Tomorrow, September 30, the European Space Agency will attempt a daunting feat of celestial maneuvering: a controlled landing of the Rosetta spacecraft on comet 67P/Churyumov-Gerasimenko. You can watch mission control. There are two ways: the ESA livestream, from 6:30–7:40 a.m. EDT, or the NASA TV livestream, which starts a little earlier (6:15 a.m. EDT).

While you won't be able to see live footage of the attempted landing (though images from the descent will be available), you'll see the scientists at mission control attempting the maneuver—and bringing to a close a mission that was in progress for decades. Beyond the science, it's bound to be emotional: expect shouts of excitement, hugs, and tears.

Rather than turning the spacecraft into a cannonball and smashing it into the comet, engineers hope to settle it on the surface, where it might sit for eons alongside Philae, its lander—a duo of tiny monuments to human exploration.

Well, maybe. The thing is, we'll never know Rosetta's fate.

Rosetta has been in orbit around comet 67P/Churyumov-Gerasimenko since August 2014. You might recall that three months later, Philae landed on 67P. It bounced (and bounced and bounced), eventually coming to rest at an angle in a shadowy area. That wasn't an ideal outcome, but Philae defied odds and made contact with Earth. Before the ESA shut it down forever in July 2016, Philae managed to do some great comet science, including the discovery on the comet of prebiotic molecules that are necessary for life.

Meanwhile, Rosetta studied the comet from afar, mapping meticulously its composition, and returning images for scientists to study back home. Among Rosetta's findings: molecular oxygen, suggesting the comet was first formed far beyond Neptune, and suggesting further that Kuiper Belt objects are not the source of Earth's water. The very shape of the comet itself was a discovery. 67P looks more like a rubber ducky than the bouncy ball they expected. And just a few weeks ago, Rosetta revealed the location of lost little Philae.

So why attempt to land Rosetta now? Because it's solar powered, and as 67P travels farther and farther away from the Sun in its orbit, the spacecraft has received less and less sunlight, and thus has diminishing power available for flight or instruments. If it's going to land, now is the time.


Comet 67P as captured by Rosetta's navigation camera on March 14, 2015. Image credit: ESA/Rosetta/NAVCAM via Flickr // CC BY-SA IGO 3.0

 
What will happen is this: Rosetta will spiral toward the asteroid in a controlled manner—descending at about 50 centimeters per second—taking ever-refined measurements and high-definition images all the way down. (It's been drawing closer for weeks, performing increasingly tight orbits.) As those measurements and images are captured, they will be sent immediately back to Earth.

At the moment of contact, the spacecraft will be instructed to switch off. That's kind of disappointing for us Earthbound viewers. But it won't be an attempt to cut away from a potential crash landing. Rather, the goal is to keep radio signals in space tidy. ESA doesn't want a spacecraft far out in the solar system blasting radio signals all over the place, because that might interfere with other spacecraft communications. Moreover, even if Rosetta were ordered to transmit until the heat death of the universe, there's virtually no chance that its antenna would survive the landing and still be properly oriented to beam signals to Earth.

This isn't the first time a space agency has attempted such a landing. In 2001, NASA landed its NEAR spacecraft on the asteroid Eros—the first time a spacecraft had landed on a small body. It wasn't a stunt, exactly—the spacecraft was going to hit Eros one way or another—but the mission's flight director was Bob Farquhar of the Johns Hopkins University Applied Physics Laboratory, who was known in life as the "grandmaster of celestial maneuvers." (Farquhar died in 2015.) He was famed for his genius at plotting incredibly complex and elaborate trajectories on space missions. He felt like he could nail the landing—and he did. The spacecraft touched down so gently that it remained fully operational for several weeks afterward, allowing scientists to capture data from a wholly unexpected vantage point.

Sadly, we won't get such satisfaction from Rosetta. Because the spacecraft will shut down before touchdown, its fate on the comet will remain a mystery. Did it land gently? Bounce away? Smash into pieces? We'll never know—unless some other space mission allows us to peer at 67P in the future.

But while Rosetta flight operations will be at an end, the work will go on. The spacecraft has already returned several years' worth of data for scientists to organize and study. On the landing approach alone, Rosetta will return an extravaganza of data covering a region of the comet known as Ma’at, characterized by 50-meter-deep pits that are actively blasting dust into space. Those pits feature "goosebump" structures that scientists believe to be "cometesimals"—the objects that came together to form the asteroid at the dawn of the solar system nearly 5 billion years ago. The spacecraft will make contact with surface area that is adjacent to a pit that has been dubbed Deir el-Medina (after a town in Egypt with a pit of similar appearance).

There's something so audacious and Star Trek–y about this: They found a deep mysterious cavern from the dawn of time, with stardust blasting from it, and now this tiny celestial voyager will peer down into the chasm before alighting on its rim. It's frontier science. 

During the livestreams from ESA and NASA, scientists and engineers will offer commentary on the mission and its legacy, and explain what is happening with the spacecraft during its final moments in operation. Over the years ahead, as scientists study the data and refine our collective understanding of the solar system, Rosetta and Philae will rest together on the celestial target of the most ambitious mission ever attempted by ESA.