A small European spacecraft called Philae became the first machine in human history to soft-land on a comet — bouncing twice in near-zero gravity before settling in a shadowed crevice on Comet 67P — after a 10-year, 6.4-billion-kilometre journey across deep space, where it transmitted scientific readings home for roughly 60 hours before its batteries finally died

Imagine throwing a refrigerator at a mountain travelling 135,000 kilometres per hour through deep space, after a ten-year flight, and asking the refrigerator to land softly enough to keep ten scientific instruments intact. This is, in essential respects, what the European Space Agency attempted in November 2014, and remarkably it almost entirely worked. The mother spacecraft, Rosetta, had launched from Kourou in French Guiana on 2 March 2004, carrying Philae folded against its side, on a trajectory that would take both vehicles past Earth three times and Mars once for gravity-assist accelerations, past two asteroids (21 Lutetia and 2867 Šteins) for scientific flybys, and across approximately ten and a half years of continuous deep-space travel — much of it in deliberate hibernation, with most systems powered down to conserve energy across the long stretches between gravity assists. The original target had been a different comet entirely, Comet 46P/Wirtanen, but a failure of an Ariane 5 rocket in late 2002 had delayed the launch and forced the European Space Agency to select a different target. The new target was Comet 67P/Churyumov-Gerasimenko, a 4-kilometre-wide irregular ice-and-dust body in a 6.5-year orbit between Jupiter and the inner solar system, named after the two Soviet astronomers who had discovered it in 1969. Rosetta and Philae arrived at the comet on 6 August 2014. Rosetta became the first spacecraft in history to enter orbit around a comet. Philae’s planned descent followed three months later.

According to the European Space Agency’s official announcement of the Philae touchdown, the descent itself began at 08:35 UTC on 12 November 2014, when Philae separated from Rosetta at a distance of approximately 22.7 kilometres from the comet’s centre. The lander then drifted slowly toward the surface for approximately seven hours at a relative velocity of about one metre per second — roughly the pace of a brisk walk. The plan, on touchdown, was for two harpoons fired from the bottom of the lander to anchor it to the comet’s surface and for a small thruster on the top of the lander to push it down against the recoil. The combined gravitational pull of the comet on the 100-kilogram lander was approximately one-hundred-thousandth of Earth’s surface gravity — so weak that any upward force on Philae’s body would, without the anchoring systems, simply send it bouncing back into space. The first touchdown occurred at 15:34 UTC at a site the mission team had named Agilkia, after the Egyptian island where the original Philae obelisk now sits.

Why the lander bounced

The anchoring systems did not work. The harpoons did not fire. The top-mounted thruster did not fire. The ice screws built into the landing legs could not penetrate the unexpectedly hard surface material of the comet — which, contrary to the team’s expectations of soft snow-like consistency, turned out to be more like solid water ice. The result was that Philae, on contact, behaved exactly as a 100-kilogram object behaves when it touches a surface at one metre per second in essentially zero gravity with no anchoring of any kind: it bounced. The first bounce sent the lander approximately one kilometre back up into the comet’s tenuous atmosphere, where it drifted slowly for nearly two hours before falling back to the surface for a second bounce, and then a final, much smaller bounce, before coming to rest at 17:31 UTC at a location the team named Abydos — about a kilometre from the originally planned landing site, against the wall of a cliff, in a position where direct sunlight reached the lander’s solar panels for only approximately 1.5 hours per 12.4-hour comet rotation period, rather than the 6 to 7 hours per rotation that the mission team had planned for.

The science programme proceeded anyway. As reported in a comprehensive reference summary of the Philae mission and its scientific operations on the comet, all ten of the lander’s onboard scientific instruments were operated at least once during the approximately 57 hours that Philae remained active on the comet’s surface — drawing on the energy stored in the lander’s primary battery, which had been charged on the journey out and was not dependent on solar power for the initial science sequence. The instruments captured the first photographs ever taken from the surface of a comet, performed the first in-situ chemical analysis of cometary nucleus material, detected organic compounds including the amino-acid precursor glycine, measured the magnetic and thermal properties of the surface, and characterised the dust environment in the immediate vicinity of the lander. At 00:08 UTC on 15 November 2014, the primary battery was exhausted. The lander entered hibernation. The mission team’s hope was that as the comet’s orbit took it closer to the Sun in the subsequent months, additional sunlight on the solar panels might allow the secondary battery to recharge sufficiently to wake the lander up. This hope was substantially realised — Philae communicated sporadically with Rosetta between 13 June and 9 July 2015 — but the contact never stabilised into the sustained science operations the team had hoped for, and after 9 July 2015 no further signals were received.

What the mission found

Per National Geographic’s overview of the scientific discoveries that emerged from the Rosetta and Philae mission across the subsequent years of analysis, the combined results from the orbiter and the lander substantially revised the existing scientific understanding of cometary composition and the role of comets in the early solar system. The water on Comet 67P, as measured by Rosetta’s mass spectrometers, has a deuterium-to-hydrogen ratio approximately three times higher than the water in Earth’s oceans — which substantially weakened the long-standing hypothesis that the bulk of Earth’s water was delivered by comet impacts during the early bombardment of the inner solar system. Cometary water, in 67P at least, does not match Earth water. Asteroids or another source must have supplied the difference. The mission also identified complex organic molecules in the comet’s outgassing material, including molecular oxygen (a finding the team had not predicted and could not initially explain), glycine, methylamine, ethylamine, and other carbon-based compounds that are precursors to the building blocks of biology. The two-lobed “rubber duck” shape of Comet 67P, photographed at high resolution by Rosetta’s cameras, turned out to be the result of two separate cometesimals having gently merged in the early solar system rather than a single body having been eroded into the irregular form.

As described in the comprehensive reference on the broader Rosetta mission and its final operational disposition, the orbiter itself was deliberately crashed into the comet on 30 September 2016, after the comet’s outbound orbit had carried it too far from the Sun for Rosetta’s solar panels to continue generating sufficient power. Philae’s exact landing location was identified in photographs taken by Rosetta’s high-resolution camera less than a month before the orbiter’s own demise — the lander wedged into a dark crack on the comet’s smaller lobe, its three legs visible in the image, exactly where the bounces had carried it on the afternoon of 12 November 2014. The two spacecraft now sit on the surface of Comet 67P together, accompanying it through its 6.5-year orbits around the Sun for the foreseeable astronomical future, the first human artefacts ever to permanently reside on the surface of a comet.