On the morning of 2 March 2004, an Ariane 5 climbed away from Kourou, French Guiana, carrying the most patient spacecraft ever built. Rosetta's task defied intuition: spend ten years looping through the solar system to catch a comet moving at tens of kilometres per second, then fly alongside it for two more. The mission had already survived a near-fatal setback. Grounded by an Ariane failure in late 2002, it missed its window to comet 46P/Wirtanen and was re-aimed at a body with a less lovely name: 67P/Churyumov-Gerasimenko.
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Getting there required celestial billiards: three gravity assists from Earth, one from Mars, and scientific detours past the asteroids Steins and Lutetia. Flying on solar power at distances where missions had always relied on nuclear sources, Rosetta travelled so far from the Sun that ESA placed it in hibernation in June 2011 and waited. For 31 months the spacecraft slept, beyond all help. On 20 January 2014 an onboard alarm clock woke it, and after a tense wait its carrier signal reached Earth. ESA announced the moment in two words: 'Hello, world!'
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Rosetta arrived at 67P on 6 August 2014 and found not a dirty snowball but a duck-shaped world of cliffs, pits and gas jets. On 12 November it released Philae, a washing-machine-sized lander, toward the surface. Philae's anchoring harpoons failed to fire and it bounced, the first rebound alone lasting nearly two hours, before coming to rest in the shadow of a cliff. Even there it delivered more than two days of science before its batteries drained: the first data ever returned from the surface of a comet.
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Rosetta escorted 67P through its closest approach to the Sun in August 2015, watching the comet wake, vent and shed. By mission's end it had travelled 7.9 billion kilometres and returned more than 100,000 images and 220 gigabytes of data. On 30 September 2016, with sunlight fading as the comet headed back out toward Jupiter's orbit, controllers set Rosetta down gently on the surface it had studied for two years, joining Philae in permanent residence.
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Our ambitious Rosetta mission has secured a place in the history books: not only is it the first to rendezvous with and orbit a comet, but it is now also the first to deliver a lander to a comet's surface.
Jean-Jacques Dordain · ESA Director General, on Philae's landing, 12 November 2014
By the numbers
Launch
2 Mar 2004, Kourou, French Guiana
Launch vehicle
Ariane 5 G+
Comet arrival
6 Aug 2014 (67P/Churyumov-Gerasimenko)
Philae landing
12 Nov 2014
Distance travelled
7.9 billion km
Mission end
30 Sep 2016, controlled descent to the comet
Why it mattered
Rosetta achieved two firsts no one had managed in nearly fifty years of spaceflight: orbiting a comet and landing on one. Its measurements of 67P's water and organic chemistry, including a deuterium ratio unlike Earth's oceans, rewrote theories of where our water came from, and its two-year escort of an awakening comet turned cometary science from snapshots into cinema. Operationally, it proved that solar power, decade-long cruises and autonomous hibernation were viable, a template every long-haul deep-space mission since has drawn on.
▸Rosetta's original target was comet 46P/Wirtanen; a December 2002 Ariane 5 failure grounded the mission for over a year and forced ESA to pick a new comet entirely.
▸It slept in deep-space hibernation for 31 months, from 8 June 2011 to 20 January 2014, then woke itself with an onboard alarm clock; ESA's announcement was simply 'Hello, world!'
▸Philae's harpoons never fired and it bounced twice, the first bounce lasting nearly two hours; its final resting place was only spotted in Rosetta's images on 2 September 2016, weeks before the mission ended.
▸The names come from the Rosetta Stone and the Philae obelisk, the keys that unlocked Egyptian hieroglyphs; the mission was pitched as deciphering the solar system's oldest records.
▸It was the first spacecraft to fly near Jupiter's orbit using solar cells as its main power source, where every predecessor at such distances had carried nuclear generators.
