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If all goes to plan, this week we should be getting our first close-up look at a brand new world. On Wednesday 6 August, the European Space Agency鈥檚 will rendezvous with the comet it has been chasing for the past 10 years. Mapping and landing on will be a challenge unlike anything space explorers have faced before.

鈥淲hen you go to Mars nowadays, you know everything,鈥� says Rosetta flight director Andrea Accomazzo. Not so for Rosetta鈥檚 target. 鈥淲e don鈥檛 know the mass, we don鈥檛 know the gravity field, we don鈥檛 know how to fly around this object.鈥�

Previous spacecraft have flown by comets at high speed. Rosetta, on receiving ESA鈥檚 commands, sent today, will slow to just 1 metre per second relative to its destination.

Because the comet鈥檚 gravity is so weak, Rosetta will initially manoeuvre around it in a series of controlled triangles. These will start at a distance of 100 kilometres and move closer in over the next few weeks as ESA learns more about the comet鈥檚 structure. Eventually it will enter a proper orbit determined only by gravity, getting as close as 10 kilometres (see diagram below).

Pictures released last month have already revealed the comet is made of two pieces fused together in a shape likened to a rubber duck. That means its gravitational pull tugs in strange and unpredictable directions, so creating a detailed 3D model of the comet is a top navigational priority. 鈥淲e are acquiring hourly images so we can see the body, reconstruct its shape and characterise features of its surface,鈥� says Accomazzo.

Missions to asteroids, like NASA鈥檚 NEAR Shoemaker, have orbited similarly lumpy rocks, but a comet offers new challenges. Gases frozen beneath Churyumov-Gerasimenko鈥檚 surface could escape explosively as the comet nears the sun and warms up. As they spurt out they will push against Rosetta鈥檚 solar panels, possibly as strongly as the comet鈥檚 gravity, but in the opposite direction.

Rosetta is in for a bumpy ride, and there will be no time for ESA researchers to catch their breath. Philae, a smaller probe that has ridden along with Rosetta, must touch down on Churyumov-Gerasimenko鈥檚 surface by 11 November: any delays will make landing much trickier as the comet becomes increasingly active on its approach to the sun.

Little time

鈥淲e have to do this at warp speed. We have very little time between the discovery of a new world and landing,鈥� says , lead researcher on Rosetta鈥檚 main camera OSIRIS, which will map the comet鈥檚 surface.

Cartographers know the difficulties involved in wrapping a flat map around a globe, but they can at least lay down lines of latitude and longitude with ease when mapping our planet. 鈥淚t is more difficult to mosaic a highly irregular body like our rubber ducky,鈥� says Sierks. 鈥淭he whole coordinate system on this body is tricky.鈥�

ESA will announce a shortlist of landing sites in September. Data from all of Rosetta鈥檚 scientific instruments and measurements of how the comet affects the probe鈥檚 trajectory will influence the eventual choice.

Dark and dusty

According to measurements released last week, the comet鈥檚 average surface temperature is -70聽掳C, around 20-30聽掳C warmer than predicted. That makes it likely that the surface is . Understanding how the surface temperature changes as the comet nears the sun will be a factor in where to land, as certain sites may become unstable.

One spot that might already be off limits is the 鈥渘eck鈥� of the duck. Despite being of great scientific interest, it may prove too difficult to touch down on, as gravity may not be acting at a right angle to the surface. 鈥淚t would be like landing on a slope,鈥� says Accomazzo.

For now, we can look forward to the first high-resolution images of Churyumov-Gerasimenko, which should be around 100 times better than those already released and will take an hour for the probe to transmit early on Wednesday morning. 鈥淚t鈥檚 the end of a 10-year trip in the solar system and the start of the exploration of a new world,鈥� says Accomazzo.

This story was updated to include the diagram of Rosetta鈥檚 orbits.