Rosetta mission ends with comet crash

Pictures of Rosettarsquo;s final landing site
Last views: pictures of the final landing site

The European Space Agency’s (ESA’s) seminal, 12-year-long Rosetta mission has concluded, as the probe made a controlled crash into the Ma’at region of comet 67P/Churyumov–Gerasimenko today. The agency confirmed the conclusion of the mission as the signal from Rosetta was lost upon impact at 11:19 GMT. Rosetta continued to take data and make measurements during its final descent, focusing on several “active pits” from which a number of the comet’s dust jets originate. During its drop, the orbiter studied the comet’s gas, dust and plasma environment very close to its surface, and took high-resolution images. All of the information has been relayed to Earth, and Rosetta researchers will now begin to dig through the last of the mission data.

“Rosetta has entered the history books once again,” says Johann-Dietrich Wörner, director general of ESA. “Today we celebrate the success of a game-changing mission, one that has surpassed all our dreams and expectations, and one that continues ESA’s legacy of ‘firsts’ at comets.”

Bumpy start

Rosetta’s mission first began more than a decade ago, when the spacecraft was launched in 2004. Its target was comet 67P/Churyumov–Gerasimenko, whose 6.5-year elliptical orbit around the Sun takes it from beyond Jupiter to between the orbits of Mars and Earth. Rosetta’s voyage included three gravity-assisted flybys of Earth and one of Mars. While en route to 67P, the spacecraft made detailed observations of two other asteroids – Šteins and Lutetia – revealing previously unknown information such as their core structures. Rosetta then spent nearly 31 months in hibernation before the researchers sent a signal waking it up in early 2014 – the year of its “rendezvous” with the comet, which was achieved that August.

In November 2014, the Rosetta team made history as they were the first not only to have a spacecraft orbit a comet, but also to land on one after their Philae probe touched down successfully on the surface. But this landing did not come without its problems. One of the probe’s thrusters malfunctioned and a harpoon that should have locked it to the surface could not lock on. As a result, the lander bounced and its final landing place was less than ideal. Philae found itself in the shadow of a crater, with not enough light hitting its solar panels. Despite this, the lander used all of its on-board devices and instruments for about 60 hours and managed to complete all of the planned observations before it entered hibernation mode on 15 November. It was then “woken up” nearly a year later as the comet approached the sun and recorded more data.

Collision course

The team only spotted Philae from Rosetta earlier this month, and the “final resting place” of the lander can be seen in images taken by Rosetta’s high-resolution camera. The images showed the tiny lander wedged into a dark crack on the comet and its orientation clearly reveals why establishing communications was so difficult, following the 2014 landing. The images were taken on 2 September by the OSIRIS narrow-angle camera, as Rosetta came within 2.7 km of the surface. Communications with Philae were officially turned off at the end of July this year, to save energy for the orbiter to run until today.

As Rosetta descended today, ESA scientists were busy monitoring the comet during the drop and retrieving data. For example, the orbiter’s Comet Pressure Sensor (COPS), found that the gas pressure around the comet’s nucleus was increasing as it descended. Also, some of the active pits in the target region had previously been spotted having intriguing, lumpy, metre-sized structures. The Rosetta team believe these could be the signatures of early cometesimals that agglomerated to create the comet in the early phases of solar system formation. “With the decision to take Rosetta down to the comet’s surface, we boosted the scientific return of the mission through this last, once-in-a-lifetime operation,” says current mission-manager Patrick Martin.

Rubber-duck

Data from the Rosetta mission have already provided a host of new insights into comets and their make-up. “One of the big surprises was the shape of our duck,” says project manager Matt Taylor, referring to the fact that the comet’s nucleus is made up of two distinct segments joined by a “neck”, giving it a “rubber-duck”-like appearance. The team now believe that the two lobes formed independently and later joined in a low-speed collision in the early days of the solar system.

Some of the earliest measurements of the levels of hydrogen isotopes on the comet have showed that much of the water it holds is heavy water. That is, the ratio of deuterium to hydrogen in the comet is much greater than the ratio found on Earth. This was a crucial finding, as it disproved the suggestion that comets supplied Earth with majority of its water.

Other measurements focussed on 67P’s coma, shape, composition, temperature, nucleus and surface features. These revealed a textured, dynamic comet that was covered in sand-dunes and ripples, spewing out jets of material. The team also discovered that the small rock has no global magnetic field.

Another major discovery, only revealed earlier this year, is that the comet contained the amino acid glycine – a key building block of life. Numerous organic compounds were also detected, both by Rosetta from orbit and by Philae on the surface. These finding suggested that critical ingredients for life may indeed have been delivered to Earth by comets, rather than being created on our planet. It also suggests similar comets could also have delivered life elsewhere in the universe.

“Just as the Rosetta Stone, after which this mission was named, was pivotal in understanding ancient language and history, the vast treasure trove of Rosetta spacecraft data is changing our view on how comets and the solar system formed,” says Taylor. “Inevitably, we now have new mysteries to solve. The comet hasn’t given up all of its secrets yet, and there are sure to be many surprises hidden in this incredible archive. So don’t go anywhere yet – we’re only just beginning.”



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