model of Chury's surface.

Surfaces become landscapes

To create the 3D model, the researchers are combining photos taken by the OSIRIS camera from at least three different positions. They use the time of the shot, the perspective of the camera, and the position of the spacecraft and comet to combine the pictures.

Then they feed the pictures into a computer which calculates a 3D model of the terrain based on the different angles.

Using these computer models, the scientists say they have been able to identify the best landing spots.

The researchers say they can see whether inclines, rocks or debris would hamper the landing.

They can examine the light situation in a given area, and calculate how long it will take for the batteries to recharge and what impact temperatures will have on the lander.

And they can even zoom in to take a closer look of the landscape.

Shape-from-shading

The Berliners worked closely with colleagues in France, who used another method called shape-from-shading.

This method creates spatial dimensions by looking at shades at different exposures of the comet to sunlight. Because they know the direction of the sun, the researchers can create a 3D model.

But this only works if the scientists make the right assumptions about how the surface reflects the light.

Both methods compliment each other.

Each of the models exposes different qualities of the comet's surface. When the results of both methods match, the researchers say they can be confident that they have understood the surface of "Chury" well.

As Rosetta neared the comet, it took even more pictures, allowing the researchers to fine-tune their models.

They made a final decision for the Philae spot mid-October - Philae will land at what was initially identified as site J. This site is located above a depression on the "head" of the duck.

A jury selected the name "Agilkia" to replace the generic letter name. Agilkia refers to an island on the Nile River where the famous Temple of Isis was moved from the island of Philae when the latter was flooded during dam building.

With luck, researchers will have gotten their calculations right.