Two years alongside a rubber-duck-shaped comet
67P/Churyumov–Gerasimenko was not merely a point of light glimpsed through a telescope: it was the first comet accompanied by a spacecraft around the Sun while its seasons changed. Rosetta and Philae recorded, at close range, how a small nucleus of ice and dust responds to sunlight by producing a coma and tails, collapsing cliffs, and releasing ancient material into space.
- Orbital period
About 6.45 years - Rotation period
About 12.4 hours - Mass
About 10 billion tonnes - Mean density
About 533 kg/m³
A small world made of two touching bodies
67P’s nucleus is not a round ball of ice. Its large lobe measures about 4.1 × 3.3 × 1.8 km and its small lobe about 2.6 × 2.3 × 1.8 km; a narrow “neck” joins the two. The shape, often compared to a rubber duck, is consistent with two small bodies meeting in a slow collision, although the details of its formation and evolution remain under study.
Rosetta’s radio-science experiment and three-dimensional shape models yielded a density of about 533 kg/m³, only a little more than half that of water. This is not because the ice-and-dust material itself is exceptionally light, but because microscopic voids make up roughly 70–80% of the interior. That does not mean one enormous cavern hollows out the nucleus: gravitational measurements support a fairly uniform porous structure without large internal voids.
An orbit reshaped by Jupiter, a surface awakened by the Sun
67P is a Jupiter-family comet. Repeated close encounters with Jupiter—especially gravitational perturbations in 1840 and 1959—shifted its orbit inward, and it now circles the Sun about every 6.45 years. Its aphelion lies near and beyond Jupiter’s orbit, while perihelion is about 1.3 AU, between the orbits of Earth and Mars.
Far from the Sun, the nucleus is dark and only weakly active. As it approaches, sunlight warms ice at the surface and in the shallow subsurface. The ice sublimates directly into gas without passing through a liquid phase. Escaping gas carries dust outward to form a coma around the nucleus; solar radiation pressure pushes the dust into a broad, curved tail, while the solar wind transports ionized gas along the magnetic-field direction into a nearly straight ion tail. Jets rising from the surface are local eruptions that supply material to these two enormous tails, not the tails themselves.
67P’s shape and tilted rotation axis distribute sunlight unevenly. The northern hemisphere experiences a long summer under weaker sunlight when the comet is farther away, whereas the south has a short, intense summer near perihelion. Rosetta observed both jets that switched on regularly as sunlight reached their source regions and sudden outbursts caused by cliff collapse or newly exposed ice.
Rosetta did not encounter an unchanged “frozen fossil.” It found an active world that preserves ancient material yet creates fresh surfaces through cracking, erosion, and collapse on every approach to the Sun.
A ten-year pursuit and three touchdowns
- 1969 — Klim Churyumov identified a new comet on photographic plates taken by Svetlana Gerasimenko.
- 2 March 2004 — ESA launched Rosetta. Three Earth gravity assists and one at Mars helped it chase the target.
- 6 August 2014 — After ten years and roughly 6.4 billion km, Rosetta arrived at 67P and became the first spacecraft to rendezvous with a comet.
- 12 November 2014 — The Philae lander separated and touched the surface. Its anchoring harpoons and screws failed to operate properly, so it bounced, made three contacts, and stopped in the shaded site Abydos.
- 13 August 2015 — The comet passed perihelion. Rosetta continuously observed activity rise toward its peak and then decline.
- 30 September 2016 — Rosetta ended its mission while sending final data during a controlled descent to the nucleus.
Philae’s landing was not a success story in which everything went to plan. Because its anchors failed, it traveled more than 1 km from the first contact point and stopped where sunlight was scarce. Even so, before its battery ran out it performed several experiments to measure the surface and interior. The unexpected journey itself provided data for comparing the strength and structure of different locations.
Terrain transformed near the Sun
Rosetta’s before-and-after images revealed growing fractures, collapsed cliffs, displaced boulders, newly exposed ice, and dust deposited elsewhere. Some dust jets could be traced back to round pits or cliffs. Around the 2015 perihelion, Rosetta also captured outbursts far brighter than ordinary jets that ended within tens of minutes. These changes show that cometary activity is not simply proportional to sunlight: terrain, season, thermal stress, and collapse act together to determine where and how strongly it occurs.
Oxygen, organics, and the water debate
Rosetta’s mass spectrometers detected molecular oxygen and nitrogen in the coma, as well as the amino acid glycine and phosphorus. These substances offer clues to the extremely cold environment and chemistry of the early Solar System. Finding ingredients associated with life, however, does not mean that life was discovered or that 67P was the direct origin of life on Earth.
The interpretation of water illustrates how scientific conclusions change as data are reexamined. Rosetta’s initial 2014 analysis reported a deuterium-to-hydrogen ratio in 67P’s water more than three times that of Earth’s oceans, strengthening the case that comets like it were not a major source of terrestrial water. A 2024 reanalysis of the full mission data, however, suggested that ice-coated dust near the nucleus could make the ratio appear higher, while measurements farther out in the coma, where dust has less influence, were close to terrestrial water.
The newer result reopens the possibility that Jupiter-family comets such as 67P delivered water to the young Earth, but it does not determine the fraction they supplied. The central lesson is that even the same mission data tell a more accurate story once researchers understand where the sample came from and how surrounding dust altered the measurement.
What did 67P teach us?
| Observation | What we learned | Question that remains |
|---|---|---|
| Gravity field and shape | A low-density, highly porous nucleus without large cavities | Exactly when and at what speed did the two lobes join? |
| Jets and surface change | Sublimation, seasons, and collapse alter the location and strength of activity | How much internal material moves during one perihelion passage? |
| Gas and dust composition | A cold formation environment and complex chemistry in the early Solar System | How much did comets and asteroids each contribute to Earth’s water and organics? |
The nucleus dimensions are given along the axes of its irregular lobes; reducing them to a single “diameter” produces a value that varies with viewing direction. Density and internal porosity are also approximate values based on shape and gravity models.