Rosetta and Philae – unlocking the secrets of comets

Talk – Tuesday 10th November 2015 – Cooch Memorial Lecture ‘Rosetta and Philae –
unlocking the secrets of comets’
Professor Andrew Coates – Head of Planetary Science, Mullard Space Science Laboratory – University College, London.
Comets are ancient members of our solar system – the surviving ‘building blocks’ of outer planet cores. Last year, ESA’s Rosetta spacecraft was the first to go into orbit around a comet (67P) and Philae landed after bouncing initially, sending back historic images and data from the surface of the comet. The comet’s closest approach to the Sun was on 13 August 2015 and the orbiter continues to send spectacular results about the increasing activity of the comet with end of mission now expected in September 2016. After a brief re-awakening with increased illumination, Philae fell silent again.
Already, Rosetta results have shown us that these types of comets are unlikely to have brought much water to Earth, molecular nitrogen and oxygen has been detected, and we have seen the best images of a comet so far. A major surprise was the ‘rubber duck-like shape of comet 67P Churyumov-Gerasimenko, this has now been shown to be due to the collision of two bodies in the early solar system. The talk included why comets, ancient building blocks of our solar system, are important, what they are made of, comet tails, the design of the mission and instrumentation, the mission profile and some of the key scientific discoveries so far. The effect on solar system formation ideas was mentioned.
Programme Details
The Rosetta mission was conceived as a result of the success of earlier projects to explore Halley’s comet during a close proximity in its orbit to Earth in 1986. Most prominent of a number of these international space probes was ESA’s Giotto which along with the others returned valuable scientific information. In the light of this it became obvious that, to shed more light on cometary composition and answer new questions posed by these results, further missions to survey comets would be necessary.
On this background it was decided that ESA should proceed with further investigations into the composition of comets and so planning began and the mission was named Rosetta. After 30 years of work and scientific innovation together with an expenditure of some £1billion information began to flow from Rosetta as it orbited comet 67P/Churyumov- Gerasimenko. In order to fund a project of this magnitude required international cooperation on a grand scale. Astrium, a company with a large representation in the UK and facilities in Portsmouth, Stevenage and Surrey University was selected as prime contractor for the spacecraft. The UK’s contribution was for the supply of the spacecraft platform which was derived from those used for communications projects by Astrium. It was manufactured in their Stevenage plant. Many other outfits and universities in the UK provided specialist parts and scientific instrumentation for a range of experiments to be conducted in the analysis of the makeup of a comet. The whole spacecraft was integrated and tested in Astrium’s German facility at Friedrichshafen on the shores of Lake Constance (of Zeppelin fame!). The comet lander “Philae” was provided by the German Aerospace Research Institute (DLR) with contributions from across Europe. This was attached to Rosetta and travelled piggyback during flight to the comet and was released for comet landing once in orbit around it. Landing on a comet had never before been attempted and posed the greatest risk to the mission. Overall Rosetta carried a payload of 16 experiments with Philae housing a further 12.
Much more could be said about the spacecraft, but to put it into perspective it’s probably better to highlight the main features of the mission in order to acquaint one with the enormity of the task presented. Those who wish to know more detail should visit ESA’s website at
Rosetta and it’s lander was to spend 10 years in orbit making many manoeuvres and planetary fly-bys during which it changed its velocity and trajectory as it extracted vital energy from the gravitational fields of Earth and Mars to enable it to complete its journey to the comet.
On the following pages are diagrams of both the Rosetta spacecraft and its attached lander together with a list of the scientific instrumentation carried by each of them. It was prudent to place instrumentation on both as one of the riskiest aspects of the mission was the landing of Philae on the comet. At least all would not be lost. Also shown are details of the mission.
Comet Selection
There are hundreds of comets flying around the Solar System, each of them a potential target for ESA’s comet-chasing Rosetta mission. As the mission took shape, the science team was faced with the difficult task of sifting through these candidates until they identified a handful of suitable objects.
Of particular interest were comets that had been observed over at least several orbits of the Sun, and which were known to be fairly active. Ideally, they had to follow orbital paths near the ecliptic plane, so that a rendezvous, prolonged survey and landing would be easier to achieve. Furthermore, the comet’s flight into the inner Solar System had to coincide with the mission timeline of Rosetta, so that they both arrived in the right place at the right time for the historic rendezvous. The favoured target for Rosetta was the periodic comet 46P/Wirtanen, but, after a previous launch failure of the latest version of the Ariane 5 vehicle to be used, it wasn’t deemed safe to proceed until the failure had been fully investigated. It was to be nearly 2 years before a suitable replacement could be found. One of the greatest risks to space programmes and they hit it!.
A delay of this length meant that comet 46P/Wirtanen could no longer be considered a suitable candidate as it would no longer be in a suitable part of its orbit to satisfy the constraints put on the programme by both timescale and spacecraft life cycle. Consequently, another regular visitor to the inner Solar System, 67P/Churyumov-Gerasimenko, was selected as a suitable replacement.
Comet 67P is one of numerous short period comets which have orbital periods of less than 20 years and a low orbital inclination. Since their orbits are controlled by Jupiter’s gravity, they are also called Jupiter Family comets.
At this point very little was known about the surface properties of the nucleus, so the selection of a suitable landing site for the Philae probe was only possible after the arrival of Rosetta in August 2014 when a detailed survey from close quarters was made.
When Rosetta arrived at the comet it was at a distance of about three Astronomical Units (450 million km) from the Sun. As it moved towards the Sun, the ice in the nucleus began to sublimate and the comet started to eject increasing amounts of dust.
Ejection of micron-sized grains starts at about 4.3 AU, but millimetre-sized grains are more likely to appear between 3.4 and 3.2 AU. This leads to the development of a coma (a diffuse cloud of dust and gas surrounding the solid nucleus) and subsequently a tail of dust that trails away from the Sun. Early observations of the comet showed some evidence for variable activity between April and June 2014, with the coma brightening rapidly and then dying down again over a period of about six weeks. The spacecraft approached from the sunward side of the comet’s orbit, in order to minimise the risk of damage from a possible large impact with the dust.
Ultraviolet Imaging Spectrometer
Comet Nucleus Sounding Experiment by Radio wave Transmission Cometary Secondary Ion Mass Analyser
Grain Impact Analyser and Dust Accumulator
Micro-Imaging Dust Analysis System
Microwave Instrument for the Rosetta Orbiter
Optical, Spectroscopic, and Infrared Remote Imaging System Rosetta Orbiter Spectrometer for Ion and Neutral Analysis
Rosetta Plasma Consortium
Radio Science Investigation
Visible and Infrared Thermal Imaging Spectrometer
Rosetta Orbitor
These comets are believed to originate from the Kuiper Belt, a large reservoir of small icy bodies located just beyond Neptune. As a result of collisions or gravitational perturbations, some of these icy objects are ejected from the Kuiper Belt and fall towards the Sun.
When they cross the orbit of Jupiter, the comets gravitationally interact with the massive planet. Their orbits gradually change as a result of these interactions until they are eventually thrown out of the Solar System or collide with a planet or the Sun.
The comet had now been observed from Earth on seven approaches to the Sun – 1969 (discovery), 1976, 1982, 1989, 1996, 2002 and 2009. Like all comets, it has a fairly small, solid nucleus which is thought to resemble a dirty snowball. The density of the nucleus seems to be much lower than that of water, indicating a loosely packed or porous object. Like other comets, its nucleus is generally blacker than coal, indicating a surface layer or crust of carbon-rich organic material.
Philae Lander
Alpha Proton X-ray Spectrometer
Panoramic and microscopic imaging system
Comet Nucleus Sounding Experiment by Radio wave Transmission Cometary Sampling and Composition experiment
Evolved Gas Analyser
Multi-Purpose Sensor for Surface and Subsurface Science
Rosetta lander Magnetometer and Plasma Monitor
Sample and Distribution Device
Surface Electric Sounding and Acoustic Monitoring Experiment
Below is a condensed description of the mission. This is the part of the programme that attracts most interest after all the hard work has been completed!
Launch date: Mission end: Launch vehicle:
Launch mass: Mission phase:
Orbit: Objectives:
2 March 2004 07:17 UT
end September 2016 (extended mission)
Ariane 5 G+ from Kourou, French Guiana
3000 kg (fully fuelled); Orbiter: 2900 kg (including 1670 kg propellant and 165 kg science payload); Lander: 100 kg
At Comet 67P/Churyumov-Gerasimenko
En route to Comet 67P/Churyumov-Gerasimenko, Rosetta completed a complex trajectory that included four gravity assist manoeuvres (3 × Earth, 1 × Mars). The spacecraft arrived at the comet on 6 August 2014. Since then, the spacecraft has been orbiting the comet. It will accompany the comet on its journey around the Sun.
To study the origin of comets, the relationship between cometary and interstellar material, and its implications with regard to the origin of the Solar System. The measurements to be made to achieve this are:
    
Global characterisation of the nucleus, determination of dynamic properties, surface morphology and composition;
Determination of the chemical, mineralogical and isotopic compositions of volatiles and refractories in a cometary nucleus;
Determination of the physical properties and interrelation of volatiles and refractories in a cometary nucleus;
Study of the development of cometary activity and the processes in the surface layer of the nucleus and the inner coma (dust/gas interaction);
Global characterisation of asteroids, including determination of dynamic properties, surface morphology and composition.
Comet 67P is classed as a dusty comet, with a dust to gas emission ratio of approximately 2:1. The peak dust production rate in 2002/03 was estimated at approximately 60 kg per second, although values as high as 220 kg per second were reported in 1982/83.
Sixty-one images of comet 67P/Churyumov-Gerasimenko were taken with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST) on 11-12 March 2003. The HST’s sharp vision enabled astronomers to isolate the comet’s nucleus from the coma. The images showed that the nucleus measures roughly five by three kilometres and has an approximately ellipsoidal (rugby ball) shape.
Comet 67P by Rosetta’s OSIRIS narrow-angle camera on 3 August 2014.
Changes in its light curve appear to be closely linked with the effective radius of the nucleus as it rotates, rather than with variations in its surface albedo (brightness). These observations indicate that it spins once in approximately 12 hours.
Comet Rendezvous
The most difficult phase of the Rosetta mission was the final rendezvous with the fast-moving comet. After the braking manoeuvre in May 2014, the priority was to edge closer to the nucleus.Since this took place before Rosetta’s cameras had imaged the comet, accurate calculations of Comet 67P/Churyumov-Gerasimenko’s orbit, based on ground-based observations, were essential.
Comet approach (January – May 2014)
The spacecraft was 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 had to avoid any comet dust and achieve good comet illumination conditions. The first camera images dramatically improved calculations of the comet’s position and orbit, as well as its size, shape and rotation. The relative speeds of the spacecraft and comet was gradually 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 showed the comet’s spin-axis orientation, angular velocity, major landmarks and other basic characteristics.
Eventually, the spacecraft was 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 mapped the nucleus in great detail. Eventually, five potential landing sites were selected for close observation.
Landing on the comet (November 2014)
Unfortunately it bounced and drifted across the comet before finally coming to rest in the shadow of a mountain. This deprived it of the power needed for a complete analysis of the surface via the installed experiments but nevertheless much valuable information was gained before the batteries were finally exhausted.
Data from Philae was transmitted back via the communication systems of Rosetta which is continuing with observations using its own installed experiments. However by the end of September 2016 with its propellant largely depleted it is

Once a suitable landing site was chosen, the lander was released from a height of about one kilometre. Touchdown took place at walking speed — less than one metre per second.

currently planned to end the mission with a touchdown of Rosetta on the comet thus ending one of the most extraordinary pieces of space exploration.
35M Antenna – New Norcia – W. Austalia
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
Deep-space communications
Ground Segment
Data processing of the scientific information although quite complex is almost straightforward compared with the equipment needed to ensure raw data receipt from deep space and its subsequent safe delivery to the various laboratories. Cooperation was needed from many different space entities around the globe to ensure maximum visibility and hence communication with Rosetta.
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.
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.
Radio communications between Rosetta and the ground will use a newly developed deep-space antenna which was built by ESA at New Norcia, near Perth in Western Australia. This 35-metre diameter parabolic antenna concentrates the energy of the radio signal in a narrow beam, allowing it to reach distances of more than 1000 million kilometres from Earth.
Signals are transmitted and received in two radio frequency bands: S-band (2 GHz) and X-band (8 GHz). The radio signals, travelling at the speed of light, will take up to 50 minutes to cover the distance between the spacecraft and Earth! ESA is building another 35-metre parabolic antenna at Cebreros in Spain. It will begin to operate in 2005 providing further coverage for Rosetta.
Massive memory
During the mission, the rate at which data can be sent from Rosetta to Earth will vary from 10 to 22 000 bits per second. However, the rotation of the Earth means that real-time communications will not always be possible.
The spacecraft will be visible from the New Norcia antenna for an average of 12 hours per day. In addition, there will be several periods of communications black-out when the spacecraft passes behind the Sun.
To overcome these breaks in communication, Rosetta’s solid-state memory of 25 Gbits capacity is able to store all scientific data and then transmit them to Earth at the next opportunity.
The talk was given by an excellent speaker who had the benefit of being intimately involved with the project and contributed as a scientist during its implementation. It was very well received by those present with quite a lively question time following tea.
The above description is, quite clearly, a condensed version of what was a 30 year project. So, for those who would like to have more detail, there is masses of information to be found on the web pages of the European Space Agency by using the following links :

David Thomas