Comet 67P: The Rubber-Duck Comet

The duck you can see from orbit

When Rosetta's OSIRIS camera resolved 67P in July and August 2014, the nucleus revealed itself as bilobate — two distinct lobes joined by a narrower neck. Scientists immediately nicknamed the large lobe the 'body' and the small lobe the 'head'; the resemblance to a rubber duck was so strong the name stuck in press briefings and papers alike.

The whole object spans about 4.34 × 2.60 × 2.12 km along its principal axes — roughly the size of a mountain, or of Manhattan's financial district stood on end. The larger lobe is about 4.1 km across, the smaller about 2.5 km, and they meet at a smooth-sided neck region named Hapi, which turned out to be one of the most active spots on the comet.

Two leading ideas explain the shape. Either 67P is a contact binary — two separate cometesimals that drifted together and merged at very low speed in the early solar system — or it was once a single body that erosion carved a waist into. The stratified layering visible in the cliffs of both lobes, which appears to wrap each lobe independently like an onion, strongly favors the two-body merger picture. If so, 67P is a fossil of gentle accretion: a collision slow enough to stick rather than shatter, frozen in place for 4.5 billion years.

Lighter than water: what 67P is made of

The single most surprising number Rosetta returned was the comet's density: about 538 kg/m³, or 0.538 g/cm³. Water is 1.0 g/cm³, so 67P would genuinely float — if you could find a bathtub big enough. Since the ices and dust that make up a comet are individually denser than water, the only way to get such a low bulk density is enormous internal emptiness. The nucleus is estimated to be 70-80% porous: mostly void, threaded with a fluffy, weakly bound mixture.

Rosetta's instruments pinned down the ingredients — the classic 'dirty snowball' recipe, in modern detail:

• Water ice, the dominant volatile, buried under a dark insulating crust.
• Carbon dioxide and carbon monoxide ices, more volatile than water, driving activity even far from the Sun.
• Refractory dust and organics — the surface is blacker than charcoal, reflecting only about 6% of sunlight.
• Complex molecules, including the amino acid glycine and the element phosphorus (detected in solid dust particles), plus molecular oxygen (O₂) that no one expected to survive since the comet's formation.

Perhaps the most talked-about result concerned water's fingerprint. The ratio of deuterium to ordinary hydrogen (D/H) in 67P's water is about three times higher than in Earth's oceans. That mismatch argued against the popular idea that Jupiter-family comets like 67P delivered most of Earth's water — pointing more toward asteroids as the likely source, though the debate is far from settled.

An icy body on a Jupiter leash

67P belongs to the Jupiter-family comets — short-period comets whose orbits are shepherded by Jupiter's gravity. It circles the Sun once every 6.44 years, swinging from a perihelion of 1.24 AU (just outside Earth's orbit) out to an aphelion of 5.68 AU (near Jupiter's distance). Its birthplace was almost certainly the frigid Kuiper Belt and scattered disk beyond Neptune, from which it was gravitationally kicked inward.

Its current orbit is young. A close pass by Jupiter in 1959 shrank 67P's perihelion from about 2.7 AU down to roughly 1.3 AU, bringing it far closer to the Sun and cranking up its activity. In cosmic terms, the comet we studied has only recently become a vigorous outgasser — a reminder that comet orbits are constantly reshuffled.

The nucleus spins once every 12.4 hours, and it is spinning slightly faster after each perihelion passage: jets of escaping gas act like tiny misaligned rocket nozzles, torquing the body. Over the 2015 perihelion, sublimation stripped material and even widened cracks in the neck, feeding speculation that 67P may eventually split its two lobes apart entirely. Each orbit, the comet sheds on the order of a meter of surface and loses millions of tonnes of ice and dust to space.

Rosetta and Philae: escorting a comet to the Sun

67P is the only comet humanity has ever orbited and landed on. ESA's Rosetta launched in 2004, looped through the inner solar system for gravity assists (including three Earth flybys and one at Mars), spent 31 months in deep-space hibernation, and finally rendezvoused with 67P on 6 August 2014 at 3.6 AU from the Sun. For over two years it flew alongside the comet as an escort, watching it wake up on approach to perihelion and fall quiet again afterward.

On 12 November 2014, Rosetta released the washing-machine-sized lander Philae — the first spacecraft ever to make a soft landing on a comet. The landing was harrowing. Philae's anchoring harpoons failed to fire and its hold-down thruster didn't work, so in the comet's near-zero gravity it bounced: a first touchdown, a two-hour hop of up to a kilometer, a second bounce, and finally a rest on its side in a shadowed crack in a region named Abydos. Starved of sunlight for its solar panels, Philae ran about 63 hours of science on battery before going silent. Its exact resting spot wasn't pinpointed until 2 September 2016, when Rosetta imaged it wedged against a cliff.

Rosetta itself ended in style. On 30 September 2016, running low on power as 67P receded from the Sun, controllers commanded a slow, deliberate descent onto the comet, gathering ever-closer images and gas readings until impact ended the signal. Its final resting place, in the Ma'at region, lies not far from Philae — two robots retired on the same tiny world.

Jets, cliffs, and a landscape that changes

Up close, 67P is astonishingly varied for a body you could walk across in an afternoon. Rosetta mapped towering cliffs, collapsed pits, dune-like ripples, boulders, and smooth dust plains, all sculpted by sublimation rather than water or wind. The near-vertical Hathor cliff plunges some 900 meters down the underside of the small lobe, while the deep Aten depression scoops a pit into the large lobe; a house-sized boulder nicknamed Cheops squats on a dusty plain like a monument.

As the comet neared the Sun, ices beneath the crust turned straight to gas (sublimation), and the escaping vapor lifted dust off the surface to build the fuzzy coma and, eventually, tails. Rosetta caught this in the act: collimated jets erupting from cliffs and pits, a sudden outburst in 2016 traced to a landslide on a cliff face, and material shifting from one hemisphere to another. Some of the comet's activity is driven by seasons: because 67P's spin axis is tilted, its southern hemisphere gets a short, ferocious summer near perihelion that erodes it far faster than the north.

A common misconception worth correcting: a comet's tail does not stream out behind its direction of travel. It is pushed by sunlight and the solar wind, so it always points away from the Sun — meaning that on the outbound leg of the orbit, the tail actually leads the comet. 67P's activity also shuts down as it retreats: past about 3 AU there is too little sunlight to sublimate much ice, and the nucleus goes dormant until the next approach.

A chance discovery, and why it matters

The comet's clumsy double name records an accidental find. In September 1969, Soviet astronomers were surveying comets at the Alma-Ata observatory in Kazakhstan. Svetlana Gerasimenko photographed comet 32P/Comas Solá; weeks later, Klim Churyumov, examining her plates back in Kyiv, spotted a second cometary smudge near the plate's edge. He first assumed it was Comas Solá — but it sat about 1.8° off the predicted position. On 22 October 1969 he realized it was an entirely new comet, and it was named for both discoverers.

Why lavish a decade-long, billion-euro mission on such a modest object? Because comets are time capsules. Formed in the cold outer solar system 4.5 billion years ago and stored in the deep freeze of the Kuiper Belt ever since, they preserve the pristine building blocks of the planets. Rosetta's headline discoveries from 67P reshaped the field:

• Comets carry prebiotic chemistry — the detection of glycine and other organics supports the idea that comets seeded the young Earth with ingredients for life, even if not with most of its water.
• Earth's oceans probably came from asteroids, given 67P's high deuterium content — a direct, in-situ constraint no telescope could provide.
• Comet nuclei are gently assembled rubble — the ultralow density and layered lobes are evidence for slow, sticky accretion rather than violent collision.

67P remains the best-characterized comet in history. Long after Rosetta and Philae fell silent on its surface, the data they returned continues to rewrite what we know about how our solar system — and possibly life itself — got its start.

Comet 67P versus a familiar solar-system rock: why a comet is nothing like an asteroid.

Property	Comet 67P	Asteroid (e.g., Bennu)
Composition	Ice + dust + organics (a 'dirty snowball')	Rock and carbon, essentially no volatile ice
Density	0.538 g/cm³ (less than water)	~1.19 g/cm³ (rubble pile, still porous)
Behavior near Sun	Sublimates, grows a coma and tails	Stays inert — no tail
Home region	Kuiper Belt / scattered disk	Main asteroid belt (Bennu is a near-Earth object)