Philae’s extraordinary comet touchdown relived

Choices, choices

Rosetta arrived on the comet on 6 August 2014, and the race was instantly on to discover a appropriate touchdown website for its lander Philae. 

The positioning wanted to supply a stability of security and distinctive science potential. Rosetta’s photographs of candidate touchdown websites have been scrutinised and debated, and inside just a few weeks the ultimate selection was made: a smooth-looking patch, later named Agilkia, situated on the smaller of the comet’s two lobes.

Intense preparations adopted, however the night time earlier than touchdown, an issue was recognized: Philae’s lively descent system, which would supply a downward thrust to forestall rebound at landing, couldn’t be activated. Philae must depend on harpoons and ice screws in its three ft to repair it to the floor. 

Nonetheless, the inexperienced gentle was given and after separating from Rosetta, Philae started its seven-hour descent to the floor of the comet. Throughout the descent, Philae started ‘sensing’ the atmosphere across the comet, taking beautiful imagery as the primary touchdown website got here into view.

Welcome to a comet

Philae’s landing at Agilkia was spot-on. The sensors on Philae’s ft felt the landing vibrations, producing the first recording of contact between a human-made object and a comet. Nevertheless it quickly grew to become clear that Philae’s harpoons hadn’t fired and it had taken flight once more. 

In the long run Philae made contact with the floor 4 instances. Due to an automated sequence that was triggered by the primary landing sign, Philae’s devices have been working whereas in flight, accumulating distinctive information that will later yield essential outcomes. It was additionally an surprising bonus that information have been collected at multiple location, offering the primary direct measurements of floor traits and permitting comparisons between the landing websites. 

For instance, Philae ‘felt’ the distinction in floor texture and hardness because it bounced from one website to a different. On the first touchdown website, it detected a tender layer a number of centimetres thick, milliseconds later encountering a a lot tougher layer. 

After colliding with a cliff, Philae scraped by its second landing website, offering the primary in situ measurement of the softness of the icy-dust inside of a boulder on a comet. The easy motion of Philae ’stamping’ an imprint in billions-of-years-old ice revealed the boulder to be fluffier than froth on a cappuccino, equal to a porosity of about 75%. 

Philae then ‘hopped’ about 30 metres to the ultimate landing website, named Abydos, the place its CIVA cameras supplied the first picture of a human-made object touching a 4.6 billion yr outdated Photo voltaic System relic. The precise location on the comet would stay hidden from view for nearly two years. 

On this location, Philae’s MUPUS hammer penetrated a tender layer earlier than encountering an unexpectedly laborious floor just a few centimetres under the floor. Philae ‘listened’ to the hammering with its ft, recording the vibrations that handed by the comet. This was the first time because the Apollo 17 mission to the Moon in 1972 that lively seismic measurements have been performed on a celestial physique. 

MUPUS additionally carried a thermal sensor, which measured the native modifications in temperature from about -180ºC to 145ºC, in sync with the comet’s 12.4 hour day – the primary time the temperature cycle of a comet had been measured at its floor.

In the meantime the CONSERT experiment, which handed radio waves between Rosetta and Philae by the comet within the first cometary sounding experiment, revealed the inside of the comet to be a really loosely compacted combination of mud and ice, with a excessive porosity of 75–85%.

In-flight science

Throughout the bouncing, Philae’s COSAC and Ptolemy devices ‘sniffed’ the comet’s fuel and dirt, essential tracers of the uncooked supplies current within the early Photo voltaic System. COSAC revealed a collection of 16 natural compounds comprising quite a few carbon and nitrogen-rich compounds, together with methyl isocyanate, acetone, propionaldehyde and acetamide that had by no means earlier than been detected in comets. The advanced molecules detected by each COSAC and Ptolemy play a key function within the synthesis of the elements wanted for all times.

Philae’s bouncing additionally allowed it to measure the magnetic area at totally different heights above the floor, exhibiting the comet is remarkably non-magnetic. Detecting the magnetic area of comets has confirmed tough in earlier missions, which have sometimes flown previous at excessive speeds, comparatively removed from comet nuclei. It took the proximity of Rosetta’s orbit across the comet and the measurements made a lot nearer to and on the floor by Philae, to supply the first detailed investigation of the magnetic properties of a comet nucleus.

In the long run some 80% of Philae’s deliberate science sequence was accomplished within the 64 hours following separation from Rosetta and earlier than the lander fell into hibernation.

Whereas Philae hibernated, Rosetta continued returning an unprecedented wealth of knowledge from the comet because it orbited across the Solar, watching the comet’s exercise attain a peak after which slowly subside once more. Philae can be heard from briefly in June–July 2015 however couldn’t be reactivated. Then, as Rosetta’s mission was drawing to its deliberate finish with its personal daring descent to the floor at a website named Sais, Philae’s ultimate touchdown website was revealed in orbiter imagery, a ultimate twist in what had change into one of many biggest tales of area exploration.

What’s subsequent?

ESA has a powerful legacy in small physique exploration, with the Rosetta-Philae double-act inspiring the subsequent technology of comet and asteroid-chasers.

ESA’s Giotto mission to fly by Comet Halley in 1986 was the primary mission to picture a comet floor. The Rosetta mission was a pure subsequent step, turning into the primary to orbit a comet, in addition to deploying a lander to its floor. Rosetta was additionally the primary to comply with a comet across the Solar, monitoring its exercise because it made its closest strategy to the Solar. 

Rosetta paves the best way for the upcoming Comet Interceptor mission, which in contrast to its predecessors, will probe a comet visiting our Photo voltaic System for the primary time. As such, the comet will include materials that has undergone minimal processing, providing a ‘cleaner’ take a look at pristine materials from the daybreak of the Photo voltaic System, earlier than it’s sculpted by the warmth of the Solar. The mission will include a major craft and two probes, offering a multi-angled view of the comet. 

ESA can also be visiting asteroids, with its flagship ‘planetary defender’ Hera on its technique to survey Dimorphos following NASA’s influence experiment to change its trajectory, a grand-scale check of planetary defence strategies. Hera’s orbit scheme is borrowed straight from Rosetta, and the mission’s two smaller satellites carry radar and dust-measuring devices primarily based on these designed for Rosetta.

In the meantime Ramses will accompany asteroid Apophis because it makes an exceptionally shut flyby of Earth in 2029. And suitcase-sized M-Argo would be the smallest spacecraft to carry out its personal impartial mission in area when it rendezvous with a small near-Earth asteroid later this decade.

Rosetta and Philae’s legacy additionally lives on within the hearts and minds of the those that labored on and adopted the mission, as revealed in our new on-line exhibition celebrating this uniquely inspiring mission. 

Rosetta was an ESA mission with contributions from its Member States and NASA. Philae was supplied by a consortium led by DLR, MPS, CNES and ASI.