67P/Churyumov–Gerasimenko

"Rosetta's comet" redirects here. For the spacecraft, see Rosetta (spacecraft).

67P/Churyumov–Gerasimenko

Greyscale photograph of Comet Churyumov–Gerasimenko taken by the Rosetta spacecraft
Discovery
Discovered by	Klim Ivanovych Churyumov Svetlana Ivanovna Gerasimenko
Discovery site	Alma-Ata, Kazakh SSR, Soviet Union Kiev, Ukrainian SSR, Soviet Union
Discovery date	20 September 1969
Designations
Alternative names	1969 R1, 1969 IV, 1969h, 1975 P1, 1976 VII, 1975i, 1982 VIII, 1982f, 1989 VI, 1988i
Orbital characteristics
Epoch 10 August 2014 (JD 2456879.5)
Aphelion	5.6829 AU (850,150,000 km)
Perihelion	1.2432 AU (185,980,000 km)
Semi-major axis	3.4630 AU (518,060,000 km)
Eccentricity	0.64102
Orbital period	6.44 yr
Mean anomaly	303.71°
Inclination	7.0405°
Longitude of ascending node	50.147°
Argument of perihelion	12.780°
Physical characteristics
Dimensions	Large lobe: 4.1×3.3×1.8 km (2.5×2.1×1.1 mi) Small lobe: 2.6×2.3×1.8 km (1.6×1.4×1.1 mi)
Volume	21.4 km3 (5.1 cu mi)
Mass	(1.0±0.1)×1013 kg
Mean density	0.533 ± 0.006 g/cm3 (0.01926 ± 0.00022 lb/cu in)
Escape velocity	est. 1 m/s (3 ft/s)
Rotation period	12.4043±0.0007 h
Axial tilt	52°
North pole right ascension	69.3°
North pole declination	64.1°
Albedo	0.06
Surface temp. min mean max Kelvin 180 230 Celsius −93 −43 Fahrenheit −135 −45

67P/Churyumov–Gerasimenko (abbreviated as 67P or 67P/C-G, and written in Cyrillic as комета Чурюмова — Герасименко) is a comet, originally from the Kuiper belt, with a current orbital period of 6.45 years, a rotation period of approximately 12.4 hours and a maximum velocity of 135,000 km/h (38 km/s; 84,000 mph). Churyumov–Gerasimenko is approximately 4.3 by 4.1 km (2.7 by 2.5 mi) at its longest and widest dimensions. It was first observed on photographic plates in 1969 by Soviet astronomers Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko, after whom it is named. It came to perihelion (closest approach to the Sun) on 13 August 2015.

Churyumov–Gerasimenko was the destination of the European Space Agency's Rosetta mission, launched on 2 March 2004. Rosetta rendezvoused with Churyumov–Gerasimenko on 6 August 2014 and entered orbit on 10 September 2014. Rosetta's lander, Philae, touched down on its surface on 12 November 2014, becoming the first spacecraft to land on a comet nucleus.

Discovery

Churyumov–Gerasimenko was discovered in 1969 by Klim Ivanovych Churyumov of the Kiev University's Astronomical Observatory, who examined a photograph that had been exposed for comet Comas Solà by Svetlana Ivanovna Gerasimenko on 11 September 1969 at the Alma-Ata Astrophysical Institute, near Alma-Ata (now Almaty), the then-capital city of Kazakh Soviet Socialist Republic, Soviet Union. Churyumov found a cometary object near the edge of the plate, but assumed that this was comet Comas Solà.

After returning to his home institute in Kiev, Churyumov examined all the photographic plates more closely. On 22 October, about a month after the photograph was taken, he discovered that the object could not be Comas Solà, because it was about 1.8 degrees off the expected position. Further scrutiny produced a faint image of Comas Solà at its expected position on the plate, thus proving that the other object was a different comet.

Shape

The two-lobe shape of the comet is the result of a gentle, low-velocity collision of two objects. The "terraces", layers of the interior of the comet that have been exposed by the partial stripping of outer layers during its existence, are oriented in different directions in the two lobes, indicating that two objects fused to form Churyumov–Gerasimenko.

Surface

There are 19 distinct regions on Churyumov–Gerasimenko. Each region is named after an Egyptian deity.

Region	Terrain	Named after
Ma'at	Dust covered	Ma'at
Ash	Dust covered	Ash
Babi	Dust covered	Babi
Seth	Pitted and brittle material	Set
Hatmehit	Large-scale depression	Hatmehit
Nut	Large-scale depression	Nut
Aten	Large-scale depression	Aten
Hapi	Smooth	Hapi
Imhotep	Smooth	Imhotep
Anubis	Smooth	Anubis
Maftet	Rock-like	Maftet
Bastet	Rock-like	Bastet
Serqet	Rock-like	Serqet
Hathor	Rock-like	Hathor
Anuket	Rock-like	Anuket
Khepry	Rock-like	Khepry
Aker	Rock-like	Aker
Atum	Rock-like	Atum
Apis	Rock-like	Apis

Gates

Features known as "Gates", twin prominences on the surface so named for their appearance, have received names by the Rosetta Science Working Team. They are named after deceased members of the Rosetta team.

Name	Named after
C. Alexander Gate	Claudia Alexander
A. Coradini Gate	Angioletta Coradini

Orbit and rotation

In February 1959, a close encounter with Jupiter moved Churyumov–Gerasimenko's perihelion inward to about 1.3 AU (190,000,000 km), where it remains today. Before that, its perihelion distance was approximately 2.7 AU (400,000,000 km).

Before Churyumov–Gerasimenko's perihelion passage in 2009, its rotational period was 12.76 hours. During this perihelion passage, it decreased to 12.4 hours, which likely happened due to sublimation-induced torque.

2015 perihelion

As of September 2014, Churyumov–Gerasimenko's nucleus had an apparent magnitude of roughly 20. It came to perihelion on 13 August 2015. From December 2014 until September 2015, it has an elongation less than 45 degrees from the Sun. On 10 February 2015, it went through solar conjunction when it was 5 degrees from the Sun and was 3.3 AU (490,000,000 km) from Earth. It crossed the celestial equator on 5 May 2015 and became easiest to see from the Northern Hemisphere. Even right after perihelion when it was in the constellation of Gemini, it only brightened to about apparent magnitude 12, and required a telescope to be seen. As of August 2015, the comet has a total magnitude of about 12.

Exploration

This animation consists of 86 images acquired by Rosetta's NavCam as it approached 67P in August 2014.

Main articles: Rosetta (spacecraft) and Philae (spacecraft)

See also: Timeline of Rosetta spacecraft

Churyumov–Gerasimenko was the destination of the Rosetta mission, launched in 2004, which rendezvoused with it in 2014 and was the first mission to land a space probe on a comet.

Advance work

As preparation for the Rosetta mission, Hubble Space Telescope pictures taken on 12 March 2003 were closely analysed. An overall 3D model was constructed and computer-generated images were created.

On 25 April 2012, the most detailed observations until that time were taken with the 2-metre Faulkes Telescope by N. Howes, G. Sostero and E. Guido while it was at its aphelion.

On 6 June 2014, water vapor was detected being released at a rate of roughly 1 L/s (0.26 USgal/s) when Rosetta was 360,000 km (220,000 mi) from Churyumov–Gerasimenko and 3.9 AU (580,000,000 km) from the Sun. On 14 July 2014, images taken by Rosetta showed that its nucleus is irregular in shape with two distinct lobes. One explanation is that it is a contact binary formed by low-speed accretion between two comets, but it may instead have resulted from asymmetric erosion due to ice sublimating from its surface to leave behind its lobed shape. The size of the nucleus is estimated to be 3.5×4 km (2.2×2.5 mi).

Rendezvous and orbit

Beginning in May 2014, Rosetta's velocity was reduced by 780 m/s (2,800 km/h; 1,700 mph) with a series of thruster firings. Ground controllers rendezvoused Rosetta with Churyumov–Gerasimenko on 6 August 2014. This was done by reducing Rosetta's relative velocity to . Rosetta entered orbit on 10 September, at about 30 km (19 mi) from the nucleus.

Landing

For more details on the comet landing, see Philae mission.

Descent of a small lander occurred on 12 November 2014. Philae is a 100 kg (220 lb) robotic probe that set down on the surface with landing gear. The landing site has been christened Agilkia in honour of Agilkia Island, where the temples of Philae Island were relocated after the construction of the Aswan Dam flooded the island. The acceleration due to gravity on the surface of Churyumov–Gerasimenko has been estimated for simulation purposes at 10⁻³ m/s², or about one ten-thousandth of that on Earth.

Due to its low relative mass, landing on the comet involved certain technical considerations to keep Philae anchored. The probe contains an array of mechanisms designed to manage Churyumov–Gerasimenko's low gravity, including a cold gas thruster, harpoons, landing-leg-mounted ice screws, and a flywheel to keep it oriented during its descent. During the event, the thruster and the harpoons failed to operate, and the ice screws did not gain a grip. The lander bounced twice and only came to rest when it made contact with the surface for the third time, two hours after first contact.

Contact with Philae was lost on 15 November 2014 due to dropping battery power. The European Space Operations Centre reestablished communications on 14 June 2015 and reported a healthy spacecraft.

Science

The composition of water vapor from Churyumov–Gerasimenko, as determined by the Rosetta spacecraft, is substantially different from that found on Earth. The ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water. This makes it unlikely that water found on Earth came from comets such as Churyumov–Gerasimenko. On 22 January 2015, NASA reported that, between June and August 2014, the comet released increasing amounts of water vapor, up to tenfold as much. On 23 January 2015, the journal Science published a special issue of scientific studies related to the comet.

Measurements carried out before Philae's batteries failed indicate that the dust layer could be as much as 20 cm (7.9 in) thick. Beneath that is hard ice, or a mixture of ice and dust. Porosity appears to increase toward the center of the comet.

The nucleus of Churyumov–Gerasimenko was found to have no magnetic field of its own after measurements were taken during Philae's descent and landing by its ROMAP instrument and Rosetta's RPC-MAG instrument. This suggests that magnetism may not have played a role in the early formation of the Solar System, as had previously been hypothesized.

The ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.6 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Also, active pits, related to sinkhole collapses and possibly associated with outbursts are present on the comet.

Measurements by the COSAC and Ptolemy instruments on the Philae's lander revealed sixteen organic compounds, four of which were seen for the first time on a comet, including acetamide, acetone, methyl isocyanate and propionaldehyde. Astrobiologists Chandra Wickramasinghe and Max Wallis stated that some of the physical features detected on the comet's surface by Rosetta and Philae, such as its organic-rich crust, could be explained by the presence of extraterrestrial microorganisms. Rosetta program scientists dismissed the claim as "pure speculation". Carbon-rich compounds are common in the Solar System. Neither Rosetta nor Philae is equipped to search for direct evidence of viable organisms.

One of the most outstanding discoveries of the mission so far is the detection of large amounts of free molecular oxygen (O
2) gas surrounding the comet. Current solar system models suggest the molecular oxygen should have disappeared by the time 67P was created, about 4.6 billion years ago in a violent and hot process that would have caused the oxygen to react with hydrogen and form water. Molecular oxygen has never before been detected in cometary comas. In situ measurements indicate that the O
2/H
2O ratio is isotropic in the coma and does not change systematically with heliocentric distance, suggesting that primordial O
2 was incorporated into the nucleus during the comet's formation. Detection of molecular nitrogen (N
2) in the comet suggests that its cometary grains formed in low-temperature conditions below 30 K (−243.2 °C; −405.7 °F).

Gallery

• 

  A reconstruction of the nucleus's shape based on 2003 Hubble observations

• 

  As seen by the Very Large Telescope on 11 August 2014

• 

  As seen by Rosetta on 22 August 2014

• 

  As seen by Rosetta on 14 September 2014

• 

  As seen by Rosetta on 7 July 2015

See also

• List of comets visited by spacecraft
• List of numbered comets

External links

• 67P/Churyumov–Gerasimenko at Cometography.com
• 67P/Churyumov–Gerasimenko at the Wayback Machine (archived 11 November 2007) by the Instituto de Astrofísica de Canarias
• 67P/Churyumov–Gerasimenko pronunciation guide by the European Space Agency

Articles

• "Rosetta's Target: Comet 67P/Churyumov–Gerasimenko" by the European Space Agency
• "Mission to Land on a Comet" by NASA

Media

• Churyumov–Gerasimenko image gallery by the European Space Agency
• ESA's Rosetta image gallery at Flickr.com
• Churyumov–Gerasimenko size comparison by the European Space Agency
• Rosetta: landing on a comet by the European Space Agency

Periodic comets (by number)
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