Philae landing on Comet Churiumov-Herasymenko: how it happened and what scientists learned

On 12 November 2014, the Philae probe, which separated from the Rosetta spacecraft, landed on Comet Churiumov-Herasymenko. In honor of the tenth anniversary of this event, the European Space Agency (ESA) has published a detailed account of the historic landing and what was learned about the comet.

In search of a landing spot

Rosetta arrived at Comet Churiumov-Herasymenko on 6 August 2014. Almost immediately, scientists began looking for a suitable place to land Philae. It had to strike a balance between safety and science. After studying the photography taken by Rosetta, the scientists opted for a smooth area located on a smaller part of the Churiumov-Herasymenko. It was named Agilkia.

Intensive preparations were made, but the night before the landing, a problem was discovered: Philae’s active descent system, which was supposed to “press” the probe to the comet at the moment of contact, could not be activated. Philae could only rely on harpoons and ice spikes in its three landing legs to secure itself on the surface.

However, operations were given the green light. After separation, Philae began a seven-hour descent to the comet’s surface. During the descent, Philae began to “probe” the environment, taking incredible images of Churiumov-Herasymenko.

Jumping on a comet

Philae’s landing proved to be precise. The sensors in the probe’s supports felt a vibration, detecting the first contact between the Earth’s messenger and the comet. But it soon became clear that Philae’s harpoons had failed. The probe bounced off the Churiumov-Herasymenko and set off again.

As a result, Philae made contact with the surface four times. Thanks to an automatic sequence of actions that started during the first touchdown signal, the probe’s instruments worked throughout the flight, collecting unique data that would later bring important results. An unexpected bonus was that they were collected in more than one location. This made it possible to conduct the first direct measurements of surface characteristics and make comparisons between different parts of the comet.

For example, Philae “felt” the difference in surface texture and hardness as it moved from one area to another. At the first landing site, it found a soft layer several centimeters thick, and milliseconds later encountered a much harder layer.

At the second contact point, Philae bumped into a rock and then scraped across the surface, providing a measurement of the softness of the ice dust inside the boulder. Further analysis showed that this boulder is “fluffier” than cappuccino foam, corresponding to a porosity of about 75%.

Then Philae travelled about 30 meters to the final landing site, called Abydos, where its cameras transmitted the first-ever image of a man-made object on the comet’s surface. However, the probe’s exact landing site was finally revealed only almost two years later.

What Philae learned about Comet Churiumov-Herasymenko

At the final landing site of Philae, the MUPUS hammer mounted on it pierced a soft layer and then collided with an unexpectedly hard surface a few centimeters below it. The probe “listened” to the hammer’s impacts with its supports, recording the vibrations passing through the comet. For the first time since the Apollo 17 mission to the Moon in 1972, active seismic measurements were carried out on a celestial body.

MUPUS also carried a thermal sensor that measured temperature fluctuations during the local 12.4-hour day. They ranged from – 180°C at night to 145°C during the day. This is the first ever measurement of the temperature cycle of a comet on its surface.

Meanwhile, the CONSERT experiment, during which radio waves between Rosetta and Philae passed through the Churiumov-Herasymenko spacecraft, showed that its interior is a very loosely compacted mixture of dust and ice with a porosity of 75% to 85%.

During Philae’s jumps, its COSAC and Ptolemy instruments “sniffed” the comet’s gas and dust to look for traces of the raw materials from which the solar system was formed. COSAC detected a set of 16 organic compounds, including many carbon and nitrogen-rich compounds, including methyl isocyanate, acetone, propanal and acetamide, which have never been found on a comet before. These complex molecules played a key role in the synthesis of the components necessary for the emergence of life on Earth.

Philae’s bouncing also allowed it to measure the magnetic field at different heights above the surface, showing that the comet is surprisingly non-magnetic. Detecting the magnetic field of comets has been particularly challenging for previous missions because they typically fly at high speeds relatively far from their nuclei. It took the proximity of Rosetta’s orbit around Churiumov-Herasymenko and Philae’s measurements much closer to the surface to provide the first detailed study of the magnetic properties of a comet’s nucleus.

Completion of Philae’s work

In the end, Philae managed to complete 80% of the planned scientific measurements. As the spacecraft sat in the shade, it could not recharge its batteries using solar panels. Therefore, 64 hours after separation from Rosetta, Philae “went to sleep”.

In the months that followed, Rosetta continued to receive an unprecedented amount of information from Churiumov-Herasymenko, observing the comet’s activity peak and then slowly decline. In June-July 2015, Philae managed to briefly “wake up” and began sending short messages. Unfortunately, because its electronics had been exposed to low temperatures for too long, engineers were unable to restore its operation.

Then, as the Rosetta mission was coming to an end and experts were already planning its own landing on the comet, they managed to find Philae’s landing site in photography. This put a formal end to the amazing story of the small craft that has taught us so much about comets.