In the climax of a decade-long mission, the European Space Agency’s Rosetta spacecraft on Wednesday is scheduled to land a small probe on the surface of a comet – something that has never been accomplished.

The probe’s descent from the mother ship to the comet’s uneven, enigmatic surface will take scientists from the ESA and NASA’s Jet Propulsion Laboratory a nerve-racking seven hours. Plenty can go wrong.

And we get to watch their reactions live.

If the lander survives, it will provide a rich trove of information about a comet known as 67P/Churyumov-Gerasimenko. The comet contains materials that formed the solar system, frozen in time. By digging them out, scientists can learn more about the origins of Earth.

“It’s a look at the basic building blocks of our solar system, the ancient materials from which life emerged,” said Kathrin Altwegg of the University of Bern in Switzerland, one of Rosetta’s project leaders. “It’s like doing archaeology, but instead of going back 1,000 years, we can go back 4.6 billion.”

Since its launch in March 2004, Rosetta has flown by Earth three times and by Mars once. At one point in 2011, the spacecraft had to hibernate because it was so far from the sun – nearly 500 million miles – that the craft’s solar panels couldn’t draw enough energy to keep it running. But in January of this year, Rosetta woke up.

Rosetta first spotted the comet in March. Since then, it has taken a multitude of readings on everything from how the comet smells to what noises it makes. Most important, Rosetta selected the perfect landing site for its payload, a 220-pound carbon-fiber robotic probe called Philae. If it lands successfully, Philae will take readings until the comet gets too warm – probably around March. Rosetta will follow the comet’s orbit until December 2015, at the earliest.

The separation of Philae from Rosetta is scheduled for 4:03 a.m. Eastern timeon Wednesday. While the public won’t be able to watch the separation itself, there will be live online video from the ESA’s control room in Germany, and probably periodic images from the spacecraft and probe.

If all goes well, Philae will land on the comet at 11 a.m. Eastern time on Wednesday – give or take 15 minutes. (That, at least, is when staff at mission control will see that the landing has occurred. Because the comet is almost 300 million miles away from Earth, there’s a communication delay of nearly a half-hour.)

There are many potential problems: Philae and Rosetta could fail to make satellite communication with each other after the separation. That would mean the robot was functionally lost.

And even though the landing site, named Agilkia, was carefully selected, it might not make for an easy touchdown. “We’re looking at the pictures of this comet and interpreting them the way we would somewhere on Earth, because we’re just not tuned to understand what they mean for comet geology yet,” said Claudia Alexander, the project scientist who is overseeing NASA’s many contributions to the effort.

The comet’s surface could be much harder or softer than expected, causing Philae to bounce or sink on impact. If Philae ends up upside-down or sinks too deeply, it has no way to right itself.

But even if the landing is a mess, Rosetta has been a resounding success, said Altwegg, who is in charge of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, a tool for detecting the comet’s molecular composition.

“The lander would be the icing on the cake,” Altwegg said. “But we’ve been receiving data on the atmosphere of the comet since August.”

If the probe lands successfully, it will first “take a panoramic picture,” Alexander said. “And then it will dig into material from the comet and break it down to sniff out the molecules in it. It’s really like a crime scene, looking for forensic evidence.”

Denton Ebel, cosmochemist at the American Museum of Natural History in New York, said that even if the landing fails, Rosetta still will trail the comet for a year – and he thinks the data from that piece of the mission will be invaluable.

“It’s super cool,” Ebel said. “We’re going to learn things we didn’t even know we needed to learn.”