For two years, the Rosetta spacecraft has been playing around with a comet shaped like a rubber ducky.
When the spacecraft makes a gentle belly flop onto the comet on Friday, it will bring to an end to the most ambitious mission ever for the European Space Agency. Since its arrival in August 2014 at Comet 67P/Churyumov–Gerasimenko, Rosetta has been sending reams of data and exquisite photographs of the comet, providing insights and surprises about one of the fragments left over from the formation of the solar system four and a half billion years ago.
On impact, Rosetta will switch off its radio transmitter, leaving silence at the end of its 12-year journey.
“As soon as we lose contact with the spacecraft, that’s the end of Rosetta,” said Matt Taylor, the mission’s project scientist. “It’s a nice appropriate line to be drawn, I think.”
Back in November 2014, Rosetta dispatched a small lander, Philae, to the surface of Comet 67P.
And as the comet has been moving away from the sun, Rosetta’s solar panels were becoming less and less able to generate enough energy to power the spacecraft, and the amount of data the spacecraft could send back has been dropping.
While it was possible to shift Rosetta into a hibernating phase, mission managers calculated that the odds were slim that Rosetta could revive when Comet 67P approached the sun again, about four years from now.
Instead, they decided to undertake some truly close-up observations as Rosetta began falling into the comet.
After Rosetta lands, with no more data coming back, mission controllers will never know the craft’s exact fate.
Shutting off the transmitter avoids any possibility of Rosetta’s interfering with the deep space communications of other spacecraft. And even if the transmitter were kept on, the antenna would almost certainly be jostled away from Earth by the impact of the landing.
“We will not be able to communicate in any case,” said Patrick Martin, the mission manager.
Then everyone working on the mission will have some time to relax. “I’m going to put my feet up,” Dr. Taylor said. “We have free time, as it were.”
There is still much work to do. Archiving all of the data will take three years, Dr. Martin said. “Not as intense as missions operations,” he said.
The scientists will also finally have time to analyze what they have collected. “We’ve only touched the tip of the iceberg in terms of digging through the data,” said Joel Parker, a scientist at the Southwest Research Institute in Boulder, Colo., and deputy principal investigator for an ultraviolet spectrograph on Rosetta. “It’s been so busy doing operations that a lot of us haven’t even had a chance to dig into our own instrument data as much as we would like.”
Here is some of what has already come out of Rosetta.
Comets Can Stick Together
The first surprise from the initial images of Comet 67P was that it was not round — it looked more like a rubber ducky. That immediately raised the question whether the strange shape was the result of erosion or whether it was originally two comets that bumped into each other and stuck.
Exposed plateaus on the surface provide an important clue that Comet 67P was originally two comets. “It’s akin to looking at the skin of an onion,” Dr. Taylor said. “Looking at the orientation of these layers, it’s clear this has to be two objects that have collided.”
That in turn has additional implications on how the interior is packed together and how much it could have heated.
Rosetta’s cameras have also captured what scientists called “goose bumps,” objects about a yard wide that are the building blocks of the comet. “This is one of the major things that came out of Rosetta, this aspect of how the comet was put together,” Dr. Taylor said. “It means comets are much more puzzling that we expected.”
Blinded by the Comet
In Rosetta’s early observations, Rosetta scientists were puzzled by the lack of much ice on the comet’s surface. After all, comets have long been known as “dirty ice balls.”
Comet 67P is not a particularly bright comet. Even when it reached its closest proximity to the sun, in August last year, it was not visible to the naked eye by people on Earth. But as expected, the comet became much more active. As the comet spun around once every 12 hours, frost formed and disappeared on parts of the comet. Periodically, jets of gas and dust erupted.
Dr. Taylor said that it appeared that dust on the surface acted as insulation, holding onto the heat during the day that warms ice below, producing gases that rise to the cold surface, freezing again as frost.
The jets appear to be the consequence of cliff walls crumbling, exposing ice that then quickly vaporized.
The bursts released so much dust that Rosetta’s star trackers, cameras that keep track of the spacecraft’s position by looking at the background of stars, were confused. After that, the mission managers moved Rosetta farther away from the comet. “We had to change the way we ran the mission,” Dr. Martin said. “This was my main challenge.”
Dr. Parker said that not all dust from Comet 67P was the same. Some particles are fluffy and fragile, and others are more solid pieces. “These different kinds of dust seem to come from different processes,” he said. The fluffy dust comes from all over the comet, he said, while the consolidated particles seem to come from specific regions.
With the comet now much quieter, Rosetta has moved closer, within 1.2 miles of the surface.
“Everything is going well in this final phase of Rosetta,” Dr. Martin said. “The spacecraft hardware is in rather good condition.”
Water Not Like the Oceans
Where did the water in Earth’s oceans came from? No one knows.
But scientists now know for sure that it did not come from comets like Comet 67P. Rosetta measured the fraction of deuterium — a heavy form of hydrogen — in the stream of water molecules coming off, and it was much higher than that found in Earth’s water.
The current six-and-a-half-year orbit of Comet 67P takes it as far out as Jupiter and even at its closest approach to the sun, it is still outside Earth’s orbit. Planetary scientists are sure Comet 67P originated much farther away, beyond Neptune’s orbit, in what is known as the Kuiper belt, and was nudged into the inner solar system, probably by a collision or close gravitational interaction with another object.
The presence of molecular oxygen, for example, suggests that the comet formed in a very cold region. “It was fully unexpected,” Dr. Taylor said. “We don’t expect to get oxygen because it likes to do stuff with other things. It’s a very friendly molecule, and it likes to jump and become something else.”
The water finding suggests that Kuiper belt comets are not the source of Earth’s water.
We Found the Lander!
The Philae lander, which bounded to the comet’s surface in November 2014, was a resounding success and a failure. Mission managers celebrated when the lander touched the surface, but were then puzzled when it appeared to still be rotating.
Two systems designed to keep Philae pressed to the surface — harpoons and a thruster — failed, and the lander bounced high before coming back down. The lander operated for a couple of days before its battery drained. Philae briefly revived a couple of times as the comet neared the sun, then was never heard from again.
No one was quite sure where Philae ended up until this month, on the very last opportunity, Rosetta finally managed to capture a photograph of Philae on the surface.
With the exact location, scientists will be able to refine the results of an experiment that sent radio transmissions through the comet. “That will really nail their results,” Dr. Taylor said.
In the final days, Rosetta is being moved into orbits that are more elliptical. One final maneuver will put it on a collision course with Comet 67P, the minuscule gravity pulling the spacecraft for a final embrace.
The landing site is on the head portion of the comet, near some intriguing pits. The aim is not to land in one of the pits, just to pass over them, but it is possible that Rosetta will end up in one of them.
Dr. Martin said he was ambivalent as Rosetta approached its end. “I’ll be very sad, of course,” he said. “We will celebrate the achievement of such an exciting mission.”
Dr. Parker said the scientists got what they came for. “We had high hopes, and it went even higher than that,” he said. “It’s been a wild ride.”