Europe's Comet Chaser
In November 1993, the International Rosetta Mission was approved as a Cornerstone Mission in ESA's Horizons 2000 Science Programme.
Since then, scientists and engineers from all over Europe and the United States have been combining their talents to build an orbiter and a lander for this unique expedition to unravel the secrets of a mysterious 'mini' ice world – a comet.
Initially scheduled for January 2003, the launch of Rosetta had been postponed due to a failure of an Ariane rocket in December 2002. The adventure began March 2004, when a European Ariane 5 rocket lifted off from Kourou in French Guiana.
During a circuitous ten-year trek across the Solar System, Rosetta will cross the asteroid belt and travel into deep space, more than five times Earth’s distance from the Sun. Its destination will be a periodic comet known as Comet 67P/Churyumov-Gerasimenko.
The Rosetta orbiter will rendezvous with Comet 67P/Churyumov-Gerasimenko and remain in close proximity to the icy nucleus as it plunges towards the warmer inner reaches of the Sun’s domain. At the same time, a small lander will be released onto the surface of this mysterious cosmic iceberg.
More than a year will pass before the remarkable mission draws to a close in December 2015. By then, both the spacecraft and the comet will have circled the Sun and be on their way out of the inner Solar System.
Historic mission
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.
- Shortly after its arrival at Comet 67P/Churyumov-Gerasimenko, the Rosetta orbiter will despatch a 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 will pass through the main asteroid belt, with the option to be the first European close encounter with one or more of these primitive objects.
- Rosetta will be the first spacecraft ever to fly close to Jupiter’s orbit using solar cells as its main power source.
Scientists will be eagerly waiting to compare Rosetta’s results with previous studies by ESA’s Giotto spacecraft and by ground-based observatories. These have shown that comets contain complex organic molecules - compounds that are rich in carbon, hydrogen, oxygen and nitrogen.
Intriguingly, these are the elements which make up nucleic acids and amino acids, the essential ingredients for life as we know it. Did life on Earth begin with the help of comet seeding? Rosetta may help us to find the answer to this fundamental question.
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Why "Rosetta" ?
The European Space Agency's unprecedented mission of cometary exploration is named after the famous 'Rosetta Stone'. This slab of volcanic basalt - now in the British Museum in London – was the key to unravelling the civilisation of ancient Egypt.
French soldiers discovered the unique Stone in 1799, as they prepared to demolish a wall near the village of Rashid (Rosetta) in Egypt's Nile delta. The carved inscriptions on the Stone included hieroglyphics – the written language of ancient Egypt – and Greek, which was readily understood. After the French surrender in 1801, the 762-kilogram stone was handed over to the British.
By comparing the inscriptions on the stone, historians were able to begin deciphering the mysterious carved figures. Most of the pioneering work was carried out by the English physician and physicist Thomas Young, and the French scholar Jean François Champollion. As a result of their breakthroughs, scholars were at last able to piece together the history of a long-lost culture.
Just as the Rosetta Stone provided the key to an ancient civilisation, so ESA's Rosetta spacecraft will unlock the mysteries of the oldest building blocks of our Solar System – the comets. As the worthy successor of Champollion and Young, Rosetta will allow scientists to look back 4600 million years to an epoch when no planets existed and only a vast swarm of asteroids and comets surrounded the Sun.
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The Rosetta Orbiter
Rosetta is a large aluminium box with dimensions 2.8 x 2.1 x 2.0 metres. The scientific instruments are mounted on the 'top' of the box (Payload Support Module) while the subsystems are on the 'base' (Bus Support Module).
On one side of the orbiter is a 2.2-metre diameter communications dish – the steerable high-gain antenna. The lander is attached to the opposite face.
Two enormous solar panel 'wings' extend from the other sides. These wings, each 32 square metres in area, have a total span of about 32 metres tip to tip. Each of them comprises five panels, and both may be rotated through +/-180 degrees to catch the maximum amount of sunlight.
In the vicinity of Comet 67P/Churyumov-Gerasimenko, the scientific instruments almost always point towards the comet, while the antennae and solar arrays point towards the Sun and Earth (at large distances, they are more or less in the same direction).
In contrast, the orbiter’s side and back panels are in shade for most of the mission. Since these panels receive little sunlight, they are an ideal location for the spacecraft’s radiators and louvres. They will also face away from the comet, so damage from comet dust will also be minimised.
Propulsion
At the heart of the orbiter is the main propulsion system. Mounted around a vertical thrust tube are two large propellant tanks, the upper one containing fuel, and the lower one containing the oxidiser.
The orbiter also carries 24 thrusters for trajectory and attitude control. Each of these thrusters pushes the spacecraft with a force of 10 Newtons, about the same as experienced by someone holding a large bag of apples. Over half the launch weight of the entire spacecraft is taken up by propellant.
| Spacecraft vital statistics | |
|---|---|
| Size | |
| Main structure | 2.8 x 2.1 x 2.0 metres |
| Diameter of solar arrays | 32 metres |
| Launch mass | |
| Total | 3,000 kg (approx.) |
| Propellant | 1,670 kg (approx.) |
| Science payload | 165 kg |
| Lander | 100 kg |
| Solar array output | 850 W at 3.4 AU, 395 W at 5.25 AU |
| Propulsion subsystem | 24 bipropellant 10N thrusters |
| Operational mission | 12 years |
An international enterprise
Rosetta’s industrial team involves more than 50 contractors from 14 European countries and the United States. The prime spacecraft contractor is Astrium Germany. Major subcontractors are Astrium UK (spacecraft platform), Astrium France (spacecraft avionics) and Alenia Spazio (assembly, integration and verification).
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Orbiter: Instruments
The Rosetta orbiter's scientific payload includes 11 experiments, in addition to the lander. Scientific consortia from institutes across Europe and the United States have provided these state-of-the-art instruments. All of them are located on the side of the spacecraft that will permanently face the comet during the main scientific phase of the mission.
For a brief overview of each instrument, follow the links on the right.
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The Rosetta Lander
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.
Lander design
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.
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Lander: Instruments
Rosetta's lander, Philae, consists of a baseplate, an instrument platform, and a polygonal sandwich construction.
Some of the instruments and subsystems are beneath a hood that is covered with solar cells. The lander carries ten instruments, with a total mass of about 21 kilograms.
For a brief overview of each instrument, follow the links on the right.
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The Long Trek
Rosetta's ten-year expedition began in March 2004, with an Ariane-5 launch from Kourou in French Guiana.
The three-tonne spacecraft was first inserted into a parking orbit, before being sent on its way towards the outer Solar System.
The cosmic billiard ball
Unfortunately, no existing rocket, not even the powerful European-built Ariane-5, has the capability to send such a large spacecraft directly to Comet 67P/Churyumov-Gerasimenko.
Instead, Rosetta will bounce around the inner Solar System like a ‘cosmic billiard ball’, circling the Sun almost four times during its ten-year trek to Comet 67P/Churyumov-Gerasimenko.
Along this roundabout route, Rosetta will enter the asteroid belt twice and gain velocity from gravitational ‘kicks’ provided by close fly-bys of Mars (2007) and Earth (2005, 2007 and 2009).
Earth fly-bys (2005, 2007 and 2009)
Rosetta first travels away from its home planet and then encounters Earth again, a year after launch, in March 2005.
Rosetta remains active during the cruise to Earth. The fly-by distance is between 300 and 14 000 kilometres. Operations mainly involve tracking, orbit determination and payload check-out. Orbit correction manoeuvres take place before and after each fly-by.
After the first fly-by of Earth in March 2005, Rosetta heads to Mars and then returns to Earth twice in November 2007 and November 2009 for its second and third fly-bys of our planet.
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Life And Survival In Deep Space
Rosetta's deep space odyssey will comprise lengthy periods of inactivity, punctuated by relatively short spells of intense activity – the encounters with Mars, Earth and asteroids.
Ensuring that the spacecraft survives the hazards of travelling through deep space for more than ten years is therefore one of the great challenges of the Rosetta mission.
Spacecraft hibernation
For much of the outward journey, the spacecraft will be placed in 'hibernation' in order to limit consumption of power and fuel, and to minimise operating costs. At such times, the spacecraft spins once per minute while it faces the Sun, so that its solar panels can receive as much sunlight as possible.
Almost all of the electrical systems are switched off, with the exception of the radio receivers, command decoders and power supply.
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Long-Distance Communication
During Rosetta’s prolonged interplanetary expedition, reliable communications between the spacecraft and the ground will be essential.
All of the scientific data collected by the instruments on board the spacecraft are sent to Earth via a radio link. The operations centre, in turn, remotely controls the spacecraft and its scientific instruments via the same radio link.
The Mission Operations Centre during Rosetta’s entire 12-year journey is the European Space Operations Centre (ESOC) in Darmstadt, Germany. ESOC is responsible for all mission operations, including:
- mission planning, monitoring and control of the spacecraft and its payload;
- determination and control of the spacecraft trajectory;
- distribution of the scientific data received from the spacecraft to the Rosetta scientific community and the Principal Investigators.
A Science Operations Centre will also be located at ESOC during the active phases of the mission. Its task will be to coordinate the requests for scientific operations received from the scientific teams supporting both the orbiter and the lander instruments.
Lander operations will be coordinated through the German Aerospace Research Centre (DLR) control centre in Cologne, and the scientific control centre of CNES, the French space agency, in Toulouse.
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The Rosetta Ground Segment
17 February 2004
A ‘ground segment’ is the system set up on Earth to manage and control a space mission, and to receive and process the data produced by a spacecraft’s instruments, and if necessary to send out and archive any generated products.
The Rosetta ground segment is designed to meet both the scientific objectives and the challenges imposed by a deep-space mission. These challenges include long turnaround times for signals (up to 100 minutes), low bit rates for data (8 bps), low power (hibernation for two years), and the requirement for particular positions of planets (Rosetta makes use of gravity-assist manoeuvres with Mars and Earth). In addition ESOC will cope with the long mission duration and the related problem in keeping expertise and experience, while minimising the overall cost.
The Rosetta spacecraft will be launched on an Ariane 5 launch vehicle from Kourou, French Guiana. After launch, the Rosetta mission will be controlled from a single control centre, the Rosetta Mission Operations Centre (RMOC) at ESOC, in Darmstadt, Germany, in conjunction with the ESA deep space ground station at New Norcia.
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Comet 67P/Churyumov-Gerasimenko
Since launch in 2004, ESA's Rosetta mission has been chasing down comet 67P/Churyumov-Gerasimenko. The comet is a regular visitor to the inner Solar System, orbiting the Sun once every 6.5 years between the orbits of Jupiter and Earth.
Like all comets, Churyumov-Gerasimenko is named after its discoverers. It was first observed in 1969, when several astronomers from Kiev visited the Alma-Ata Astrophysical Institute in Kazakhstan to conduct a survey of comets.
On 20 September, Klim Churyumov was examining a photograph of comet 32P/Comas Solá, taken by Svetlana Gerasimenko, when he noticed another comet-like object. After returning to Kiev, he studied the plate very carefully and eventually realised that they had indeed discovered a new comet.
The comet has now been observed from Earth on seven approaches to the Sun: in 1969, 1976, 1982, 1989, 1996, 2002 and 2009. It was also imaged by the Hubble Space Telescope in 2003, which allowed estimates of its size and shape to be made – an irregular object roughly 3 x 5 km across.
Most of the time, however, its faint image is drowned in a sea of stars, making observations with Earth-based telescopes extremely difficult.
However, during its short-lived excursions to the inner Solar System, the warmth of the Sun causes ices on its surface to evaporate and jets of gas to blast dust grains into the surrounding space.
Although this enveloping ‘coma’ of dust and gas increases 67P/Churyumov-Gerasimenko’s brightness, it also completely hides the comet’s nucleus to ground-based observers.
Rosetta's task is to rendezvous with the comet while it still lingers in the cold regions of the Solar System, and to deploy a lander to reveal in close-up detail exactly what the surface looks like.
Observations indicate that, if the activity of 67P is consistent from orbit to orbit, then Rosetta may return images of an active nucleus when it rendezvous with the comet when it is about 3.5 AU from the Sun.
Over an entire year, as it approaches the Sun, Rosetta will orbit the comet, mapping its surface and studying changes in its activity.
As its ices evaporate, instruments on board the orbiter will study the dust and gas particles that surround the comet and trail behind it as streaming tails, as well as their interaction with the solar wind.
Rosetta will also help identify which regions of the nucleus are more active than others. As is the case with most comets, activity is not evenly distributed on the surface of the nucleus and the coma of 67P is fed by several dust jets – at least three prominent active areas were identified during the 2009 apparition. If the comet behaves as in 2003 and 2009, the main jets should become visible a month before perihelion, i.e. mid-July 2015.
To ensure the spacecraft is kept safe during times of high activity, its orbit around the comet will be adjusted accordingly.
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Comet Rendezvous
The most difficult phase of the Rosetta mission is the final rendezvous with the fast-moving comet. After the braking manoeuvre in May 2014, the priority will be to edge closer to the nucleus.
Since this takes place before Rosetta's cameras have imaged the comet, accurate calculations of Comet 67P/Churyumov-Gerasimenko's orbit, based on ground-based observations, are essential.
Comet approach (January – May 2014)
The spacecraft is re-activated prior to the comet rendezvous manoeuvre, during which the thrusters fire for several hours to slow the relative drift rate of the spacecraft and comet to about 25 metres per second.
As Rosetta drifts towards the heart of the comet, the mission team will try to avoid any comet dust and achieve good comet illumination conditions. The first camera images will dramatically improve calculations of the comet’s position and orbit, as well as its size, shape and rotation.
The relative speeds of the spacecraft and comet will gradually be reduced, slowing to 2 metres per second after about 90 days.
Comet mapping and characterisation (August 2014)
Less than 200 kilometres from the nucleus, images from Rosetta show the comet’s spin-axis orientation, angular velocity, major landmarks and other basic characteristics.
Eventually, the spacecraft is inserted into orbit around the nucleus at a distance of about 25 kilometres. Their relative speed is now down to a few centimetres per second.
The orbiter starts to map the nucleus in great detail. Eventually, five potential landing sites are selected for close observation.
Landing on the comet (November 2014)
Once a suitable landing site is chosen, the lander is released from a height of about one kilometre. Touchdown takes place at walking speed — less than one metre per second.
Once it is anchored to the nucleus, the lander sends back high-resolution pictures and other information on the nature of the comet’s ices and organic crust.
The data are relayed to the orbiter, which stores them for transmission back to Earth at the next the period of contact with a ground station.
Around the Sun (November 2014 – December 2015)
The orbiter continues to orbit Comet 67P/Churyumov-Gerasimenko, observing what happens as the icy nucleus approaches the Sun and then travels away from it.
The mission ends in December 2015. Rosetta will once again pass close to Earth’s orbit, more than 4000 days after its adventure began.
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Source: Rosetta Official Site
Rosetta is pretty awesome, isn't it?
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