Rosetta's comet
Comet 67P/Churyumov–Gerasimenko
No comet has been studied as closely as 67P/Churyumov–Gerasimenko. Europe's Rosetta spacecraft reached it in 2014, stayed alongside for more than two years as it approached the Sun and moved away again, and sent down the Philae lander: the first human-made object to touch down on a comet's nucleus.
Perihelion passage of 13 August 2015, from the 3D simulation
- Nucleus
- 3.4 km
- Orbital period
- 6.44 years
- Perihelion
- 1.21 AU
- Aphelion
- 5.71 AU
- Discovered
- 1969
- Next perihelion
- 9 April 2028
Nucleus and tail
The nucleus of 67P is made of two lobes joined by a thin neck, and looks like a rubber duck: it measures about 4.3 km at its widest and has the volume of a sphere 3.4 km across. It is as dark as coal and surprisingly light: its density is about half that of water, because more than 70% of its interior is empty space. Like Arrokoth, it was born from the slow merger of two separate bodies.
As it nears the Sun, ice beneath the surface sublimates and escapes in jets that drag dust along: Rosetta watched them switch on at dawn and off at dusk, witnessed the collapse of cliffs a hundred metres high and, around perihelion, measured a water loss of up to 300 kg per second. It is nonetheless a weakly active comet: its tail is short, and it is never visible to the naked eye from Earth.
Orbit
67P is a Jupiter-family comet: its aphelion, 5.7 AU from the Sun, grazes the orbit of the giant planet, which controls its fate. It completes one lap in 6.44 years and at perihelion comes to 1.21 AU, between the orbits of Earth and Mars. Its orbit is tilted 3.9° to the ecliptic.
It was not always so. Until 1840 its perihelion lay at about 4 AU, too far for the ice to sublimate; two close encounters with Jupiter, the latest in February 1959, brought it to its present distance. Before 1959 its perihelion was at 2.7 AU: it is only since then that the comet has been as active as we see it today. The nucleus rotates in about 12.4 hours.
Discovery and exploration
It was discovered in 1969: the astronomer Klim Churyumov spotted it in Kyiv while examining a photographic plate taken by Svetlana Gerasimenko on 11 September in Alma-Ata, in present-day Kazakhstan, while they were looking for another comet. It was not Rosetta's original target: it was chosen in 2003, after a launch delay put the intended comet, 46P/Wirtanen, out of reach.
Rosetta, launched on 2 March 2004, reached the comet on 6 August 2014 after a ten-year journey. On 12 November it released Philae, which bounced twice and came to rest in a shadowed spot, working for about 60 hours before its batteries ran out. The spacecraft followed the comet for two more years and on 30 September 2016 ended its mission by setting down on the nucleus itself.
Frequently asked questions
What did Rosetta discover?
Many unexpected things. That the nucleus is porous and made of two lobes; that it contains molecular oxygen and complex organic molecules, including glycine, an amino acid, and phosphorus; and that a comet's activity changes hour by hour. Above all, it observed an entire perihelion passage from close range, something never done before.
What happened to Philae?
It is still on the comet. After landing on 12 November 2014 it bounced and ended up wedged on its side beneath a rock wall, where its solar panels received too little light. It transmitted for about 60 hours, woke up briefly in June 2015 and was found in Rosetta's images on 2 September 2016, a few weeks before the mission ended.
Did Earth's water come from comets?
Not from comets like this one. Rosetta measured in 67P's water an amount of deuterium, heavy hydrogen, about three times higher than in Earth's oceans. The result strengthened the idea that Earth's water came mainly from asteroids, although other comets have shown values closer to ours.
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