Rosetta (spacecraft)

• 1. Introduction

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  Rosetta was a space probe built by the European Space Agency that, on the 12th of November 2014, placed a lander named Philae on the surface of a comet for the first time in human history. That comet was 67P/Churyumov-Gerasimenko, a peculiar two-lobed object tumbling through space hundreds of millions of kilometres from Earth. The mission cost approximately 1.3 billion euros. It took a decade of flight to reach its target. And when it ended on the 30th of September 2016, it ended on purpose, the spacecraft guided down to a controlled impact on the comet's surface. What drove engineers and scientists to attempt something so audacious? What did they find when they got there? And what does comet dust, billions of years old, have to do with the origins of life on Earth?

• 2. Named for a Stone

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  The probe was named after the Rosetta Stone, a stele of Egyptian origin bearing a single decree written in three different scripts. That stone cracked open the mystery of Egyptian hieroglyphics; the spacecraft was expected to crack open mysteries about the early Solar System. The lander, Philae, took its name from the Philae obelisk, whose bilingual Greek and Egyptian hieroglyphic inscription helped catalyse the deciphering of the Egyptian writing system when its hieroglyphs were compared with those on the Rosetta Stone. The naming was not merely decorative. Rosetta also carried a micro-etched pure nickel disc donated by the Long Now Foundation, inscribed with 6,500 pages of language translations, a physical echo of the multilingual legacy its namesake represents.

• 3. A Mission Rebuilt from Scratch

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  The mission that became Rosetta was not the mission originally planned. In the early 1990s, both ESA and NASA were developing comet probes cooperatively. NASA's project was called the Comet Rendezvous Asteroid Flyby mission, or CRAF, while ESA was developing the Comet Nucleus Sample Return mission. Both were to share the Mariner Mark II spacecraft design to keep costs down. In 1992, NASA cancelled CRAF because of budget constraints, and ESA found itself alone. By 1993, ESA concluded that the ambitious sample return concept was beyond its existing budget and redesigned the mission entirely, arriving at a plan that resembled the cancelled CRAF: an asteroid flyby, a comet rendezvous, in-situ science, and a lander. The original target was comet 46P/Wirtanen, with a launch planned for the 12th of January 2003. That plan collapsed when an Ariane 5 ECA rocket failed during the launch of Hot Bird 7 on the 11th of December 2002. In May 2003, a new target was chosen: comet 67P/Churyumov-Gerasimenko, with a revised launch date in February 2004. The larger mass of the new comet forced engineers to redesign the landing gear for a harder impact velocity. Both co-discoverers of that comet, Klim Churyumov and Svetlana Gerasimenko, were present at the spaceport on launch day.

• 4. Ten Years Through the Solar System

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  Rosetta finally lifted off on the 2nd of March 2004 at 07:17 UTC from the Guiana Space Centre in Kourou, French Guiana, on an Ariane 5 G+ rocket. To gather enough speed to intercept 67P, the spacecraft used a sequence of gravity assists, swinging around planets to accelerate without burning extra fuel. The first Earth flyby occurred on the 4th of March 2005. During its second Earth flyby in November 2007, Rosetta was briefly mistaken for a near-Earth asteroid roughly 20 metres in diameter by an astronomer working with the Catalina Sky Survey, and was given a provisional asteroid designation before another astronomer, Denis Denisenko, recognised the trajectory as matching Rosetta. The most dangerous moment of the cruise phase came on the 25th of February 2007, when Rosetta skimmed past Mars at an altitude of only 250 km. For 15 minutes the spacecraft was in the planet's shadow, its solar panels useless, flying on batteries that were not originally designed for such a role. The manoeuvre was nicknamed "The Billion Euro Gamble." It worked. In September 2008, Rosetta flew within 800 km of asteroid 2867 Steins at a relative speed of 8.6 km/s. In July 2010, it passed asteroid 21 Lutetia at a minimum distance of 3,168 km, capturing 462 images covering roughly half of the asteroid's surface. In June 2011, Rosetta was put into hibernation for a planned 31 months, most of its electronics switched off to conserve power during the long cold journey toward Jupiter's orbit.

• 5. Arrival and the Philae Landing

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  Rosetta woke from hibernation in January 2014 and began a series of eight engine burns starting in May to slow its approach to 67P. The burns reduced the relative velocity between the spacecraft and the comet from 775 metres per second to just 7.9 metres per second. On the 6th of August 2014, the spacecraft entered orbit, the first time any probe had done so around a comet nucleus. Before Rosetta's arrival, the shape of 67P was unknown. Images from the onboard OSIRIS camera revealed an irregular, two-lobed body. Five potential landing sites were identified by the 25th of August 2014; on the 15th of September, ESA announced Site J, renamed Agilkia after a public contest, as the destination for Philae. On the 12th of November 2014, at 08:35 UTC, Philae detached from Rosetta and descended toward the comet at roughly 1 metre per second. It touched down initially at 15:33 UTC, but the harpoons designed to anchor it to the surface failed to fire. The comet's escape velocity was only around 1 metre per second, and Philae bounced twice before coming to rest at 17:33 UTC in the shadow of a cliff, tilted at about 30 degrees. Unable to collect adequate solar power from that position, Philae's batteries ran out within three days. Despite that, the lander still managed to identify sixteen organic compounds in the comet's atmosphere, including acetamide, acetone, methyl isocyanate, and propionaldehyde, four of which had never before been detected on a comet.

• 6. What the Comet Revealed

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  Among the first surprises from the mission was a discovery about 67P's magnetic field. Instruments detected that it oscillated at 40-50 millihertz. A German composer turned the measured data into an audible rendition, and observers compared the resulting sound to Continuum for harpsichord by the composer Gyorgy Ligeti. Later analysis from the Philae landing showed the comet's nucleus itself has no magnetic field; the oscillating signal Rosetta detected was likely produced by the solar wind. On the chemistry front, the VIRTIS spectrometer found nonvolatile organic macromolecular compounds across the surface of 67P, with little visible water ice. Carbon appeared to be present in polyaromatic organic solids mixed with sulfides and iron-nickel alloys. The COSAC and Ptolemy instruments on Philae also detected glycine, the only amino acid found, along with its precursor molecules methylamine and ethylamine. One longstanding question about life's origins involves why the amino acids used by living organisms are almost exclusively left-handed molecules. A hypothesis proposed in the 1980s by William A. Bonner and Edward Rubenstein of Stanford University suggested that circularly polarised radiation from a supernova could have selectively destroyed one chiral type, seeding space with the surviving handedness. Rosetta could not test this hypothesis, because glycine is non-chiral, presenting no preference. Later asteroid sample return missions Hayabusa2 and OSIRIS-REx found largely equal mixtures of left- and right-handed amino acids on their target asteroids, suggesting that whatever selected the left-handed bias for life on Earth happened here, not in space. On water, early Rosetta data showed the ratio of deuterium to hydrogen in 67P's water vapour was roughly three times that found on Earth, seeming to rule out comets as the source of Earth's oceans. A re-analysis published in November 2024 identified a significant measurement error in that original data and concluded that 67P's water isotopic composition is actually similar to Earth's, keeping the comet-delivery hypothesis alive.

• 7. End of Mission

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  Between November 2014 and December 2015, Rosetta escorted comet 67P around the Sun, conducting riskier and closer observations as the comet warmed and became more active. As 67P moved away from the Sun again, the solar panels on Rosetta received progressively less light. Engineers considered putting the spacecraft into a second hibernation but concluded there was no reliable way to keep it from freezing during the comet's aphelion. On the 23rd of June 2015, ESA confirmed a mission extension and announced the spacecraft would be guided to a controlled impact on the comet's surface at the end of September 2016. A 19 km descent began with a 208-second thruster burn at approximately 20:50 UTC on the 29th of September 2016. The final data packet was transmitted by the OSIRIS instrument at 10:39:28.895 UTC and received at the European Space Operations Centre in Darmstadt at 11:19:36.541 UTC. The spacecraft struck the surface of 67P at an estimated speed of 3.2 km/h, landing in a region called Ma'at, at a site the operations team named Sais after the original temple home of the Rosetta Stone. At the moment of contact, Rosetta's computer entered safe mode, turning off its radio transmitter in accordance with International Telecommunication Union rules. From mission control, Spacecraft Operations Manager Sylvain Lodiot said: "All stations and the briefing room, we've just had loss of signal at the expected time. This is another outstanding performance by flight dynamics. This is the end of the Rosetta mission. Thank you, and goodbye." In September 2016, before impact, Rosetta came close enough to the comet to finally photograph Philae's exact resting place, providing the context needed to properly interpret the two days of science the lander had transmitted two years earlier.

• 8. Rosetta in Culture

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  The Rosetta mission generated a public following unusual for a deep-space probe, partly through a deliberate media strategy. ESA commissioned an animated web series titled Once upon a time..., produced by German studio Design & Data GmbH, which gave both spacecraft anthropomorphic personalities. Rosetta was depicted as an older sister and Philae as her younger brother. The characters, designed by ESA cartoonist Carlo Palazzari, were inspired by JAXA's anime-style spacecraft mascots, and ESA employees role-played as the characters on Twitter throughout the mission. Twelve videos were produced from 2013 to 2016, with a 25-minute compilation released in December 2016. In 2019, Design & Data adapted the series into a 26-minute planetarium show commissioned by the Swiss Museum of Transport and distributed to eighteen planetariums across Europe. A separate short film, Ambition, was produced with Polish visual effects company Platige Image and shot in Iceland. It stars Irish actor Aidan Gillen and Irish actress Aisling Franciosi, both known for Game of Thrones, and was directed by the Oscar-nominated Polish director Tomasz Baginski. The film premiered at the British Film Institute on the 24th of October 2014, three weeks before Philae landed. At that premiere, science fiction author Alastair Reynolds told the audience that future generations "may look back to Rosetta with the same sense of admiration that we reserve for, say, Columbus or Magellan." On the 23rd of September 2016, the musician Vangelis released a studio album titled Rosetta in honour of the mission, which was used during the ESA's "Rosetta Grand Finale" livestream event on the day of impact.