Rendezvous with a comet: 10 years to the historic Philae comet landing

On 12 November 2014, after a 10-year journey through the solar system and over 500 million kilometers from Earth, the European Space Agency's Rosetta mission's lander Philae made space exploration history by touching down on Comet 67P/Churyumov-Gerasimenko, marking the first time a man-made object landed on a comet, generating the first recording of contact between a human-made object and a comet.

The feat of landing on a comet has not yet been repeated. 

On January 13, 2003, the mission was ready to embark on its planned journey to comet Wirtanen. However, only a few hours before the launch, a shock announcement came: Due to a problem with another launch, the manufacturer had withdrawn the launch authorization for the Ariane 5. This meant the launch window for the mission had closed, comet Wirtanen became unreachable, and a new mission target had to be found. Due to the performance of the Ariane 5 and the known comet orbits, comet 67P/Churyumov-Gerasimenko was finally selected for exploration.

A concern arose in the lander team: The new comet was up to four times larger and much more active than the original mission target. However, the team decided to take this risk. 

After Rosetta arrived at the comet on August 6, 2014, the team raced to find a suitable landing site for Philae, ultimately choosing a smooth-looking patch named Agilkia, located on the smaller of the comet's two lobes. Within a few weeks after arrival, the final landing site was chosen. The safe landing in a selected area still posed the greatest headache, as the landing site needed to offer a balance of safety and unique scientific potential. The most interesting spots on the surface were unfortunately unreachable for a passive drop due to the orbital dynamics. Therefore, the landing site J was ultimately agreed upon, which is now known as Agilkia.

Nervous tension prevailed in the lander control center in Cologne on November 12, 2014. At that time, the comet was 510 million kilometers from Earth, and radio signals took about 28 minutes to reach Earth. Due to the French-German leadership structure in the lander, the data from the ESA control center in Darmstadt were first routed through a server in Toulouse, which meant an additional ten minutes' delay in Cologne. At 9:35 AM Central European Time, Philae was released from Rosetta to begin the seven-hour landing approach.

During the descent, a problem was identified: Philae's active descent system, which would provide a downward thrust to prevent rebound at touchdown, could not be activated. It was uncertain how the shape would look and how the surface would be structured. Despite the problem with the harpoons, the green light was given for the landing.

At 5:03 PM, jubilation broke out in Darmstadt. The lander reported ground contact, and people embraced on the stage. In Cologne, the reaction was more restrained because they were still waiting for data from the landing system due to the time delay. Eventually, champagne was served in Cologne, and the mood lightened. However, the joy was short-lived because the instruments displayed data that did not match a successful landing.

An analysis revealed that the spring in the needle system had aged too much during the ten years of flight time to open the extra strong protective membrane on the gas tank. The cold gas system could not be activated. The lander was not on the ground but had lifted off again and was still rotating. It appeared that the anchor projectiles had not fired.

After the initial touchdown, Philae bounced several times, as its anchoring harpoons failed to deploy. Philae's touchdown at Agilkia was spot-on. Philae would have to rely on harpoons and ice screws in its three feet to fix it to the surface. Philae "felt" the difference in surface texture and hardness as it bounced from one site to another.

Philae made contact with the surface four times, collecting data about each bounce location and the properties of the comet's gas and dust. Eventually, Philae landed at its final touchdown site, named Abydos, after "hopping" about 30 meters. At Abydos, Philae's MUPUS hammer penetrated a soft layer before encountering an unexpectedly hard surface a few centimeters below. Philae "listened" to the hammering with its feet, recording the vibrations that passed through the comet. Philae's CIVA cameras provided the first image of a human-made object touching a 4.6 billion-year-old solar system relic.

Despite the problem with the harpoons, Philae managed to send valuable data back to Earth before its batteries ran out. In the end, about 80% of Philae's planned science sequence was completed in the 64 hours following separation from Rosetta and before the lander fell into hibernation.

While Philae hibernated, Rosetta continued returning an unprecedented wealth of information from the comet as it orbited around the sun. Rosetta observed the comet's activity reach a peak and then slowly subside again. To everyone's surprise, Philae reestablished contact on June 15, 2015, as apparently enough solar energy was available, but could not be reactivated afterward. It was not until September 5, 2016, that Philae was found in the last photo of the suspected landing site near a cliff, revealed in orbiter imagery as Rosetta's mission was drawing to its planned end.

The Rosetta spacecraft was intentionally crash-landed on Comet 67P/Churyumov-Gerasimenko at the end of the mission on September 30, 2016, as the probe had reached a distance from the sun where the power supply would not have been secured for long.

Philae's measurements showed that Comet 67P/Churyumov-Gerasimenko is remarkably non-magnetic. Philae measured the temperatures of the comet's surface, finding fluctuations between -292 degrees Fahrenheit to 293 degrees Fahrenheit across the comet's 12.4-hour day.

Philae's bouncing allowed it to measure the magnetic field at different heights above the surface. Data were collected at more than one location, providing the first direct measurements of surface characteristics and allowing comparisons between the touchdown sites, including some with much softer surface textures than others. Philae's COSAC and Ptolemy instruments "sniffed" the comet's gas and dust, important tracers of the raw materials present in the early solar system. The organic molecules found included methyl isocyanate, acetone, propionaldehyde, and acetamide, which can have a significant impact on the origin of life and had never before been detected in comets.

"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," said Matt Taylor, the Rosetta project scientist, in 2016. "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."

The insights gained will help in subsequent missions like Comet Interceptor, planned to launch in 2029, and in the exploration of Jupiter's icy moons by the spacecraft JUICE. One of the upcoming missions is the Comet Interceptor mission, which hopes to visit a long-period comet visiting our Solar System for the very first time. The Comet Interceptor mission will consist of a primary craft and two probes, providing a multi-angled view of the comet.

Sources: Der Standard, Newsweek, Phys.org

This article was written in collaboration with generative AI company Alchemiq