It's time to say goodbye to Rosetta, the historic comet-chasing spacecraft.
In the early morning hours of Sept. 30, mission scientists at the European Space Agency initiated instructions to send the 9-foot-wide spacecraft on a collision course with Comet 67P/Churyumov-Gerasimenko. During its death dive, which ended with a belly flop of sorts into the comet's dusty surface, Rosetta collected and sent back data and images. And then, after more than 12 years in space, it went silent.
"There's mixed emotions here," Matt Taylor, the mission's project scientist, told NPR. "You know, people have invested their lives and their mentality, I think, as well — their psychology — on this mission. I really couldn't tell you what I'm going to feel."
Rosetta's end is something of a reunion with Philae, a small comet lander that descended from the spacecraft back in 2014. Despite a landing that didn't go as planned, Philae was able to conduct critical science experiments before shutting down only a few days into its mission.
Below are just a few of the amazing discoveries Rosetta was able to record during its two years circling Comet 67P.
A comet of dunes and ripples
Instead of a featureless, icy landscape, researchers were surprised to discover the northern hemisphere of Comet 67P covered in dunes, dusty ripples and wind tails behind giant boulders. Despite lacking an atmosphere, the comet features a dynamically shifting landscape of dust thanks to gases flowing from jets on its nucleus.
"You might sink into the smooth dust where we find the thick snow-field like layers, other areas might be robust enough to carry you," Holger Sierks, the principal investigator of OSIRIS told Space.com. "The dust is dry like powder."
Amateur astronomer Jacint Roger Perez selected and processed this view by combining three images taken in different wavelengths. (Photo: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; J. Roger)
On Oct. 1, 2018, the ESA released this image taken on Sept. 22, 2014 that shows two regions, Seth and Hapi, and the Aswan cliff. A large chunk of the cliff broke off in August 2015 when the comet traveled closer to the sun. The photo also shows layers of dust along the two regions.
The current theory is that the dust, measuring several meters thick in some regions, plays a key role in insulating the comet's icy interior from further exposure to the sun. "This could explain why 67P and other comets, which penetrate into the inner solar system, are so long-lived and survive many orbits around the Sun," explained Paul Hartogh from the Max Planck Institute for Solar System Research.
A very dark surface
Despite photos that appear to show Comet 67P with a gray nucleus, the surface is actually on par with the color of coal or charcoal. Even more surprising to researchers is that the color appears rather uniform, with very few of the bluish patches of ice one might expect for a "dirty snowball."
With 67P's surface reflecting only 4 to 6 percent of the light that hits it, it's necessary for scientists to increase the exposure time of Rosetta's camera to capture surface detail. During processing, they further adjust brightness and contrast. Without such tweaks, Comet 67P, one of the darkest objects in the our solar system, would appear as little more than a ghost.
It smells pretty awful
If we could somehow bottle Comet 67P's smell, it's safe to say that it would not be very popular. Rosetta's two mass spectrometers have "sniffed" 67P's gases and discovered that they smell completely awful.
"The perfume of 67P/C-G is quite strong, with the odour of rotten eggs (hydrogen sulphide), horse stable (ammonia), and the pungent, suffocating odour of formaldehyde," Kathrin Altwegg, a principal investigator on the mission, wrote in a blog post. "This is mixed with the faint, bitter, almond-like aroma of hydrogen cyanide. Add some whiff of alcohol (methanol) to this mixture, paired with the vinegar-like aroma of sulphur dioxide and a hint of the sweet aromatic scent of carbon disulphide, and you arrive at the 'perfume' of our comet."
A cache of primordial oxygen
While analyzing the fog of gas surrounding Comet 67P, scientists were shocked to find oxygen molecules included in the mix. They theorize that the oxygen is likely primordial, roughly around 4.6 billion years old, and originating from trapped water ice ejected from the comet's core.
"This is the most surprising discovery we have made so far," Altwegg said last November. "When we first saw it, we all went into a little bit of denial."
That denial was fueled in part by the fact that interstellar oxygen molecules are extremely hard to find in the cosmos. Up until Comet 67P, its presence had only been detected in two locations busy in the formation of stars. Now that comets are known to contain molecular oxygen, it confirms the theory that they are preserved time capsules from the earliest days of the solar system.
"We had never thought that oxygen could 'survive' for billions of years without combining with other substances," added Altwegg.
A surprising twin
Initial impressions that Comet 67P was round were blown away when Rosetta sent back the first images showing off something that looked more like a rubber duck. After studying exposed plateaus on the surface, researchers come to the conclusion that 67P is actually two comets that "gently" collided and fused together.
"It's akin to looking at the skin of an onion," Matt Taylor, the mission's project scientist, told The New York Times. "Looking at the orientation of these layers, it's clear this has to be two objects that have collided."
Hey, little buddy!
After an emotional farewell to Rosetta's lost Philae comet lander in July, ESA scientists were thrilled when images taken of Comet 67P revealed the little spacecraft's final resting place.
"This remarkable discovery comes at the end of a long, painstaking search," Patrick Martin, ESA's Rosetta mission manager, said. "We were beginning to think that Philae would remain lost forever. It is incredible we have captured this at the final hour."
The lander's location, in the unfortunate shadow of a giant outcropping, gives the scientists some extremely valuable context for the brief three days of science it was able to conduct before going quiet.