Europe's Rosetta spacecraft reached a comet on Wednesday - something that has never been attempted before.
The tiny probe has met up with one of the strangest objects in the solar system. Previous missions have only managed to pass comets at high speed.
After a decade travelling through space, Rosetta aims to orbit the comet for at least a year with the aim of determining whether comets brought the water and carbon vital for life on Earth. It will try to deploy a lander on to its icy surface.
The latest in a series of manoeuvres brought Rosetta to within 100km of Comet 67P/Churyumov-Gerasimenko. One of the scientists leading this European Space Agency (Esa) venture described it as "the sexiest, most fantastic mission ever", the BBC reports.
Throughout human history, comets lighting up the night sky have triggered fascination and fear - but their speed and distance have made them difficult to investigate.
One theory is that they delivered water, carbon and other essential building-blocks for life to the early Earth. Previous missions have had to be fly-bys - brief encounters crossing a comet's path to gather data or collect samples of dust.
By contrast, Rosetta is designed to fly around comet 67P in a form of orbit for more than a year, its 20 instruments providing unprecedented information about the comet's structure and composition.
If all goes according to plan by November this year, mission managers will pick a spot for what will be an audacious attempt to send a lander, known as Philae, to touch down.
For the moment, all eyes were on Wednesday's landmark manoeuvre to bring Rosetta into a controlled flight in a triangular pattern around the comet.
With 67P hurtling along at 55,000km/h, the spacecraft's speed was adjusted so that in relative terms it was flying beside the comet at a slow walking pace of 1m/sec (3.6km/h).