Rosetta (spacecraft)

Rosetta

Artist's illustration of Rosetta
Mission type	Comet orbiter/lander
Operator	European Space Agency
COSPAR ID	2004-006A
SATCAT №	28169
Website	esa.int/rosetta
Mission duration	Elapsed:
Spacecraft properties
Manufacturer	Astrium
Launch mass	Orbiter: 2,900 kg (6,400 lb) Lander: 100 kg (220 lb)
Dry mass	Orbiter: 1,230 kg (2,710 lb)
Payload mass	Orbiter: 165 kg (364 lb) Lander: 27 kg (60 lb)
Dimensions	2.8 × 2.1 × 2 m (9.2 × 6.9 × 6.6 ft)
Power	850 watts at 3.4 AU
Start of mission
Launch date	2 March 2004, 07:17 UTC (2004-03-02UTC07:17Z)
Rocket	Ariane 5G+ V-158
Launch site	Kourou ELA-3
Contractor	Arianespace
Flyby of Mars
Closest approach	25 February 2007
Distance	250 km (160 mi)
Flyby of 2867 Šteins
Closest approach	5 September 2008
Distance	800 km (500 mi)
Flyby of 21 Lutetia
Closest approach	10 July 2010
Distance	3,162 km (1,965 mi)
67P/Churyumov–Gerasimenko orbiter
Orbital insertion	6 August 2014, 09:06 UTC
Orbit parameters
Periapsis	29 km (18 mi)
Transponders
Band	S band (low gain antenna) X band (high gain antenna)
Bandwidth	from 7.8 bit/s (S band) up to 91 kbit/s (X band)
Instruments ALICE Ultraviolet Imaging Spectrometer CONSERT Comet Nucleus Sounding Experiment by Radio wave Transmission COSIMA Cometary Secondary Ion Mass Spectrometer GIADA Grain Impact Analyser and Dust Accumulator MIDAS Micro-Imaging Dust Analysis System MIRO Microwave Spectrometer for the Rosetta Orbiter OSIRIS Optical, Spectroscopic, and InfraRed Remote Imaging System ROSINA Rosetta Orbiter Spectrometer for Ion and Neutral Analysis RPC Rosetta Plasma Consortium RSI Radio Science Investigation VIRTIS Visible and Infrared Thermal Imaging Spectrometer

Rosetta is a space probe built by the European Space Agency launched on 2 March 2004. Along with Philae, its lander module, Rosetta is performing a detailed study of comet 67P/Churyumov–Gerasimenko (67P). On 6 August 2014, the spacecraft reached the comet and performed a series of manoeuvres to be captured in its orbit. On 12 November, the lander module performed the first successful landing on a comet. As of 2015, the mission continues to return data from the spacecraft in orbit and from the lander in the comet's surface. During its journey to the comet, the spacecraft flew by Mars and the asteroids 21 Lutetia and 2867 Šteins.

The probe is named after the Rosetta Stone, a stele of Egyptian origin featuring a decree in three scripts. The lander is named after the Philae obelisk, which bears a bilingual Greek and Egyptian hieroglyphic inscription. A comparison of its hieroglyphs with those on the Rosetta Stone catalysed the deciphering of the Egyptian writing system. Similarly, it is hoped that these spacecraft will result in better understanding of comets and the early Solar System. In a more direct analogy to its namesake, the Rosetta spacecraft also carries a micro-etched nickel alloy Rosetta disc donated by the Long Now Foundation inscribed with 13,000 pages of text in 1200 languages.

Mission overview

Comet Churyumov–Gerasimenko in September 2014 as imaged by Rosetta

Rosetta was launched on 2 March 2004 from the Guiana Space Centre in French Guiana on an Ariane 5 rocket and reached Comet Churyumov–Gerasimenko on 6 August 2014, becoming the first spacecraft to orbit a comet. (Previous missions had conducted successful flybys of seven other comets.) It is one of ESA's Horizon 2000 cornerstone missions. The spacecraft consists of the Rosetta orbiter, which features 12 instruments, and the Philae lander, with nine additional instruments. The Rosetta mission will orbit Comet Churyumov–Gerasimenko for 17 months and is designed to complete the most detailed study of a comet ever attempted. The spacecraft is controlled from the European Space Operations Centre (ESOC), in Darmstadt, Germany. The planning for the operation of the scientific payload, together with the data retrieval, calibration, archiving and distribution, is performed from the European Space Astronomy Centre (ESAC), in Villanueva de la Cañada, near Madrid, Spain. It has been estimated that in the decade preceding 2014, some 2,000 people assisted in the mission in some capacity.

In 2007, Rosetta made a Mars gravity assist (flyby) on its way to Comet Churyumov–Gerasimenko. The spacecraft also performed two asteroid flybys. The craft completed its flyby of asteroid 2867 Šteins in September 2008 and of 21 Lutetia in July 2010. Later, on 20 January 2014, Rosetta was taken out of a 31-month hibernation mode as it approached Comet Churyumov–Gerasimenko.

Rosetta's Philae lander successfully made the first soft landing on a comet nucleus when it touched down on Comet Churyumov–Gerasimenko on 12 November 2014. Astrophysicist Elizabeth Pearson said that although the future of the lander Philae is uncertain, Rosetta is the workhorse of the mission and its work will carry on.

History

Background

During the 1986 approach of Halley's Comet, international space probes were sent to explore the comet, most prominent among them being ESA's Giotto. After the probes returned valuable scientific information, it became obvious that follow-ons were needed that would shed more light on cometary composition and answer new questions.

Both ESA and NASA started cooperatively developing new probes. The NASA project was the Comet Rendezvous Asteroid Flyby (CRAF) mission. The ESA project was the follow-on Comet Nucleus Sample Return (CNSR) mission. Both missions were to share the Mariner Mark II spacecraft design, thus minimising costs. In 1992, after NASA cancelled CRAF due to budgetary limitations, ESA decided to develop a CRAF-style project on its own. By 1993 it was evident that the ambitious sample return mission was infeasible with the existing ESA budget, so the mission was redesigned and subsequently approved by the ESA, with the final flight plan resembling the cancelled CRAF mission: an asteroid flyby followed by a comet rendezvous with in-situ examination, including a lander. After the spacecraft launch, Gerhard Schwehm was named mission manager; he retired in March 2014.

Mission firsts

The Rosetta mission planned to achieve many historic firsts.

On its way to comet 67P, Rosetta passed through the main asteroid belt, and made the first European close encounter with several of these primitive objects. Rosetta was the first spacecraft to fly close to Jupiter's orbit using solar cells as its main power source.

Rosetta is the first spacecraft to orbit a comet nucleus, and is the first spacecraft to fly alongside a comet as it heads towards the inner Solar System. It is planned to be the first spacecraft to examine at close proximity how a frozen comet is transformed by the warmth of the Sun. Shortly after its arrival at 67P, the Rosetta orbiter dispatched the Philae lander for the first controlled touchdown on a comet nucleus. The robotic lander's instruments obtained the first images from a comet's surface and made the first in-situ analysis of its composition.

Design and construction

The Rosetta bus is a 2.8 × 2.1 × 2.0 m (9.2 × 6.9 × 6.6 ft) central frame and aluminium honeycomb platform. Its total mass is approximately 2,900 kg (6,400 lb), which includes the 100 kg (220 lb) Philae lander and 165 kg (364 lb) of science instruments. The Payload Support Module is mounted on top of the spacecraft and houses the scientific instruments, while the Bus Support Module is on the bottom and contains spacecraft support subsystems. Heaters placed around the spacecraft keep its systems warm while it is distant from the Sun. Rosetta's communications suite includes a 2.2 m (7.2 ft) steerable high-gain parabolic dish antenna, a 0.8 m (2.6 ft) fixed-position medium-gain antenna, and two omnidirectional low-gain antennas.

Electrical power for the spacecraft comes from two solar arrays totalling 64 square metres (690 sq ft). Each solar array is subdivided into five solar panels, with each panel being 2.25 × 2.736 m (7.38 × 8.98 ft). The individual solar cells are made of silicon, 200 μm thick, and 61.95 × 37.75 mm (2.44 × 1.49 in). The solar arrays generate a maximum of approximately 1,500 watts at perihelion, a minimum of 400 watts in hibernation mode at 5.2 AU, and 850 watts when comet operations begin at 3.4 AU. Spacecraft power is controlled by a redundant Terma power module also used in the Mars Express spacecraft, and is stored in four 10-A·h NiCd batteries supplying 28 volts to the bus.

Main propulsion comprises 24 paired bipropellant 10 N thrusters, with four pairs of thrusters being used for delta-v burns. The spacecraft carried 1,719.1 kg (3,790 lb) of propellant at launch: 659.6 kg (1,454 lb) of monomethylhydrazine fuel and 1,059.5 kg (2,336 lb) of dinitrogen tetroxide oxidiser, contained in two 1,108-litre (244 imp gal; 293 US gal) grade 5 titanium alloy tanks and providing delta-v of at least 2,300 metres per second (7,500 ft/s) over the course of the mission. Propellant pressurisation is provided by two 68-litre (15 imp gal; 18 US gal) high-pressure helium tanks.

Rosetta was built in a clean room according to COSPAR rules, but "sterilisation [was] generally not crucial since comets are usually regarded as objects where you can find prebiotic molecules, that is, molecules that are precursors of life, but not living microorganisms", according to Gerhard Schwehm, Rosetta's project scientist. The total cost of the mission is about €1.3 billion (US$1.8 billion).

Launch

Trajectory of the Rosetta space probe

Rosetta was set to be launched on 12 January 2003 to rendezvous with the comet 46P/Wirtanen in 2011.

This plan was abandoned after the failure of an Ariane 5 carrier rocket during Hot Bird 7's launch on 11 December 2002, grounding it until the cause of the failure could be determined. A new plan was formed to target the comet Churyumov–Gerasimenko, with a revised launch date of 26 February 2004 and comet rendezvous in 2014. The larger mass and the resulting increased impact velocity made modification of the landing gear necessary. After two scrubbed launch attempts, Rosetta was launched on 2 March 2004 at 7:17 GMT from the Guiana Space Centre in French Guiana. Aside from the changes made to launch time and target, the mission profile remained almost identical.

Deep space manoeuvres

To achieve the required velocity to rendezvous with 67P, Rosetta used gravity assist manoeuvres to accelerate throughout the inner Solar System. The comet's orbit was known before Rosetta's launch, from ground-based measurements, to an accuracy of approximately 100 km (62 mi). Information gathered by the onboard cameras beginning at a distance of 24 million kilometres (15,000,000 mi) were processed at ESA's Operation Centre to refine the position of the comet in its orbit to a few kilometres.

The first Earth flyby was on 4 March 2005.

On 25 February 2007, the craft was scheduled for a low-altitude flyby of Mars, to correct the trajectory. This was not without risk, as the estimated altitude of the flyby was a mere 250 kilometres (160 mi). During that encounter, the solar panels could not be used since the craft was in the planet's shadow, where it would not receive any solar light for 15 minutes, causing a dangerous shortage of power. The craft was therefore put into standby mode, with no possibility to communicate, flying on batteries that were originally not designed for this task. This Mars manoeuvre was therefore nicknamed "The Billion Euro Gamble". The flyby was successful, with Rosetta even returning detailed images of the surface and atmosphere of the planet, and the mission continued as planned.

The second Earth flyby was on 13 November 2007 at a distance of 5,700 km (3,500 mi). In observations made on 7 and 8 November, Rosetta was briefly mistaken for a near-Earth asteroid about 20 m (66 ft) in diameter by an astronomer of the Catalina Sky Survey and was given the provisional designation 2007 VN₈₄. Calculations showed that it would pass very close to Earth, which led to speculation that it could impact Earth. However, astronomer Denis Denisenko recognised that the trajectory matched that of Rosetta, which the Minor Planet Center confirmed in an editorial release on 9 November.

The spacecraft performed a close flyby of asteroid 2867 Šteins on 5 September 2008. Its onboard cameras were used to fine-tune the trajectory, achieving a minimum separation of less than 800 km (500 mi). Onboard instruments measured the asteroid from 4 August to 10 September. Maximum relative speed between the two objects during the flyby was 8.6 km/s (19,000 mph; 31,000 km/h).

Rosetta's signal received at ESOC in Darmstadt, Germany, on 20 January 2014

Rosetta's third and final flyby of Earth happened on 12 November 2009.

On 10 July 2010, Rosetta flew by 21 Lutetia, a large main-belt asteroid, at a minimum distance of 3,168±7.5 km (1,969±4.7 mi) at a velocity of 15 kilometres per second (9.3 mi/s). The flyby provided images of up to 60 metres (200 ft) per pixel resolution and covered about 50% of the surface, mostly in the northern hemisphere. The 462 images were obtained in 21 narrow- and broad-band filters extending from 0.24 to 1 μm. Lutetia was also observed by the visible–near-infrared imaging spectrometer VIRTIS, and measurements of the magnetic field and plasma environment were taken as well.

In May 2014, Rosetta began a series of eight burns. These reduced the relative velocity between the spacecraft and 67P from 775 m/s (2,540 ft/s) to 7.9 m/s (26 ft/s).

Orbit around 67P

In August 2014, Rosetta rendezvoused with the comet 67P/Churyumov–Gerasimenko (67P) and commenced a series of manoeuvres that took it on two successive triangular paths, averaging 100 and 50 kilometres (62 and 31 mi) from the nucleus, whose segments are hyperbolic escape trajectories alternating with thruster burns. After closing to within about 30 km (19 mi) from the comet on 10 September, the spacecraft entered actual orbit about it.

The surface layout of 67P was unknown before Rosetta's arrival. The orbiter mapped the comet in anticipation of detaching its lander. By 25 August 2014, five potential landing sites had been determined. On 15 September 2014, ESA announced Site J, named Agilkia in honour of Agilkia Island by an ESA public contest and located on the "head" of the comet, as the lander's destination.

Philae lander

File:Rosetta and Philae (crop).jpg

Rosetta and Philae

Main article: Philae (spacecraft)

Philae detached from Rosetta on 12 November 2014 at 08:35 UTC, and approached 67P at a relative speed of about 1 m/s (3.6 km/h; 2.2 mph). It initially landed on 67P at 15:33 UTC, but bounced twice, coming to rest at 17:33 UTC. Confirmation of contact with 67P reached Earth at 16:03 UTC.

On contact with the surface, two harpoons were to be fired into the comet to prevent the lander from bouncing off as the comet's escape velocity is only around 1 m/s (3.6 km/h; 2.2 mph). Analysis of telemetry indicated that the surface at the initial touchdown site is relatively soft, covered with a layer of granular material about 0.82 feet (0.25 meters) deep, and that the harpoons had not fired upon landing. After landing on the comet, the Philae had been scheduled to commence its science mission, which included:

• Characterisation of the nucleus
• Determination of the chemical compounds present, including amino acid enantiomers
• Study of comet activities and developments over time

Philae landed oddly, likely in the shadow of a nearby cliff or crater wall and canted at an angle of around 30 degrees. This made it unable to adequately collect solar power, and it lost contact with Rosetta when its batteries ran out after two days, well before much of the planned science objectives could be attempted. Contact was briefly and intermittently reestablished several months later at various times between 13 June and 9 July, before contact was lost once again.

Results

One of the first discoveries was that the magnetic field of 67P oscillated at 40–50 millihertz. Scientists modified the signal by speeding it up 10,000 times so that people could hear a rendition of it. While a natural phenomenon, it has been described as a "song", and has been compared to Continuum for harpsichord by György Ligeti. However, results from Philae's landing show that the comet's nucleus has no magnetic field, and that the field originally detected by Rosetta likely resulted from the influence of solar winds.

The isotopic signature of water vapour from comet 67P, as determined by the Rosetta spacecraft, is substantially different from that found on Earth. That is, 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 very unlikely that water found on Earth came from comets such as comet 67P, according to the scientists. On 22 January 2015, NASA reported that, between June and August 2014, the rate at which water vapor was released by the comet increased up to tenfold.

On 2 June 2015, NASA reported that the ALICE spectrograph on Rosetta determined that electrons within 1 km (0.62 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.

Instruments

For the instruments on Philae, see Philae (spacecraft) § Instruments.

Nucleus

The investigation of the nucleus is done by three optical spectrometers, one microwave radio antenna and one radar:

• ALICE (an ultraviolet imaging spectrograph). The ultraviolet spectrograph will search for and quantify the noble gas content in the comet nucleus, from which the temperature during the comet creation could be estimated. The detection is done by an array of potassium bromide and caesium iodide photocathodes. The 3.1 kg (6.8 lb) instrument uses 2.9 watts and was produced in the USA, and an improved version is used in the New Horizons spacecraft. It operates in the extreme and far ultraviolet spectrum, between 700 and 2,050 ångströms (70 and 205 nm).
• OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System). The camera system has a narrow-angle lens (700 mm) and a wide-angle lens (140 mm), with a 2048×2048 pixel CCD chip. The instrument was constructed in Germany.
• VIRTIS (Visible and Infrared Thermal Imaging Spectrometer). The Visible and IR spectrometer is able to make pictures of the nucleus in the IR and also search for IR spectra of molecules in the coma. The detection is done by a mercury cadmium telluride array for IR and with a CCD chip for the visible wavelength range. The instrument was produced in Italy, and improved versions were used for Dawn and Venus Express.
• MIRO (Microwave Instrument for the Rosetta Orbiter). The abundance and temperature of volatile substances like water, ammonia and carbon dioxide can be detected by MIRO via their microwave emissions. The 30 cm (12 in) radio antenna was constructed in Germany, while the rest of the 18.5 kg (41 lb) instrument was provided by the USA.
• CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission). The CONSERT experiment will provide information about the deep interior of the comet using a radar. The radar will perform tomography of the nucleus by measuring electromagnetic wave propagation between the Philae lander and the Rosetta orbiter through the comet nucleus. This allows it to determine the comet's internal structure and deduce information on its composition. The electronics were developed by France and both antennas were constructed in Germany.
• RSI (Radio Science Investigation). RSI makes use of the probe's communication system for physical investigation of the nucleus and the inner coma of the comet.

Gas and particles

• ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis). The instrument consists of a double-focus magnetic mass spectrometer DFMS and a reflectron type time of flight mass spectrometer RTOF. The DFMS has a high resolution (can resolve N₂ from CO) for molecules up to 300 amu. The RTOF is highly sensitive for neutral molecules and for ions. ROSINA was developed at the University of Bern in Switzerland.
• MIDAS (Micro-Imaging Dust Analysis System). The high-resolution atomic force microscope will investigate several physical aspects of the dust particles which are deposited on a silicon plate.
• COSIMA (Cometary Secondary Ion Mass Analyser). COSIMA analyses the composition of dust particles by secondary ion mass spectrometry, using indium ions. It can detect ions up to a mass of 6500 amu.
• GIADA (Grain Impact Analyser and Dust Accumulator). GIADA will analyse the dust environment of the comet coma measuring the optical cross section, momentum, speed and mass of each grain entering inside the instrument.

Solar wind interaction

• RPC (Rosetta Plasma Consortium).

Search for organic compounds

Previous observations have shown that comets contain complex organic compounds. These are the elements that make up nucleic acids and amino acids, essential ingredients for life as we know it. Comets are thought to have delivered a vast quantity of water to Earth, and they may have also seeded Earth with organic molecules. Rosetta and Philae will also search for organic molecules, nucleic acids (the building blocks of DNA and RNA) and amino acids (the building blocks of proteins) by sampling and analysing the comet's nucleus and coma cloud of gas and dust, helping assess the contribution comets made to the beginnings of life on Earth. Before succumbing to falling power levels, Philae's COSAC instrument was able to detect organic molecules in the comet's atmosphere, and may be able to continue its investigation if it can establish a stable contact with Rosetta.

Two enantiomers of a generic amino acid. The mission will study why one chirality of some amino acids seems to be dominant in the universe.

Amino acids

Upon landing on the comet, Philae will also test some hypotheses as to why essential amino acids are almost all "left-handed", which refers to how the atoms arrange in orientation in relation to the carbon core of the molecule. Most asymmetrical molecules are oriented in approximately equal numbers of left- and right-handed configurations (chirality), and the primarily left-handed structure of essential amino acids used by living organisms is unique. One hypothesis that will be tested was proposed in 1983 by William A. Bonner and Edward Rubenstein, Stanford University professors emeritus of chemistry and medicine respectively. They conjectured that when spiralling radiation is generated from a supernova, the circular polarisation of that radiation could then destroy one type of "handed" molecules. The supernova could wipe out one type of molecules while also flinging the other surviving molecules into space, where they could eventually end up on a planet.

Preliminary results

The VIRTIS spectrometer on board the Rosetta spacecraft has provided evidence of nonvolatile organic macromolecular compounds everywhere on the surface of comet 67P with little to no water ice visible. Preliminary analyses strongly suggest the carbon is present as polyaromatic organic solids mixed with sulfides and iron-nickel alloys. In turn, the Philae lander's COSAC instrument detected organic molecules in the comet's atmosphere as it descended to its surface. In contrast, the COSIMA instrument on-board Rosetta collected and analyzed comet grains from the near-nucleus region and the inner coma, and has not yet found carbon-bearing compounds.

Reaction control system problems

In 2006, Rosetta suffered a leak in its reaction control system (RCS). The system, which consists of 24 bipropellant 10-newton thrusters, is responsible for fine tuning the trajectory of Rosetta throughout its journey. The RCS will operate at a lower pressure than designed due to the leak. This may cause the propellants to mix incompletely and so burn 'dirtier' and less efficiently, though ESA engineers are confident that they have sufficient fuel reserves to allow successful completion of the mission.

Rosetta's reaction wheels are showing higher than expected vibration, though testing revealed the system can be operated more efficiently resulting in less wear on the wheels. Before hibernation, two of the spacecraft's four reaction wheels began exhibiting "noise". Engineers turned on three of the wheels after the spacecraft awoke, including one of the bad wheels. The other improperly functioning wheel will be held in reserve. Additionally, new software was uploaded which would allow Rosetta to function with only two active reaction wheels if necessary.

Timeline of major events and discoveries

Main article: Timeline of Rosetta spacecraft

2004

• 2 March – ESA's Rosetta mission was successfully launched at 07:17 UTC (04:17 local time) from Kourou, French Guiana.

2005

• 4 March – Rosetta executed its first planned close swing-by (gravity assist passage) of Earth. The Moon and the Earth's magnetic field were used to test and calibrate the instruments on board of the spacecraft. The minimum altitude above the Earth's surface was 1,954.7 km (1,214.6 mi).
• 4 July – Imaging instruments on board observed the collision between the comet Tempel 1 and the impactor of the Deep Impact mission.

2007

• 25 February – Mars flyby.
• 8 November – Catalina Sky Survey briefly misidentified the Rosetta spacecraft, approaching for its second Earth flyby, as a newly discovered asteroid.
• 13 November – Second Earth swing-by at a minimum altitude of 5,295 km (3,290 mi), travelling 45,000 km/h (28,000 mph).

2008

• 5 September – Flyby of asteroid 2867 Šteins. The spacecraft passed the main-belt asteroid at a distance of 800 km (500 mi) and the relatively slow speed of 8.6 km/s (31,000 km/h; 19,000 mph).

2009

• 13 November – Third and final swing-by of Earth at 48,024 km/h (29,841 mph).

Hubble view of P/2010 A2

2010

• 16 March – Observation of the dust tail of asteroid P/2010 A2. Together with observations by Hubble Space Telescope it could be confirmed that P/2010 A2 is not a comet, but an asteroid, and that the tail most likely consists of particles from an impact by a smaller asteroid.
• 10 July – Flew by and photographed the asteroid 21 Lutetia.

2014

Comet 67P seen from 10 km (6 mi)

• May to July – Starting on 7 May, Rosetta began orbital correction manoeuvres to bring itself into orbit around 67P. At the time of the first deceleration burn Rosetta was approximately 2,000,000 km (1,200,000 mi) away from 67P and had a relative velocity of +775 m/s (2,540 ft/s); by the end of the last burn, which occurred on 23 July, the distance had been reduced to just over 4,000 km (2,500 mi) with a relative velocity of +7.9 m/s (18 mph). In total eight burns were used to align the trajectories of Rosetta 67P with the majority of the deceleration occurring during three burns: Delta-v's of 291 m/s (650 mph) on 21 May, 271 m/s (610 mph) on 4 June, and 91 m/s (200 mph) on 18 June.
• 14 July – The OSIRIS on-board imaging system returned images of Comet 67P which confirmed the irregular shape of the comet.
• 6 August – Rosetta arrives at 67P, approaching to 100 km (62 mi) and carrying out a thruster burn that reduces its relative velocity to 1 m/s (3.3 ft/s). Commences comet mapping and characterisation to determine a stable orbit and viable landing location for Philae.
• 4 September – The first science data from Rosetta's ALICE instrument was reported, showing that the comet is unusually dark in ultraviolet wavelengths, hydrogen and oxygen are present in the coma, and no significant areas of water-ice have been found on the comet's surface. Water-ice was expected to be found as the comet is too far from the Sun to turn water into vapour.
• 10 September 2014 – Rosetta enters the Global Mapping Phase, orbiting 67P at an altitude of 29 km (18 mi).
• 12 November 2014 – Philae lands on the surface of 67P.
• 10 December 2014 – Data from the ROSINA mass spectrometers show that the ratio of heavy water to normal water on comet 67P is more than three times that on Earth. The ratio is regarded as a distinctive signature, and the discovery means that Earth's water is unlikely to have originated from comets like 67P.

2015

• 14 April 2015 – Scientists report that the comet's nucleus has no magnetic field of its own.
• 2 July 2015 – Scientists report that active pits, related to sinkhole collapses and possibly associated with outbursts, have been found on the comet.
• 11 August 2015 – Scientists release images of a comet outburst that occurred on 29 July 2015.
• 28 October 2015 – Scientists publish an article in Nature reporting high levels of molecular oxygen around 67P.

Future milestones

• November 2014 to December 2015 – Rosetta will escort the comet around the Sun and would perform riskier investigations.
• September 2016 - Mission ends by slow crash-landing on the comet. Although Philae sent back some data during its descent, Rosetta has more powerful and more varied sensors and instruments. The orbiter will also descend much more slowly than Philae did, allowing it to gather more data and better images.

Media coverage

The entire mission was featured heavily in social media, with a Facebook account for the mission and both the satellite and the lander having an official Twitter account portraying a personification of both spacecraft. The hashtag "#CometLanding" gained widespread traction. A Livestream of the control centres was set up, as were multiple official and unofficial events around the world to follow Philae's landing on 67P.

• Video reports by the German Aerospace Center
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  About Rosetta's mission
  (9 min., 1080p HD, English)

• About Philae's landing
  (10 min., 1080p HD, English)

See also

• Deep Impact (spacecraft)
• Halley Armada
  • Giotto (spacecraft)
• Hayabusa – successful sample-return mission to an asteroid
• Stardust (spacecraft)
• Timeline of Solar System exploration

External links

• Rosetta website at ESA.int
• Rosetta website at NASA.gov
• Rosetta operations site at ESA.int
• Rosetta mission profile at NASA.gov
• Rosetta Latest science news from the craft at ESA.int

Media

• Rosetta outreach image gallery at ESA.int
• Rosetta raw image gallery at ESA.int
• Rosetta image gallery at Flickr.com
• Rosetta's orbital journey at YouTube.com
• Rosetta: landing on a comet at ESA.int
• How to land on a comet by Fred Jansen at TED