DARMSTADT, Germany—A robotic probe has landed on a comet, in an unprecedented moment for space exploration that could provide a trove of insights into what comets are made of and how they behave.

Rocket scientists at the European Space Agency’s mission control here erupted in cheers, as they received the first signal that the probe that was released from the Rosetta spacecraft had touched down more than 300 million miles away from earth on the forbidding landscape of a small comet known as 67P/Churyumov-Gerasimenko.

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