Choices, selections

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 steadiness of security and distinctive science potential. Rosetta’s photographs of candidate touchdown websites had been scrutinised and debated, and inside a couple of weeks the ultimate selection was made: a smooth-looking patch, later named Agilkia, positioned on the smaller of the comet’s two lobes.

Intense preparations adopted, however the evening earlier than touchdown, an issue was recognized: Philae’s lively descent system, which would offer 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 mild 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 gorgeous 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 primary recording of contact between a human-made object and a comet. But it surely quickly turned 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 occasions. Due to an automated sequence that was triggered by the primary landing sign, Philae’s devices had been working whereas in flight, accumulating distinctive information that will later yield essential outcomes. It was additionally an sudden bonus that information had 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 smooth layer a number of centimetres thick, milliseconds later encountering a a lot more durable layer. 

After colliding with a cliff, Philae scraped by way of 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 straightforward 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 offered the first picture of a human-made object touching a 4.6 billion 12 months 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 smooth layer earlier than encountering an unexpectedly exhausting floor a couple of centimetres under the floor. Philae ‘listened’ to the hammering with its ft, recording the vibrations that handed by way of the comet. This was the first time for the reason that Apollo 17 mission to the Moon in 1972 that lively seismic measurements had 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 way of 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 complicated molecules detected by each COSAC and Ptolemy play a key function within the synthesis of the components wanted for all times.

Philae’s bouncing additionally allowed it to measure the magnetic subject at totally different heights above the floor, displaying the comet is remarkably non-magnetic. Detecting the magnetic subject of comets has confirmed tough in earlier missions, which have usually 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 data 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 house exploration.

What’s subsequent?

ESA has a powerful legacy in small physique exploration, with the Rosetta-Philae double-act inspiring the subsequent era 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 encompass a main craft and two probes, offering a multi-angled view of the comet. 

ESA can be visiting asteroids, with its flagship ‘planetary defender’ Hera on its option to survey Dimorphos following NASA’s influence experiment to change its trajectory, a grand-scale take a look at of planetary defence methods. Hera’s orbit scheme is borrowed straight from Rosetta, and the mission’s two smaller satellites carry radar and dust-measuring devices based mostly 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 house 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 individuals, as revealed in our new on-line exhibition celebrating this uniquely inspiring mission. We’re additionally reliving the important thing moments round Philae’s touchdown on the comet and Rosetta group members’ experiences on X. Be a part of us by following #CometLandingRelived.

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