The exploration of space allows people to find out more about the origin of the universe and the Earth. There is a theory, according to which the Earth was previously a comet, which traveled through the outer space until it gained the necessary weight for staying in the solar system. Therefore, people should familiarize themselves with the “Rosetta” mission that allows investigating the properties of the comet and possibly learn more about the origin of the Earth.
It is considered that the date of the discovery of the comet 67P/C-G was 23 October 1969, although the astronomers received the first image of the comet on 11 September 1969 (Churyumov, 2004). The survey was conducted in the vicinity of Alma-Ata, where K. Churyumov and S. Gerasimenko arrived as the members of the expedition, sponsored by the Kiev State University, which was aimed at observing the comets. The astronomers carried out the observations using several telescopes. Thus, during that expedition they photographed a dozen of well-known comets (Churyumov, 2004). During the night from 11th to 12th of September, S. Gerasimenko, working on the 500-mm telescope, received a snapshot of the famous periodic comet Comas-Sola. While expositing the plate, she noticed a small, bright spot (13m) near the comet, which at first was mistakenly taken for the plate defect. A week later, K. Churyumov shot another two photographic plates of that sky section. A week later, after the image processing conducted in Kiev, the astronomers revealed that a spot significantly shifted in relation to the stars’ positioning. On the same day, the scientists sent a message to the Central Bureau of Astronomical Telegrams. Brian Marsden processed those images and calculated the orbit of the suspicious body, which was actually the previously unnoticed comet (Churyumov, 2004).
The orbit of the comet 67P/C-G is relatively small: its perihelion lies between the orbits of the Earth and Mars, and the aphelion lies beyond the orbit of Jupiter (Churyumov, 2004). There are only about 150 comets with the similar parameters, called the Jupiter family comets.
The orbital period of the 67P/C-G around the Sun was equal to 6.55 years at the moment of its detection (Churyumov, 2004). The discovery of such kind of a comet is a great success for any astronomer. Among the annually discovered dozens of comets, the overwhelming majority (and even one in several years) is either the long-period comets with an orbital period around the Sun of more than 200 years, or parabolic, which arrive from the distant outskirts of the solar system, like Ison Comet.
Just like all comets of the Jupiter family, 67P/Churyumov-Gerasimenko incurs the influence of the largest planet of the solar system (Churyumov, 2004). Jupiter is constantly changing the orbit of the comet. The scientists found out that shortly before the discovery its orbit was much more elongated, and the orbital period around the Sun was more than three years longer being equal to 9.3 Earth years. But in 1959, 67P/C-G approached the Jupiter too close and the planet reduced its period to modern 6.5 years with its force of gravity (Churyumov, 2004).
Initially, the ESA scientists planned to send “Rosetta” to the 46P/Wirtanen comet, which is the short-period comet as well but has a smaller size than the comet 67P/C-G (Schwehm & Schulz, 1999). Its core’s diameter is only 1.2 km. However, right before the spacecraft launch in January 2003, the problems with the heavy lift launch vehicle “Ariane-5” start occurred. Thus, the opportunity was missed and the scientists had to find a new space object for the research. For this reason, “Rosetta” was launched on 2 March 2004 to approach the 67P/C-G comet. And after the flight, which lasted about ten years, the spacecraft approached the comet and came over its orbit in summer of 2014 (Balsiger & Schwehm, 2015).
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It is worth noting that the “Rosetta” flight program was extremely complicated. It included four gravitational maneuvers nearby the Earth and Mars, which in the case of even minor deviation could greatly affect the success of the mission (Balsiger & Schwehm, 2015). After the large-scale maneuvers, which included three flights around the Earth, the hibernation of the “Rosetta” occurred, and lasted for the whole duration of the probe flight in the deep space before it met the comet. During this period, “Rosetta” drifted away for 800 million kilometers from the Sun (Balsiger & Schwehm, 2015). The space probe was re-activated before approaching the 67P/C-G and started the maneuver, during which it activated the thrusters and slowed it drift velocity according to the comet’s speed (about 25 meters per second) (Balsiger & Schwehm, 2015).
The device consists of two functional parts: Rosetta space probe and Philae Lander, which were both named in honor of the ancient Egyptian writings and their decryption. Thus, the space probe name originated from the famous Rosetta Stone (Sol, 2014). It was a stone panel with three texts of the identical meaning carved on it, two of which were written in the ancient Egyptian language (one was written in hieroglyphs, and the other – in the demotic script), and the third one was written in ancient Greek (Sol, 2014). Comparing the texts of the Rosetta Stone, the scientists were able to decipher the ancient Egyptian hieroglyphics. The name of the Lander is associated with decryption of the ancient Egyptian writings as well (Sol, 2014). On the Philae Island over the Nile River, the archaeologists found an obelisk with the hieroglyphic inscription, which mentioned the Ptolemy VIII, Cleopatra II, and Cleopatra III. Those writings helped to decipher the ancient Egyptian hieroglyphics. Thus, using the “Rosetta” spacecraft, the scientists of ESA hope to discover the view of the solar system of the period when the planets had not yet been formed (Sol, 2014).
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Thus, the purpose of the “Rosetta” mission was the research of the 67P/C-G that included the identification of the parameters of the comet’s core, the study of its chemical composition and the investigation of the comet’s activity changes over the time (Balsiger & Schwehm, 2015).
The main unit of the Rosetta space probe has the dimensions of 2.8 x 2.1 x 2.0 meters, with all basic equipment encased in it (Balsiger & Schwehm, 2015). There two 14-meter panels with the solar cells cover the total area of 64 square meters. When the mission started, the space probe weighted about 3,000 kilograms (with the full refueling), including 1,670 kg of the propellant, 165 kg of the payload of the orbiter and 100 kilograms of the Lander (Balsiger & Schwehm, 2015).
The new solar manufacturing technology used on two huge solar panels of the orbiter allows them to operate even at a distance of 800 million kilometers from the Sun, where the level of the solar energy accounts for only 4% of the one on Earth. Hundreds of thousands of specially designed anti-reflective silicon cells can generate up to 8,700 watts in the inner solar system and about 400 watts at the meeting point with 67P/C-G in the deep space (Berner et al., 2002).
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The probe consists of the base, platform with the devices, and the polygonal construction of the layered type made of the carbon fiber (Berner et al., 2002). Some appliances are located under the hood covered with the solar panels. Using the antenna, the probe sends its signals to the orbiter, which retransmits them to the Earth. A large complex of the scientific equipment was located on the front side of the unit, directly facing the comet’s surface. Before undocking, the Lander was mounted on the back of the orbiter to the large powerful antenna (Berner et al., 2002).
As soon as the Lander touched the 67P/C-G comet’s surface, two harpoons consolidated it on the comet (Berner et al., 2002). The self-adjusting props provided the vertical position of the Lander regardless of the roughness of the surface and then fixed the Lander on the ground. These devices were designed for the compensation of the low gravity of the comet.
The weight of the Lander is 100 kg. The payload of the unit is 26.7 kg (Berner et al., 2002). It consists of ten scientific instruments, which were designed for ten experiments used for studying the composition and the structure of the comet’s core materials, in particular, the elemental composition, trace minerals, the isotopic composition of the surface and the subsurface layer. It also enables to explore the strength, density, texture, porosity, ice phases and thermal characteristics of the comet’s surface. The Lander allows investigating the microscopic particles and the whole pellets as well (Berner et al., 2002).
The pyrolyzer, the gas chromatograph, and the mass spectrometer constitute the basis of the analytical laboratory of the Lander. The MIRO tool is a small spectrometer, which is used to display the amount, temperature and the rate, at which the water vapor and other molecules leave the core of the comet (Berner et al., 2002). It can measure the temperature under the surface (at the depth of a few centimeters) as well.
COSIMA (Cometary Secondary Ion Mass Analyzer) collects and studies the comet’s dust particles. It is equipped with 24 gold plates the size of which is 1×1 cm, covered with a thin layer of black gold (Berner et al., 2002). Collection procedure of the dust is the following: one of the 24 plates opens and is bumped by the particles at the speed of 1 to 10 meters per second. The method of mass-spectrometry of the secondary ions is applied in COSIMA. According to this method, the indium ion beam irradiates the small grains. As a result, it creates a beam of the secondary ions, which the spectrometer can easily analyze (Berner et al., 2002).
The scientists are now able to study the shape of the 67P/C-G comet’s surface using OSIRIS scientific camera installed on the “Rosetta” (Berner et al., 2002). They divided the surface of the comet into 19 areas, which differ from each other by their appearance. They named these areas in honor of the Egyptian gods. In general, one can identify five main types of the surface: the land covered with the dust, the plain areas, large-scale depression, the areas with the holes and ring structures, and the rocky areas.
After the landing of “Rosetta”, some parameters of the comet were updated. The scientists knew previously that the comet consists of the “head” and the “body” connected with the “neck”. Thus, the size of the comet’s “head” is about 2.6 × 2.3 × 1.8 km, and the size of the “body” – 4.1 × 3.3 × 1.8 km (Balsiger & Schwehm, 2015). The total volume of the celestial body is about 21.4 cube kilometers. The weight is about 10 billion tons. In such a way, the researchers obtained the density value of about 470 kg/m3. Considering the fact that the basic structure of the comet contains the water ice and the dust, whose density is higher than the obtained one, the researchers concluded that 67P/C-G’s porosity is extremely high being approximately 70-80% (Balsiger & Schwehm, 2015).
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In the structure of the rocky “walls” of the comet, one can identify a lot of cracks (Balsiger & Schwehm, 2015). The scientists associate their formation with the day and night changes, which happen relatively quickly. Some areas are covered with the small rocks, which can be compared with crawls. Unfortunately, their origin is still unknown.
MIRO allowed investigating the temperature on the surface of the 67P/C-G. In September 2014, the comet’s temperature at the depth of one centimeter was changing ranging from -243 to -83 °C (or from 30 to 190 Kelvin) during the day (Balsiger & Schwehm, 2015). The seasonal changes are the key factor, which influenced the changes in temperature. At some latitudes of the comet it is warmer, and in some other areas, it is colder. For example, the lowest temperature was recorded in the south polar area of the celestial body, which had not seen the Sun rays for a few years (Balsiger & Schwehm, 2015). Meanwhile, the highest temperature was observed at the northern latitudes.
The change of day and night affects the temperature fluctuations as well. However, the degree of these fluctuations varies depending on the latitude. Thus, in the southern polar region in the autumn of 2014, the fluctuations were close to zero, while at the northern latitudes, the changes of the temperature were about 50 Kelvin (Balsiger & Schwehm, 2015).
The ESA scientists found out that at a certain time of the day, specific areas of the comet release greater amount of gas. Thus, in the period from June to September 2014, the largest volume of the gas was spewed from the comet’s “neck” in the second half of the day, which lasted 12.4 hours (Balsiger & Schwehm, 2015). Additionally, during the observation period, the researchers marked the significant increase of the water vapor amount emitted by the 67P/C-G. In June 2014, the comet released about 300 ml of water every second and in August of the same year, its surface evaporated 1.2 liters of water per second (Sol, 2014).
However, the gas streams were recorded arising from the depressions (Sol, 2014). The eruption of the gas plays a crucial role in the transportation of the dust throughout the comet’s surface. The researchers assume that the dust can function as the “protector” of the ice from evaporation, hiding it from the Sun rays.
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The major part of the northern hemisphere of the 67P/C-G is covered with the dust. While heating, the ice turns into gas, which forms the comet’s atmosphere (Sol, 2014). The dust leaves the core with the vapor as well, but its speed is slower. The scientists conducted the investigations of the cosmic dust located on the comet’s surface with the use of the data collected by COSIMA during the period from August to October 2014 (Sol, 2014). During that period, the 67P/C-G approached the Sun by 85 million kilometers. During the collection of the particles, the researchers noticed that a lot of large particles crumbled into the smaller fragments. The fact that the specks of the dust crumbled into pieces so easily made the scientists conclude that they were not interconnected in a proper way. Thus, they did not contain the ice fragments, which could prevent the erosion (Balsiger & Schwehm, 2015).
The analysis of the particles showed the amount of the sodium in their content (Balsiger & Schwehm, 2015). Based on this fact, the researchers assumed that those particles are the interplanetary dust ones, which could get to the comet’s surface when it moved through the meteor showers, for instance, the Perseids (Tubiana, 2008). The scientists believe that the fixed particles do not belong to the comet. They landed on its surface after the passage through the previous perihelion, after which the gas streams subsided, and nothing more could raise dust particles from the 67P/C-G surface. Therefore, the comet simply “collected” the interplanetary space dust for a few years of its traveling (Balsiger & Schwehm, 2015).
There are several hypotheses about the past of the 67P/C-G core (Tubiana, 2008). The “body” and the “head” of the comet have a similar composition. Thus, one can assume that they formed one body in the past, which destroyed over time and gradually acquired the modern shape. At the same time, the latest data does not allow the scientists to exclude the alternative scenario, according to which the comet’s core could be formed as a result of the collision of two celestial bodies (Tubiana, 2008).
The comet 67P/C-G was discovered on 23 October 1969 by K. Churyumov and S. Gerasimenko, although the astronomers received the first image of the comet on 11 Sep. 1969 in the vicinity of Alma-Ata. The orbit of the comet is relatively small. Its perihelion lies between the orbits of the Earth and Mars, while the aphelion lies beyond the orbit of Jupiter thus, the orbital period of the 67P/C-G around the Sun is equal to 6.55 years, although it was around ten years before its discovery.
ESA sent the spacecraft “Rosetta” on 2 March 2004 to the 67P/C-G with the purpose of researching the comet. The research included the identification of the parameters of the comet’s core, the study of its chemical composition and the investigation of the changes of the comet’s activity over time. To research the elemental and isotopic composition and explore the strength, density, texture, porosity, ice phases and thermal characteristics of the comet’s surface, the scientists use ten scientific instruments on the Philae Lander, including pyrolyzer, the gas chromatograph, and the mass spectrometer. Using this equipment, the scientists have determined the real comet’s dimensions and the porosity of its structure. They have also discovered that the surface of the comet is not homogeneous, and there are the areas covered with the dust, the plain areas, large-scale depressions, the areas with the holes and ring structures, and the rocky areas.
The scientists investigated the dust as well and determined its content, according to which they assumed that the dust does not define the comet’s properties because it has the interplanetary origin, caused by movement through the meteor showers. However, they presumed that while 67P/C-G moves through the perihelion, the dust will completely leave the core with the gas that evaporates due to the temperature rise, which forms the comet’s atmosphere. The researchers found out that the temperature fluctuations at the surface of the comet have both daily and seasonal nature.