In the climax of a decade-long mission, the European Space Agency's Rosetta spacecraft is expected to drop a lander on the surface of a comet Wednesday morning. But the journey is far from over: The feat of landing a probe on a comet's uneven, enigmatic surface — which has never before been accomplished — will take scientists from the ESA and NASA's Jet Propulsion Laboratory a grueling seven hours.

And we get to watch it all unfold live.

Since its launch in March of 2004, Rosetta has flown by Earth three times, as well as making one Mars flyby. At one point in 2011, the spacecraft even had to hibernate. Flying nearly 500 million miles away from the sun (close to Jupiter's orbit) the craft's solar panels couldn't leech enough energy to keep it running. But in January of this year, Rosetta woke up — and quickly approached its target.

Rosetta first spotted the comet known as 67P/Churyumov-Gerasimenko back in March. Since then, it's taken a multitude of readings of the comet, telling scientists everything from how it smells to what noises it makes. Most importantly, Rosetta had to select the perfect site for its payload, a 220-pound carbon-fiber robotic lander called Philae. Once stationed on the comet, Philae will take readings until the comet is too warm -- probably around March. Meanwhile, Rosetta will follow the comet's orbit until December.

(ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)


The separation of Philae from Rosetta herself is scheduled for 4:03 a.m. Eastern Time on Wednesday. While the public won't be able to see video of the separation itself, there will be live video from the ESA's control room online, and probably periodic images from the spacecraft. Once Philae separates from its mother ship, it's got a seven-hour descent to the surface.

If all goes well, Philae will land at 11 a.m. Eastern Time on Wednesday — give or take 15 minutes. Of course, that's just when staff at mission control will see that the landing has occurred. Because 67P/Churyumov-Gerasimenko is almost 300 million miles away from Earth at the moment, there's a communication delay of nearly half an hour.

But even in the final seven hours of a 10-year endeavor, plenty can go wrong: Philae and Rosetta could fail to make satellite communication with each other after the separation. Since Rosetta is Philae's relay to Earth, a failure to connect the probe to the spacecraft would mean the robot was functionally lost.


Philae's landing site isn't perfect, but it's as close as the ESA could get. (ESA)

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's overseeing NASA's many contributions to the effort.

The surface could be much harder or softer than they're expecting, causing Philae to bounce or sink on impact. If Philae ends up upside-down or sinks too deep into soft ground, it has no way to right itself.

But even if the landing is a mess, Rosetta has been a resounding success, said Kathrin Altwegg of the University of Bern in Switzerland. Altwegg is in charge of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), 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."

Alexander agreed. "Even during the descent itself we'll be taking readings that will move our understanding of the comet forward leaps and bounds," she said.

That isn't to say there isn't much to learn from a successful landing. "Right away the lander will 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."

The comet contains the materials that originally formed the solar system, frozen in time. By digging them out, we can learn more about the origins of our own planet.

"It's a look at the basic building blocks of our solar system, the ancient materials from which life emerged," Altwegg said. "It's like doing archaeology, but instead of going back 1,000 years, we can go back 4.6 billion."


A selfie of Rosetta and the comet, taken from 10 miles away. (ESA/Rosetta/Philae/CIVA)

An image of a portion of Comet 67P/Churyumov-Gerasimenko obtained on Oct. 30, 2014 from a distance of approximately 30 kilometers. (ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Rosetta mission poster showing the deployment of the Philae lander to comet 67P/Churyumov–Gerasimenko. (ESA/Rosetta/NavCam)