ESA’s Rosetta Cometary Rendezvous Mission
The European Space Agencie’s (ESA) Rosetta Mission began on 2004 and rendezvoused with comet 67P/Churyumov-Gerasimenko. The spacecraft studied the physical and chemical properties of the comet’s nucleus and the evolution of the coma during perihelion approach. In addition to these scientific investigations, the Rosetta lander Philae was deployed onto the comet’s surface. Although it carried out a landing different to the one which was planned, touching down on an area of the comet with insufficient sunlight, causing the lander to enter hibernation due to insufficient power.
In November 1993 ESA’s Science Programme Committee approved the International Rosetta Mission. Rosetta was the Planetary Cornerstone Mission in ESA’s long-term space science programme. The main target of the mission was originally Comet 46P/Wirtanen. Due to postponement of the first launch a new comet needed to be selected as target: Comet 67P/Churyumov-Gerasimenko. Finally, Rosetta was launched on 2 March 2004 along with Philae: its lander module. 
“Rosetta’s main objectives were:
- A global characterization of the nucleus.
- The determination of its dynamic properties.
- The surface morphology and composition.
- The determination of chemical, mineralogical and isotopic compositions of volatiles and refractories in the cometary nucleus.
- The determination of the physical properties and the interrelation of volatiles and refractories in the cometary nucleus.
- Studies of the development of cometary activity and the processes in the surface layer of the nucleus and inner coma that is dust/gas interaction.
- Studies of the evolution of the interaction region of the solar wind and the outgassing comet during perihelion approach.” 
Figure I: Labelled photograph of Hale-Bopp comet, showing its coma, its yellow dust tail and its blue plasma tail. 
Comets provide information about the origins of the Solar System as their composition is similar to that of the pre-solar nebula. It was for this reason that Rosetta was sent to study Comet 67P’s composition as it is thought comets were key in planetary evolution. This is due to the fact that there were more frequent cometary impacts in the early stages of the Solar System. These impacts are thought to have provided Earth’s water and organic molecules for evolution. 
Comets provide tantalizing clues to the early history of the Solar System. Due to both their small masses and long orbital periods, they have undergone less internal and external evolution than other members of the Solar System.
The icy conglomerate model proposed by Whipple (1950,1951) provides the basis for our present conception of a comet nucleus, namely, that it is a solid but fragile, ice-dust mixture of diameter 0.1 to 10 km. As this so-called dirty snowball transits the Solar System towards perihelion the surface is subjected to a variety of erosive processes, with the most prolonged exposure being to cosmic rays, solar wind ions and surface photoelectron currents.
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After a comet passes within 3 AU of the Sun, volatiles sublimed from the surface by solar radiation form an expanding atmosphere of gas and dust called the coma. The formation of a dust and/or plasma tail may or may not follow. The type I, plasma or ion tail is formed by the interaction of ions in the coma with the solar wind magnetic field which folds onto the comet ionosphere and is presumed to accelerate ions from the coma in the anti-solar direction to form the plasma tail. The type II tail is comprised of dust formerly embedded in the nucleus and released as the surface ices vaporize in the solar radiation field. These different parts of a comet are depicted in Figure I. 
According to Gerard Kuiper’s 1951 theory, there is a dis-like belt of icy bodies beyond Neptune’s orbit. In this location (near Pluto) many dark comets orbit the Sun. A small gravity change causes these comets to be put into orbit closer to the Sun. These comets are called short period comets due to their orbits being less than 200 years. However, the so-called long period comets originate in a region called the Oort Cloud (100,000 AU from the Sun). These comets can have orbital periods of 30 million years.
Figure II: Image depicting the duck-shaped Comet 67P/Churyumov-Gerasimenko. 
Comet 67P/Churyumov-Gerasimenko (Figure II) was first observed in 1969. Its orbital period is of 6.5 years. Comet 67P is a short period comet with low orbital inclination which belongs to the so-called Jupiter Family as Jupiter’s gravity controls its own orbit. These comets originated in a region of space beyond Neptune’s orbit called the Kuiper Belt. This region is filled with colliding icy bodies, these collisions cause some of the bodies to exit the Kuiper Belt’s orbit and move towards the Sun. 
- THE MISSION
The original target of the Rosetta mission was comet 46P/Wirtanen. In December 2002 this target comet was changed to comet 67P due to a failure of the Ariane rocket, causing the January 2003 launch to be rescheduled. Rosetta was finally launched by an Ariane-5 G+ launch vehicle from the Guyana Space Center in Kourou, French Guyana, on March 2, 2004. 
Milestones of the Rosetta Mission 
Rosetta was a 3-axis stabilised spacecraft with a box-type main structure of 2.8 x 2.1 x 2.0 m. A schematic view of the spacecraft can be seen in Figure III. This primary structure was made up of a central aluminium honeycomb cylinder with shear panels. These connected the lateral panels onto which the spacecraft and scientific instruments were mounted. The principal propulsion system was located in the centre of the structure along with two propellant tanks mounted around a vertical thrust tube. In order to allow for attitude and trajectory corrections and additional 24 10N thrusters were added to the spacecraft. The scientific payload was mounted on the top panel, with the high gain antenna and Philae attached to the opposite (bottom) panel. Both 14m solar panels were therefore attached to the remaining two sides of Rosetta, making the total span total 32 m. Rosetta’s total launch mass was estimated to be 2900 kg, which included the propellant, the scientific payload and the lander Philae (see Table II). 
Spacecraft Properties 
Figure III: Schematic view of the Rosetta spacecraft. The payload was mounted on the panel pointing to the comet’s direction. 
The solar panels were composed of specially developed silicon cells for low-intensity, low-operation (LILT). The spacecraft thermal control was particularly demanding due to the strong variation in solar radiation during the mission (up to a factor of 25). Heaters and radiators covered by louvers were placed in strategic points (i.e pipelines, thrusters and fuel tanks) in order to compensate for the thermal imbalance. Multi-layered insulation blankets (MLI) which covered Rosetta were also used for this same purpose.
Figure IV: A schematic view of the Rosetta spacecraft and its scientific payload. 
Two radio transmission frequency bands were used to enable data transfer between the spacecraft and the lander and the spacecraft and Earth. This communication was achieved using S-band and X-band transmitters (2 GHz and 8GHz respectively) and an S-band receiver.
The Rosetta orbiter’s scientific payload included 12 experiments, including the lander. These were provided and designed by different consortia from institutes around Europe and the United States (see Figure IV)]. Some of these included:
- ALICE- an ultraviolet imaging spectrometer used to analyse the gases in the tail and coma. It also measured the production rates of H2O, CO2 and CO. It will also provide information of the surface composition of the nucleus.
- CONSERT- comet nucleus radio wave sounding experiment, which probed the nucleus’ interior using radio waves.
- COSIMA- secondary ion mass spectrometer designed to analyse the characteristics of the dust grains of the comet, i.e. their composition and their organic nature. 
The 100kg lander was carried on the side of the orbiter until arrival to comet 67P. Philae’s structured comprised various elements made of carbon fibre, i.e. a base plate, platform for its instruments and a “polygonal sandwich construction”. Some of the scientific payload was covered using a solar cell covered casing. 
Philae carried 10 experiments (see Figure V), some of which included:
- APXS- alpha and X-ray spectrometer deployed to the surface to measure the elemental composition of the surface material.
ROMAP- magnetometer and plasma monitor to explore the magnetic and plasma environment of the landing site and its interaction with the solar wind. 
Figure V: Labelled diagram of the Philae lander and its scientific payload. 
LAUNCH AND TRAJECTORY
Rosetta’s ten year expedition began in March 2004 with an Ariane 5 launch. The 2900 kg spacecraft was first inserted into a parking orbit and then sent on its journey towards the target comet.
Unfortunately, no existing rocket had the capability to send such a large spacecraft directly to comet 67P. Instead, various deep space manoeuvres had to be carried out in order for the spacecraft to gain sufficient energy to rendezvous with the comet. Rosetta entered the asteroid belt twice to gain velocity from the gravity assists provided by close flybys of Mars and Earth (see Table I).
Figure VI: diagram that depicts Rosetta’s 10 year trajectory through deep space. 
On March 2005, after travelling away from Earth, Rosetta encountered Earth again with a flyby distance between 300 and 14 000 km. During this first gravity assist, a few operations were carried out: tracking, orbit determination and payload verification. After another gravity assist from Mars on February 2007, Rosetta travelled back to Earth (November 2007 and 2009) for another two assists from our planet (see Figure VI).
After the previously mentioned gravity assists, Rosetta was put into cruise mode towards the asteroid belt. According to plan, Rosetta flew by asteroids Steins (September 2008) and Lutetia (July 2010) in order to study them and test the spacecraft measurement devices for better calibration. The spacecraft was then put into electronic hibernation in order to save energy. During this, Rosetta’s maximum distances from the Sun and Earth were recorded (around 800 million km and 1000 million km). Rosetta exited this deep space hibernation mode on 20 January 2014 in order to finally rendezvous with Comet 67P. 
ARRIVAL AND LANDING
On August 2014 Rosetta arrived at comet 67P. The whole approach phase was very critical for the mission because of the delicate navigation required.
Before landing on the comet, the characterisation phase had to take place. This was due to the need to develop an engineering model of the comet in order to allow for the start of the proper orbit phase and the selection of the possible Philae landing sites.
To design and plan the next phase it was necessary to have an accurate model of the comet’s gravitational field and attitude with an associated reference frame. The first step consisted of cataloguing the comet’s evident features of its surface that could easily be recognised in the images and used as navigation references. “These were then fed into the orbit determination system which consisted of an estimator of:
- Spacecraft position and velocity.
- Comet position and velocity.
- Comet spin axis.
- Comet attitude evolution (thus rotation period).
- Comet gravity potential (mass) and position of the centre of mass.
- Comet shape.” 
Figure VIII: Schematic showing Philae’s landing trajectory scenario. 
Figure VII Diagram showing the pyramid orbits carried out by Rosetta in order to obtain an accurate model of comet 67P. 
The collection of these measurements was carried out using a ‘pyramid orbit’, this consisted of a sequence of three hyperbolic arcs flown in front of the comet at a distance varying between 115 and 90km (see Figure VII). In this way the spacecraft was not captured by the comet’s gravity but was still in a position to have its orbit affected by it, in this manner the spacecraft was also able to view the comet from different angles. During the next 10 days, Rosetta flew in this triangular trajectory. 7 days later another triangular orbit was flown but now at a shorter distance from the comet (70 to 50km). 
During the characterisation phase, various landing sites were chosen and assessed. The most important aspect of selection was the sun illumination, as Philae would rely on the power generated by the solar generator. A site was finally chosen for landing and was named Agilkia. 
With the landing site selected, Rosetta then carried out several manoeuvres in order to reduce its distance from the comet to about 22.5 km. Philae was separated from the orbiter with an adjustable ejection device and was stabilized during descent by an internal fly-wheel (see Figure VIII) . How accurately engineers predicted Rosetta’s speed and position would determine whether Philae arrived on target. The calculations had to factor winds streaming from the comet and the irregular gravitational field produced by comet 67P’s shape. 
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However, after touching down on the target region as planned, Philae did not secure itself to the comet, and it bounced to a few locations on the comet. There were three methods used to secure Philae after landing: ice screws, harpoons and a small thruster. The ice screws had been designed for landing on a soft material (as a comet’s surface was thought to be soft), but the surface was in fact hard and they unfortunately did not penetrate the surface.
The harpoons could work in both soft and hard material. They were meant to fire contact and lock the lander to the surface, while a thruster on top would push it down to counteract the harpoon’s recoil. Attempts to the thruster the night before failed: it is thought that the seal did not open. Tests were carried out the night before to arm the thruster, however these possibly failed due to the seal not opening. However sensor failure could have also been the case. 
Although Philae operated for nearly 57 hours (less than the 64 hours planned to complete its primary activities) it managed to return results from all 10 of its instruments . The lander went into electronic hibernation on the evening of 14-15 November 2014, due to insufficient sunlight at its new location.
As the comet moved closer to the Sun, on 13 of June 2015, Philae established contact with Rosetta. After the 13 of June, Philae made a further seven intermittent contacts with the spacecraft (the last one being on 9 July). However, these contacts were too short and unstable to allow any scientific measurements to be received. 
Since no signal had been received from Philae since July 2015, it was decided on July 2016 to switch off the interface used for communications between Rosetta and Philae. This was part of the preparation for the end of the mission in order to save power. 
Figure IX: Image taken by OSIRIS camera on Rosetta which revealed Philae wedged into a dark crack on the comet. 
With less than a month until mission end, on 5 September 2016, Rosetta’s high-resolution camera OSIRIS revealed the Philae lander wedged into a dark crack on Comet 67P (see Figure IX). These images provided proof of Philae’s orientation. Having acquired this knowledge was very important as Philae’s three days of science could now be put into proper context. 
The mission came to an end on 30 September 2016 when Rosetta carried out its final manoeuvre: colliding with the comet from an altitude of 19 km. The aim of this was to study the comet’s gas, dust and plasma environment very close to its surface, as well as to take very high-resolution images of the comet’s surface.
This was purposely done in order to study some of the comet’s characteristics as close to the surface as possible: the comet’s gas, dust and plasma environment. During collision with the comet, Rosetta took very high resolution images close to the comet’s surface. 
Since Rosetta arrived at Comet 67P/Churyumov-Gerasimenko on 2014, it has helped scientists decipher the make-up of comets. The most important and remarkable findings were:
- The comet harbours organic compounds– several organic compounds such as glycine (the simplest amino acid) were found in the comet’s nucleus. These are believed to be the chemical building blocks of life. This suggests comets could have helped create life on Earth by supplying out planet with the necessary raw materials. 
- Comets did not bring Earth its water– it was found that water on comet 67P contained a higher deuterium-to-hydrogen ratio than water on Earth. Therefore ruling out Kuiper Belt comets from bringing water to Earth. 
- Detection of free molecular oxygen gas– molecular oxygen had never been detected before in cometary comas. The creation of molecular oxygen is key to understanding the evolution of the universe and the origin of life on Earth. It was initially thought that the oxygen was frozen in the nucleus and escaped to the coma. However, further analysis of the data revealed it had indeed been created due to interactions of water molecules in the coma and particles from the Sun. 
Rosetta was developed over a decade ago. At that time space technology was much less advanced, making the Rosetta mission one of the greatest spaceflight challenges ever faced. The mission provided a great amount of information and data on the origin of comets and their characteristics. Making future missions better informed and better adapted. (such as:…)
Although Philae’s landing did not go according to plan, the lander still managed to serve its purpose and provide outstanding results. After Philae went into hibernation Rosetta still stayed in rendezvous with Comet 67P and still provided more images as the comet approached its perihelion.
The data obtained has allowed scientists to analyse it for the past years and will keep allowing interpretation for the following decades.
Rosetta managed to be the first ever spacecraft to put a lander on a comet and recorded the smallest distance from a comet.
Overall, I would consider this mission a huge success as it not only provided outstanding findings about comets but it also enabled ESA to put itself on the map as one of the leading space agencies.
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