Astrophysicists believe that

• Comets contain ancient ice and dust left behind while building of the Solar System around 4.6 billion years ago.

• So Comets hold key to the very origin of Solar system and mankind

In November 1993 therefore , the Rosetta Mission was approved through an International co-operation understanding to unravel the secrets of a mysterious ice world of a comet.

Critrion to choose a suitable comet ( out of hundreds ) was Launch time Comet path should be near ecliptic plane Should have been studied with fair accuracy ( implying several visits i.e. low orbital

period The landing time on comet should not be too near or far from Sun (around 3AU from

Sun )

Based on this a rendezvous was planned with Comet 46P/Wirtanen with a 2003 launch.Butthis launch could not happen. So, with 2004 launch window the Comet 67P/Churyumov-Gerasimenko was selected . ( We will refer to it as 67P/C-S henceforth for simplicity ).

67P/C-S is a periodic comet which completes one orbit around Sun ( extending just beyond Jupiter as shown in adjacent figure) in 6.44 years. The comet head spins on its axis in 12.5 hrs.

The mission is named after the famous 'Rosetta Stone‘, the slab of volcanic basalt with inscriptions, that unravelled the hitherto unknown facts of ancient Egyptian civilisation.

Initially scheduled for January 2003 , Rosetta was launched on 2 March 2004 aboard an Ariane 5G+ from Kourou.

During the long and serpentine 10 years journey it was deviated by using four gravity assists: one by Mars ( Feb’07)and three by Earth ( March ‘05, Nov ’07 and Nov’09) .

Gravity assist is the term used when a spacecraft passing near a large body uses the gravity of large body to gain speed. As an example see how ( in March ‘05) Earth pulled Rosetta, increasing its speed from 30Kms/s to over 38 Kms/sec. However the retrorockets were used to control the final speed to 34 km/sec which is required to turn this orbit to a Trans Martian Orbit carrying the spacecraft to Mars distance around Sun.

The figure above shows how Rosetta started at A, Used Earth gravity assist at B, then Marsassist C and Earth Gravity assist D in Nov’07. These two assists made it a 2 year orbit aroundSun bringing it back to Earth in Nov’09 when third Earth assist F made it 6 year orbit inwhich it caught up with Comet 67P at J in May’14.

During this period it also made close observations of a couple of Asteroids viz. 2867 Steins(in 2008) and 21 Lutetia (in 2010). In June 2011 it went into hibernation for about 2.5 yearsand was ‘woken up’ in January 2014

It reached its destination viz. Comet 67P/C-G in May 2014. Since then it has been following the comet from distances as long as 100s of kms to almost a touching distance of about 8 kms.Currently it has taken steps to go 30 kms away from comet , and on 12th Nov it will turn towards 67P/C-G and when it is at 22.5 kms, it will release the lander Philae with speed between 0.05 m/s and 0.51 m/s ( exact speed will be decided by onboard computers ). A detailed timeline of this historic operation is available in next slide. It is a highly complex and involved operation lasting almost a day. Philae will permanently remain steadfast at its landing site while Rosetta will remain in close proximity of comet for providing communication between Earth and Philae and it will also conduct measurements to compliment with those being carried out by Philae.

It is expected that Rosetta/Philae will continue measurements for about a year. By then, both the spacecraft and the comet would have circled the Sun. The comet will become highly active during its passage nearest to Sun ( in August 2015 ) and Rosetta/Philae duo will obtain images from its surface in that crucial state and also continue with observations for about an year.

Lander switch-on

Start lander flywheel operation

Rosetta maneuver for orientation start

Start Switch on of Science instruments on Philae

Lander on internal battery now

Lander separation , click two farewell photos of mother-ship

Rosetta divert maneuver … causes loss of communication with Earth

Communication Resumes

Data download starts

Start imaging landing site, switch on ADS

Philae Touchdown on Comet, Harpoons fire, Flywheel off

Science observations start. It obtains Panoramic Photographs of surroundings

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Figure shows the paths of various bodies ( SUN, Earth, Mars, 67P/C-G and Rosetta ) about solar system. Notice that after meeting the Comet in August

’14 the Rosetta follows it continuously and is expected to follow upto Dec ‘ 15 ( i.e. near ‘16 in top left corner ) covering the crucial phase (bright green line )

near comet’s perihelion in August 2015, at 186 million kilometres.

In the grand finale to this complex journey it has been in rendezvous with Comet for last few days finalizing the most suitable ( out of 5 already shortlisted out of 10, tagged ‘A’ thru ‘J’ in figure below ) site and surveying the 67P/C-G surface to decide on the best strategy to land and anchor its lander Philae on it.

( Even as I write this the news is that the site ‘ J ‘ has been finalized for landing and has now been named Agilkia. )

To read detailed technicalities in site selection see this

Four examples of the complex maneuvers that Rosetta had undergone and the very important final flyby to release the Philae lander on 12th Nov 2014.Top Left : Entry to rendezvous with 67P/C-GBottom Right: Path during Philae release

Original Image from ESA web-portal: http://sci.esa.int

The comet landing lab ‘ PHILAE ‘ ( top hanging black portion ) being integrated with Rosetta. One can imagine the actual size of spacecraft in comparison with persons standing nearby.

Original Image from ESA web-portal : http://sci.esa.int

Sketch of Rosetta when deployed fully . The blue ‘ bulge ‘ on the surface facing the reader is the lander Philae which will descend on comet and anchor itself firmly to it.

The Rosetta mission will achieve many historic firsts.

- Rosetta will be the first spacecraft to orbit a comet’s nucleus.

- It will be the first spacecraft to fly alongside a comet as it heads towards the inner Solar System.

- Rosetta will be the first spacecraft to examine from close proximity how a frozen comet is transformed by the warmth of the Sun.

- This week the Rosetta orbiter will despatch the robotic lander for the first controlled touchdown on a comet nucleus.

- The Rosetta lander’s instruments will obtain the first images from a comet’s surface and make the first in situ analysis to find out what it is made of.

On its way to Comet 67P/Churyumov-Gerasimenko, Rosetta passed through the main asteroid belt of Solar system..It was the first spacecraft ever to fly close to Jupiter’s orbit using solar cells as its main power source.Scientists will now compare Rosetta’s results with previous studies by ESA’s Giotto spacecraft and by ground-based observatories.

Previous observations from long distance had shown the presence of complex organic molecules - compounds that are rich in carbon, hydrogen, oxygen and nitrogen.

These are the very elements which make up nucleic acids and amino acids, the essential ingredients for life as we know it.

SO DID LIFE ON EARTH BEGIN WITH THE HELP OF COMET SEEDING?

We expect an answer to this fundamental question with the help of Rosetta observations.

Comet 67 P/Churyumov-Gerasimenko belongs to a group, or family of comets known as the Jupiter family. These are comets that are controlled by Jupiter's gravity and have short orbital periods (the time taken to complete an orbit), generally less than 20 years. Comet 67P has a current orbital period of 6.45 years, although this has changed in the past as the result of interactions with Jupiter. It was first discovered in 1969 by astronomers KlimChuryumov and Svetlana Gerasimenko and has been observed 6 more times since discovery.Comet 67P is classed as a dusty comet. This is to say that, during the period in which it is emitting, it will release approximately two times as much dust as gas. In 1982-1983 this was released at a rate of up to 220 kg per second. Thus far, the observations that have been made suggest that the nucleus of the comet is approximately ellipsoidal in shape with dimensions of 5 x 3 km and spins once in approximately 12 hours. The density of the nucleus, significantly lower than that of water, indicates that comet 67P is fairly porous and spectroscopic tests have shown it to be exceptionally dark. The latter observation suggests a covering of carbon-rich organic material.

ObjectivesThe target comet

Original Image from ESA web-portal: http://sci.esa.int

The orbiter consists of a box of 2.8 × 2.1 × 2.0 metres with two rotatable wings, measuring 32 m in length. Solar panels mounted on these wings will face the sun at all times during the interactions with comet 67P and will provide the power to run the onboard instruments. The wings on the opposite side to the solar panels contain radiators.

The box itself supports a 2.2 metre communications dish to send signals back to Earth. The 11 science experiments that will operate in orbit around comet 67P are mounted on the top of the box with the subsystems are located in the base. The orbiter will collect data relating to the comet nucleus, as well as the gas and dust ejected from the comet during its journey around the sun.

To find out more about the individual instruments on board the Rosetta orbiter, use the sidebar links.

The Rosetta orbiter will analyse comet 67P/Churyumov-Gerasimenko and its environment using a suite of 11 instruments:ALICE: Ultraviolet Imaging Spectrometer – (characterising the composition of the comet nucleus and coma)CONSERT: Comet Nucleus Sounding Experiment by Radio wave Transmission (studying the internal structure of the comet with lander Philae)COSIMA: Cometary Secondary Ion Mass Analyser (studying the composition of the dust in the comet’s coma)GIADA: Grain Impact Analyser and Dust Accumulator (measuring the number, mass, momentum and velocity distribution of dust grains in the near-comet environment)MIDAS: Micro-Imaging Dust Analysis System (studying the dust environment of the comet)MIRO: Microwave Instrument for the Rosetta Orbiter (investigating the nature of the cometarynucleus, outgassing from the nucleus and development of the coma)

OSIRIS: Optical, Spectroscopic, and Infrared Remote Imaging System Camera (a dual camera imaging system consisting of a narrow angle (NAC) and wide angle camera (WAC) and operating in the visible, near infrared and near ultraviolet wavelength range)ROSINA: Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (determining the composition of the comet's atmosphere and ionosphere, and measuring the temperature, velocity and density of the gas flow, comprising: DFMS (Double-focusing mass spectrometer), RTOF (Reflectron Time-Of-Flight mass spectrometer) and COPS (Comet Pressure Sensor))RPC: Rosetta Plasma Consortium (studying the plasma environment of the comet, comprising: ICA (Ion Composition Analyser), IES (Ion and Electron Sensor), LAP (Langmuir Probe), MAG (Fluxgate Magnetometer), MIP (Mutual Impedance Probe), PIU (Plasma Interface Unit))RSI: Radio Science Investigation (tracking the motion of the spacecraft to infer details of the comet environment and nucleus)VIRTIS: Visible and Infrared Thermal Imaging Spectrometer (studying the nature of the comet nucleus and the gases in the coma)

Earth got its water is certainly one of the most important unresolved questions about the early solar system.In tracing where the water came from, scientists attempt to recreate the conditions of the protosolar nebula, the cloud of gas and dust that formed the Sun and planets. They agree that the nebular disk was hotter and denser toward the center and cooler and less dense away from the center. The varying degrees of temperature throughout the protosolar disk clearly affected where water and icy particles existed. The central region would have contained high concentrations of metals and silicates, whereas icy particles could have existed in far greater quantities away from the center. They also believe the earliest solid particles were tiny; these objects accreted into larger ones by sticking together through countless collisions. Where plentiful oxygen existed, carbonaceous chondrite meteorites formed, which can contain up to 10 percent water. But comets, on the icy perimeter, contain as much as 80 percent water by mass.

Comets are believed by astrophysicists to be ancient ice and dust left from the building of the Solar System around 4.6 billion years ago

As hard as it is to believe when one stands on the shore of a great ocean, Earth has a small amount of water by mass -- only 0.02 percent in its oceans and a little more than that below ground on continents. Despite the small fraction of water on Earth compared to its total mass, our planet has plenty of water. For a planet at our distance from the Sun, it is exceedingly rich in water, containing far more than might exist here.

CIVA (the Comet Infrared and Visible Analyser) is a set of cameras split into two groups. The first experiment, CIVA-P, consists of seven identical cameras that will produce a panoramic image of the comet as seen from Philae. CIVA-P will characterise the landing site, mapping the surface topography and the albedo (reflectivity) of the surface. Two of the camera are aligned so as to produce stereoscopic images.CIVA-M, the second experiment, combines two miniaturised microscopes, one of which operates in visible light and the other in infrared. These are mounted on the base plate of the philae lander and will analyse samples delivered by the SD2 system for texture, albedoand mineral composition. As these analyses are non-destructive, it is possible that the samples could subsequently be analysed on COSACor Ptolemy.

The APXS (Alpha Proton X-ray Spectrometer) is an experiment designed to determine the chemical composition of the Philae landing site. The instrument will be lowered to ~ 4 cm from the ground and will detect alpha particles and X-rays.

The data collected from the APXS system will be used to determine the chemical composition of the comet dust component. This will be compared with known meteorite compositions and put into context using data collected from other instruments on both the orbiter and lander.

The APXS (Alpha Proton X-ray Spectrometer) is an experiment designed to determine the chemical composition of the Philae landing site. The instrument will be lowered to ~ 4 cm from the ground and will detect alpha particles and X-rays.

The data collected from the APXS system will be used to determine the chemical composition of the comet dust component. This will be compared with known meteorite compositions and put into context using data collected from other instruments on both the orbiter and lander.

COSAC (Cometary Sampling and Composition) is a system specially designed for the detection of complex organic (carbon-bearing) molecules. Material from the surface of the comet will be fed into the instrument from the SD2 instrument, combusted, and the resultant gas fed into the analysis section, consisting of a gas chromatograph and a mass spectrometer. In principle it is similar to the Ptolemyinstrument also found on the Philae lander.

COSAC represents an attempt to miniaturise a considerably sized instrument to fit on a space probe while retaining similar analytical precision. The data from COSAC will help determine whether some of the organic material on Earth was brought here by comets.

Ptolemy operates in a similar fashion to the COSAC instrument. Samples will be taken from the comet surface using the SD2 system and delivered to one of three ovens on Ptolemy. A fourth oven will collect volatile gases from the atmosphere of the comet. Samples are heated and the resultant gas is purified, quantified and sent to the mass spectrometer.Ptolemy is specialised for the analysis of so-called light elements, comprising carbon,

nitrogen and oxygen. It can also be used to analyse volatiles such as water, carbon monoxide and noble gases, as well as light organic compounds.An impressive aspect of this instrument is the sheer scale of the miniaturisation

involved. Ptolemy fits the level of analysis of two room-sized mass spectrometry systems into a system with similar dimensions to a shoebox and weighing less than 5 kg.

PHILAE the ultimate in Laboratory Miniaturiztion

The 100-kilogram Rosetta lander is provided by a European consortium under the leadership of the German Aerospace Research Institute (DLR). Other members of the consortium are ESA and institutes from Austria, Finland, France, Hungary, Ireland, Italy and the UK.The box-shaped lander is carried on the side of the orbiter until it arrives at Comet 67P/Churyumov-Gerasimenko. Once the orbiter is aligned correctly, the lander is commanded to self-eject from the main spacecraft and unfold its three legs, ready for a gentle touchdown at the end of the ballistic descent.On landing, the legs damp out most of the kinetic energy to reduce the chance of bouncing, and they can rotate, lift or tilt to return the lander to an upright position.Immediately after touchdown, a harpoon is fired to anchor the lander to the ground and prevent it escaping from the comet’s extremely weak gravity. The minimum mission target is one week, but surface operations may continue for many months.=====The lander structure consists of a baseplate, an instrument platform, and a polygonal sandwich construction, all made of carbon fibre. Some of the instruments and subsystems are beneath a hood that is covered with solar cells.An antenna transmits data from the surface to Earth via the orbiter. The lander carries nine experiments, with a total mass of about 21 kilograms. It also carries a drilling system to take samples of subsurface material.

Original Image from : ESA webportal, http://sci.esa.int

Rosetta will deploy the Philae lander to the surface of comet 67P/Churyumov-Gerasimenko for in situ analysis with its 10 instruments:APXS: Alpha Proton X-ray Spectrometer (studying the chemical composition of the landing site and its potential alteration during the comet's approach to the Sun)CIVA: Comet Nucleus Infrared and Visible Analyser (six cameras to take panoramic pictures of the comet surface)CONSERT: COmet Nucleus Sounding Experiment by Radiowave Transmission (studying the internal structure of the comet nucleus with Rosetta orbiter)COSAC: The COmetary SAmpling and Composition (detecting and identifying complex organic molecules)PTOLEMY: Using MODULUS protocol (Methods Of Determining and Understanding Light elements from Unequivocal Stable isotope compositions) to understand the geochemistry of light elements, such as hydrogen, carbon, nitrogen and oxygen.MUPUS: MUlti-PUrpose Sensors for Surface and Sub-Surface Science (studying the properties of the comet surface and immediate sub-surface)ROLIS: Rosetta Lander Imaging System (providing the first close-up images of the landing site)ROMAP: Rosetta Lander Magnetometer and Plasma Monitor (studying the magnetic field and plasma environment of the comet)SD2: Sampling, drilling and distribution subsystem (drilling up to 23 cm depth and delivering material to onboard instruments for analysis)SESAME: Surface Electric Sounding and Acoustic Monitoring Experiment (probing the mechanical and electrical parameters of the comet)

MUPUS (Multi Purpose Sensors for Surface and Subsurface Science) consists of a number of temperature sensors attached to a 35 cm long penetrator that will be deployed away from the landing module. As the penetrator is hammered into the ground, the progress per hammering stroke and the temperature of the subsurface will be measured. In combination these will provide an indication of the properties of the comet’s surface (i.e. how resistant to penetration the surface is) and a profile of the temperature change with depth. The sensors can also operate in a heating mode which will allow the thermal properties of the comet (such as the heat conductivity) to be investigated.

In addition to the sensors on the penetrator, MUPUS also has two heat sensors mounted on the harpoons that will secure the lander to the comet, providing an indication of the subsurface heat to a depth of ~ 1.5 m. Finally, an infrared sensor known as the thermal mapper (TM), mounted on thePhilae lander itself, will measure heat emitted from the surface of the comet over a small area.

ROLIS (Rosetta Lander Imaging System) is an imaging system consisting of a miniaturised CCD camera. Its primary purpose is to operate as an imaging device during the decent of Philae, obtaining increasingly high resolution images of the landing site.

Once on the surface, ROLIS will take pictures of the surface below the lander. A series of light emitting diodes will allow this to be done using several wavelengths. In addition it will provide support to the drilling instruments and to APXS, by imaging the resultant boreholes and by imaging the target locations respectively.

ROMAP (Rosetta Lander Magnetometer and Plasma Monitor) is an experiment designed to determine the plasma environment and any residual magnetic field present on comet 67P.

The main electronics for the instrument are located within the Philae lander. The sensors themselves are placed on a 60 cm long retractable rod. One of these sensors will measure the magnetic field while the other will measure the abundance of the ions and electrons which make up the plasma environment of the comet.

SD2 (Sample Drilling and Distribution) is less of an instrument in itself and more of a system to provide some of the other instruments with material for analysis. It contains a drill capable of boring down to 230 mm and collecting samples, a carousel and 26 ovens. The entire system weighs ~ 5 kg.

The principle purpose of SD2 is to provide material for ÇIVA, COSAC and Ptolemy to analyse. The ovens will be used to heat samples to medium (~ 180ºC) and high (~ 800 ºC) temperatures and will serve to provide the gases required for analyses with COSAC and Ptolemy.

SESAME (Surface Electric Sounding and Acoustic Monitoring Experiment) is not an instrument in itself, but rather an experiment formed from combining three instruments that will work together in order to help understand how comets formed. The three instruments are:

The Cometary Acoustic Surface Sounding Experiment (CASSE)

The Permittivity Probe (PP)

The Dust Impact Monitor

The Cometary Acoustic Surface Sounding Experiment (CASSE) is an experiment to investigate the surface and subsurface of comet 67P. A set of sensors have been built into the feet of the lander which will have two functions:To listen for noise produced within the nucleus of the comet caused by various sources

(expansion and contraction from heating and cooling, impacts or seismic events). In this way it will operate in a similar way to a seismometer on Earth, used to characterise earthquakes.To generate sound and use the reflections of that sound to provide information about the

material that it has passed through, like a sonar. This will provide information on any layering within the comet, as well as holes and other features.

The Permittivity Probe consists of 5 electrodes incorporated into parts of Philae. There are 3 transmitter electrodes, one placed into one of the lander feet, one in APXS and one attached to the penetrator ofMUPUS, visible as the brown mesh around MUPUS, shown in the image. The other 2 electrodes are receivers placed in the remaining two feet of the lander.

The transmitter electrodes send an electric signal through the surface of the comet. This helps determine the electrical conductivity of the surface down to a depth of ~ 2 m. This can then be used to determine the presence and abundance of water in the surface.

The Dust Impact Monitor (DIM) is an instrument designed to measure the impact of cometary particles. In principle, DIM is intended to monitor particles that have been volatilised and released from the surface of the comet, but do not have sufficient velocity to escape the gravity of the comet.

DIM is mounted on one of the upper surfaces of Philae and will measure the impact of these particles from three directions. The measurements it makes will be used to infer the velocity at which the particles were ejected from the surface.