Two Mercury-bound spacecraft left their mother ship behind on Thursday and headed down the home stretch of their nearly decade-long voyage to the solar system’s smallest and closest-to-the-sun planet.

The linked European and Japanese spacecraft separated as planned from their cruiser, but controllers had to wait two anxious hours to receive confirmation via radio signals. The crucial operation was broadcast from the European Space Agency’s control center in Germany, more than 124 million miles (200 million kilometers) away.

“It’s a brilliant moment,” said lead project scientist Geraint Jones.

If all goes well, the conjoined spacecraft known as BepiColombo will enter orbit around Mercury in November and split from each other in December to map and study the entire planet from different altitudes. As of Thursday, the spacecraft still had 2 million miles (3.3 million kilometers) to go before reaching its destination.

Launched in 2018, the BepiColombo mission is named for the Italian mathematician who took part in NASA’s Mariner 10 mission to Mercury in the 1970s. Only one other spacecraft, NASA’s Messenger, has explored the puzzling, scorching planet that is just a little bigger than our moon.

“How tiny Mercury is and still it holds some of the biggest mysteries of our solar system,” mission manager Santa Martinez said earlier this week.

Added Jones: “There’s a huge amount still left to be learned.”