Rosetta is about to drop a spacecraft on a comet: What could go wrong?

The countdown has begun. In less than 24 hours, the European Space Agency’s Rosetta mission will attempt to put a laboratory on a comet.

On Wednesday at 12:35 a.m. PST, three mechanical screws that have been holding the Philae lander to the Rosetta orbiter’s side for the past 10 years will activate, and gently push the lander away from the orbiter.

The lander will then free fall from the orbiter to the comet. Because the gravity of the comet is just 1/60,000th of the gravity of Earth, scientists say it will take an agonizing 7 hours for Philae to complete the journey.

If everything goes according to plan, the washing machine-sized Philae lander will use sensors on its feet to tell scientists what the comet’s surface feels like, a thermometer will take its temperature, drills on the lander will scoop up material and let scientists know what the comet is made of, and cameras will give humanity our best look yet at a cometary landscape.

But there is no guarantee that everything will go according to plan. Landing an orbiter on a comet may be high reward, but it is also high risk.

Some of that risk can be controlled. At a press conference last week, Andrea Accomazzo, flight director for the European Space Operations Center (ESOC) in Darmstadt, Germany, outlined what he called “three pillars of danger.”

The first has to do with navigation. In order for the Philae lander to hit the chosen landing site, and not miss the comet completely, the lander needs to leave the orbiter’s side at exactly the right place and time, going at the right velocity and at the right altitude.

Because it will take Philae seven hours to drop to the comet’s surface, even the smallest error will grow larger over time.

“If there is an error in the velocity by 1 centimeter per second, it will be propagated over the seven hours so that it winds up being an error of a couple hundred meters,” said Accomazzo. “This is the first pillar.”

It is also essential that the two spacecraft stay synchronized. Philae cannot communicate directly with Earth. It sends signals to Rosetta, which then sends a signal back to our planet.

But the danger that Accomazzo most fears is the one he can’t control -- the touchdown. The surface of comet 67P/Churyumov-Gerasimenko is rough and strewn with boulders. It is not an ideal place to land. There is no way to steer Philae once it leaves Rosetta’s side.

“This is the part that worries me the most because we have no control over it,” said Accomazzo. “All the other processes are our responsibility and we are prepared.”

Fortunately, all is not lost if Philae tumbles on its landing. Fred Jansen, Rosetta mission manager, explains that while the whole ESA team is excited about the possibility of successfully landing on a comet, over the course of the mission just 20% of the scientific data are expected to come from Philae, while 80% will come from Rosetta.

A successful landing would be amazing, but the feats of the Rosetta mission have already been amazing--successfully catching up with a comet after 10 years in space, taking the highest-resolution images yet of a comet from space, gobbling up its dust emissions and sniffing its stinky gas cloud.

“The science is already meeting or even exceeding our expectations,” he said.

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