Philae is already transmitting scientific data back home, but we're still waiting to see whether the probe is in a stable position. Until we know it's anchored tight, it could roll onto its back and never get back up.

Tensions were high in the European Space Agency's German mission control center, especially as the landing window approached. Because the comet that Philae landed on is so far from Earth, there's a communications delay of 28 minutes. So as the minutes ticked by, the Rosetta team knew that Philae had already either landed or failed — and there was nothing they could do but wait for the data to reach them. Those following the video online were nearly as desperate for news, and Twitter became a sounding chamber of anticipation and excitement.

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But a few minutes after 11 a.m., the stern, cautious expressions of the mission control team melted into smiles. And just like that, the world swiveled from anxiety to elation: Philae was on the surface of the comet and ready to do some science.

The comet contains the materials that originally formed our solar system, frozen in time. By digging them out, we can learn more about the origins of our planet. The Rosetta spacecraft has made invaluable observations about the comet's attributes, and it will continue to do so as it follows it around the sun for the next year. But Philae will be able to look more closely at the comet's physical and molecular composition.

"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 the Rosetta project's lead researchers. "It's like doing archaeology, but instead of going back 1,000 years, we can go back 4.6 billion."

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It's no easy thing to land on a comet's surface: These chunks of rock and ice are constantly spinning, and Comet 67P/Churyumov-Gerasimenko, which was discovered in 1969, orbits the sun at a speed of about 85,000 mph. It's irregularly shaped — like a toddler's play-dough impression of a duck, or something — and its surface is uneven and pitted. And in a universe of unimaginable proportion, Rosetta's target is just 2.5 miles in diameter — smaller than Northwest Washington's Columbia Heights neighborhood.

So Rosetta has taken an onerous journey to get in sync with the comet's orbit, which would allow it to drop down a lander. In 2004, the spacecraft began what would be three looping orbits around the sun, altering its trajectory as it skimmed Mars, just 150 miles from the surface, and enduring 24 minutes in the planet’s shadow to align with Churyumov-Gerasimenko. The cumulative distance traveled by the craft – with all its looping and gravity assists – is a stunning 4 billion miles. “When the Rosetta signal reappeared after the passage behind Mars, shortly after the end of the ‘shadow’ period, there was a collective sigh of relief,” ESA said.

At one point in 2011, the spacecraft even had to hibernate for nearly three years. It flew so far from the sun — nearly 500 million miles — that its solar panels couldn't leech enough energy to keep the spacecraft operational. But in January of this year, Rosetta woke up, and quickly approached its target.

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The last leg of this landing has not been without its bumps. Even as the mission approached its most critical moment, controllers at the European Space Agency on Tuesday night reported a problem with the thruster on the lander that could make for a rough landing. The gravity of the problem — and the extent to which it threatened the mission — remained unknown. “We’ll need some luck not to land on a boulder or a steep slope,” blogged Stephan Ulamec, lander manager for the project.

Following its separation early Wednesday morning, Philae made a seven-hour-long drop to the surface. After successfully making a satellite connection with Rosetta (without which the probe would have been functionally lost), Philae sent home a goodbye picture of its mother-ship.

Then, we watched and waited. We worried we'd have to watch mission control experience the agony of an upside-down rover, lost forever. If the surface of the comet was too hard, the probe might have bounced on impact and landed badly.

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"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.

Luckily, the probe's sensors detected a soft landing. But mission control hasn't gotten confirmation that the probe's harpoons deployed — let alone that they stuck. The Rosetta team is concerned that the probe might roll out of place, but they're considering re-firing the harpoons to try again. If the surface is too soft, it's possible that the probe won't be able to anchor itself at all — which would make it hard to stay upright. And if the team refires the harpoons, it's possible that the probe will actually just get knocked into a bad position.

If Philae flips over, it has no way to right itself.

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But even if the morning ends with disappointment, Rosetta has been — and will continue to be — a resounding success, researchers say.

"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.

And no matter what, Rosetta will continue to trail the targeted comet as it orbits the sun this year, no doubt giving scientists unprecedented insight into its composition.

But whether or not Philae lives up to its full potential as a scientific instrument, it has landed on a moving comet and shown us that humanity is capable of incredible things.