Philae’s status is uncertain after a mechanical failure following its 4-billion mile journey, officials say. The craft’s historic landing was bumpy, and the European Space Agency (ESA) doesn’t “fully understand” its condition.

The robotic craft may have floated away from the comet after landing due to a failure with a  harpoon system meant to anchor it to Comet 67P. The ESA says that it will have to analyze incoming data, and might not know the exact status of the craft until morning.

"Maybe we didn't just land once, we landed twice," Philae Lander project manager Dr. Stephan Ulamec said, according to a CNN report. "Did we land in a soft sandbox or is there something else happening? We don't fully understand what happened."

Ulamec told the BBC that the agency thought that Philar was stable according its last radio contact. The ESA would now know “until tomorrow morning” the exact status of the craft, which took 10 years to reach its destination.

The comet landing is “a big step for human civilization,” ESA Director General Jean-Jacques Dordain said when it was confirmed, echoing Neil Armstrong’s first words when stepping onto the surface of the moon. Dordain called it a “great, great day, not only for ESA, but... I think for the world,” according to the BBC.

Philae had sunk about four centimeters into the comet's surface, the ESA said, meaning the comet likely had a soft surface. The agency said that harpoons meant to hold it onto the space-faring body never fired.

Thrusters meant to hold the robotic craft down also failed to ignite. The ESA said Philae transmitted radio data that meant it had drifted off of the comet’s surface, and began to turn for nearly two hours. That eventually stopped, leading to what the agency called a “second landing.”

Comet 67P/Churyumov-Gerasimenko is more than 4 billion years old, according to reports.