Science has been the mother of all inventions and discoveries. More and more discoveries are being made, thus increasing the amount of knowledge that scientists have about the surrounding environment and the universe at large. One of the areas that have reached the most significant milestones in the matters of discovery is astronomy. Astronomy is the study of the universe at large, including all the celestial objects and processes that originate outside the Earth’s atmosphere. From learning about planets, their satellites and orbits to landing the first man on the moon, scientists have continued to make groundbreaking discoveries. These discoveries include studying celestial objects such as comets, meteorites and asteroids. Astronomers use various spaceships, rockets and artificial satellites to study these bodies. A comet named Comet 67P is one of such subjects of investigation. The exploration of this comet is known as the Rosetta comet mission. This mission was aimed at studying the components and composition of the comet. This paper discusses the development, launch and achievements of the Rosetta comet mission.
Introduction
The Rosetta Comet Mission is the exploration of a comet, the 67P/ Churyumov-Gerasimenko. This mission was spearheaded by the European Space Agency (ESA). Hand in hand with Philae (the lander of the spacecraft), it landed a probe to explore the comet. On the 6th of August 2014, Rosetta reached the 67P and performed several maneuvers to be caught in its orbit. Philae, the lander module, recorded its first successful landing on the 12th of November the same year. From the comet’s surface, Philae sends data together with Rosetta as part of its mission. During the journey, Rosetta went through the asteroids 2867 Steins and Lutetia and Mars. According to Auster et al. (2010), magnetic field measurement was made during the flyby of the 2867 Steins. Measurements of the plasma environment as well as the magnetic field were also conducted during the 21 Lutetia flyby (Sierks 2011). This shows that throughout the journey, probes were launched on various celestial bodies.
Naming of the spacecraft and its lander is very significant. The name of the spacecraft, Rosetta, derives from a stele of Egyptian origin, the Rosetta Stone. The lander, on the other hand, received its name from the Philae obelisk. It has a hieroglyphic carving of both the Egyptians and Greeks. Both the hieroglyphics from the Rosetta Stone and the Philae obelisk aided the decryption of the Egyptian system of writing. The significance of these two names is that they facilitated accurate description of the early Solar System and the comets. In a more direct analogy of the naming, the spacecraft is in possession of a small nickel alloy called the Rosetta disc that is made up of thirteen thousand pages written in 1200 languages. The disc was a donation from the Long Now Foundation.
In the past, various international space probes were launched, especially in 1986, during the approach of Halley’s comet. The most successful among them was the ESA probe called the Giotto. This probe helped to obtain critical information about the comet. However, more questions arose, and thus another follow-up investigation was needed to shed light on these answers as well as on the composition of the comet. Therefore, NASA in cooperation with ESA started the development of new probes. NASA was working on a mission called CRAF while ESA settled CNSR. To minimize the development costs, the two missions used the Mariner Mark II probe strategy. However, in 1992, due to monetary constraints, NASA cancelled the CRAF mission, and ESA adopted the design to develop their space probe. In addition, this shift took place due to the infeasibility of the CNSR. The final model was a spacecraft resembling the CRAF mission: a flyby of an asteroid and a rendezvous with a comet, which was developed inclusive of its lander that supports on-site examination.
Rosetta’s launch was done at the Guiana Space Center in French Guiana on 2nd March 2004. The Ariane 5 rocket aided the launch and reached the Comet 67P five months later, on 6th August 2014. The spacecraft became the first to enter the orbit of the comet. The probe was composed of the main orbiter featuring twelve instruments and the lander featuring nine instruments. To complete the most detailed exploration of any comet ever recorded, the mission is set to stay in orbit for 17 months. The control of the spacecraft is done by the European Space Operations Centre (ESOC) located in Darmstadt, Germany. The European Space Astronomy Center (ESAC) located in Villanueva de la Canada in Spain retrieves, calibrates, archives and distributes the data received as well as plans the Scientific’s operation payload. The first landing of Philae on the comet’s nucleus was made on 12th November 2014.
Development and Launch
The Rosetta comet mission was significant due to several reasons. The first achievement made by the spacecraft was a flyby made across the asteroid belt, which was the first time a European spacecraft encountered these celestial objects. Again, Rosetta was the first probe to fly closest to Jupiter’s orbit while being powered by solar cells. It also became the first probe to enter the orbit of a nucleus of a comet and to fly against a comet heading to the inner Solar System. Moreover, it was the first spacecraft that examined transformation of a frozen comet by the sun’s warmth at a very close range. Philae, the spacecraft’s lander, aided the first controlled landing on a nucleus of a comet. The instruments of the lander captured images of the comet’s surface, and enabled performing the first on-site analysis of the composition of the nucleus of a comet.
The Rosetta bus weighs 2.9 tons, including the scientific instruments that are 0.165 tons and the lander module that is 0.1 tons. The spacecraft is a composed of a central frame that has the dimensions 2.8 x 2.1 x 2.0 meters and a honeycomb platform. The module supporting the payload is situated on the roof of Rosetta. It is a place where the scientific instruments are stored. On the other hand, there is a module supporting the bus, which is positioned on the floor of the probe and controls the spacecraft’s support subsystems. Since Rosetta is designed for conducting investigations far from the sun, heaters are installed throughout the whole spacecraft to keep the systems warm in conditions of lack of solar energy. For communication, Rosetta has installed two omnidirectional low-gain antennas, a steerable 80cm medium-gain antenna, which has a fixed position, and a 220 cm high-gain parabolic antenna.
The spacecraft uses two solar arrays that have an area of 64m2. Each of them is divided further into five solar panels, each of which is 2.25 meters in width and 2.736 meters in length. Each cell is made of silicon, having the thickness of 200 micrometers, the length of 61.95 millimeters and the width of 37.75 millimeters. During hibernation at 5.2 AU, solar arrays produce a minimum of 400 watts; approximately 850 watts are produced when the operations of the comet begin at 3.4 AU. Solar arrays product a maximum of about 1500 watts at perihelion.
To propel the spacecraft, a combination of 24 paired bipropellant thrusters, each producing a thrust force of 10N are used. For delta-v-burns, four pairs of thrusters are used. At launch, the spacecraft carries 1.7191 tons of propellant: 659.6 kg of monomethyl hydrazine fuel and 1.0595 tons of dinitrogen tetroxide oxidizer, which is contained in two 108-liter grade 5 tanks made of titanium alloy. The fuel provides the delta-v at least 2300 m/s velocity during the mission. Apart from that Rosetta contains two 68-liter helium tanks of high pressure that provide pressurization of the propellants.
The development of the spacecraft occurred in a sanitized environment in reference to COSPAR regulations. However, sterilization during the development was not significant because scientists regard comets as objects where prebiotic molecules can be found, as it was described by Gerhard Schwehm, who was the lead scientist of the project (“No Bugs Please, This Is a Clean Planet” 2002). The whole project cost was 1.8 billion USD, which is equivalent to 1.3 billion euros.
Initially, Rosetta was to be launched on 12 January 2003 to meet with another comet. This plan was halted after a failed launch of the Hot Bird 7 that should have been aided by an Ariane 5 carrier rocket, which used the same model as the one that was to be used to support the launch of Rosetta. As a result, a new strategy that targeted the 67P was developed and a new launch date, the 26th of February 2004, was set (Ulamec et al., 2006). Modification of the landing gear was made due to the increased impact velocity that resulted from the larger mass. After two unsuccessful attempts to launch Rosetta, the spacecraft was launched on the 2nd of March 2004.
Orbiting and Detachment
The probe used maneuvers assisted by gravity for acceleration in the Solar System to acquire the velocity required to meet with the comet 67P. The orbit of the comet was known before the launch of Rosetta, approximately 100 km. On-board cameras that were installed on Rosetta sent back information to the ESA’s Operation Center, which was situated at the distance of 24 million km, and the information was used to estimate the comet’s position concerning its orbit.
The actual meeting with the celestial body was not until August 2014. Rosetta then began various maneuvers, taking it on two paths that were successive and triangular with an average of a hundred and fifty kilometers from the nucleus. The segments of the maneuvers were escaped trajectories that were hyperbolic in shape and alternated with thruster burns. Rosetta started orbiting the comet on September 10 after reaching about 30 km from the comet.
Since the surface of the comet was not known before the spacecraft’s arrival, the orbiter mapped it in the preparation of the detachment of Philae, the lander module. Five potential landing spots have been made by 25th August 2014, and 15th September of the same year Spot J, given the name Agilkia, was announced as Philae’s landing site. Agilkia was located on the head of the comet.
The detachment of the lander module occurred on 12th November 2014. The velocity with which it approached the comet was 1 m/s. After the initial landing, Philae bounced twice before finally resting on the comet. The plan was to shoot two harpoons into the comet’s surface after successful landing to prevent Philae from bouncing off (Knapton 2014). However, the analysis conducted on the data from telemetry showed that the surface of the first touchdown was soft, made up of granular substances about 250 cm deep and the harpoons did not fire after landing. The schedule that developed for Philae after landing was threefold: to characterize the nucleus, to study the activities of the comet and how it developed over time, and to determine the chemical components present. The landing of Philae was not very successful because it landed in an odd position, probably in the shadow of a crater wall or cliff, and then tilted at an approximate angle of 300. The angle made it almost impossible to power up the batteries, and after two days, the batteries ran out of power, leading to a loss of communication with Rosetta. Contact was reestablished on several occasions between 13th June and 9th July the next year and was then lost again. Since then contact was lost.
Results
During orbiting the comet, several discoveries were made. First, the magnetic field of the comet revolved at between 40 and 50 MHz. The signal from Rosetta was magnified by being sped up ten thousand times till the rendition of the oscillation could be heard. Results from the Philae’s landing showed that no magnetic field exists on the comet’s nucleus. This brought the astronomers to conclude that the resultant magnetic field that had been initially detected by the spacecraft might have been a result of solar winds.
In addition, it was discovered that the isotopic composition of water vapor found on the comet is different from that found on Earth. The deuterium to hydrogen’s ratio is thrice that of water on the Earth’s surface. According to the astronomers, the water found on the Earth’s surface is unlikely to have come from comets. In a report by NASA on 22nd January 2015, water vapor release rate between June and August 2014 increased ten times. According to a report from 2nd June 2015, the ultraviolet imaging spectrograph on the spacecraft aided in drawing a conclusion that electrons in a height of 1 km above the nucleus of the comet led to the degrading of molecules of carbon dioxide and water sent to the coma. These electrons are produced as a result solar radiation that photoionize the water molecules. This contrasts a previously held belief that electrons are produced as a result of photons from the sun.
Instruments Used
One of the core reasons for the spearing of the Rosetta comet mission was to obtain more answers to questions that arose after the first study of the comet. A follow-up on the nucleus’s components was required. To explore the nucleus, three optical spectrometers, one radar and one microwave radio antenna are required. Some of the instuments used in this exploration of the nucleus include ALICE, OSIRIS, VIRTIS, MIRO, CONSERT and RSI.
These instruments were used to conduct various tests and studies of the composition of the comet. The first instrument is ALICE. An ultraviolet imaging spectrograph (ALICE) uses the ultraviolet spectrograph to search and quantify the content of noble gasses in the nucleus (Feldman et al. 2015). Using these measurements, the temperatures during the creation of the comet were estimated. To detect noble gasses in the comet’s nucleus, an array of cesium iodide photocathodes and potassium bromide were used. Another instrument used is Optical, Spectroscopic and Infrared Imaging System (OSIRIS). It is a camera system that has both a wide-angle lens and a narrow-angle lens with a CCD chip of 2048 x 2048 pixels (Thomas et al. 1998). According to Coradini et al. (1996), VIRTIS makes the nucleus’s pictures in the Infrared. It also conducts searches in the coma for infrared spectra molecules. For infrared detection, an array of mercury cadmium telluride is used. The MIRO detects temperatures and the abundance of carbon dioxide, water and ammonia using microwave emissions. Lastly, the RSI uses Rosetta’s communication system to physically investigate the comet’s inner coma and the nucleus.
Another goal of the Rosetta comet mission was to study gasses and particles around the comet. This was done using various equipment, including ROSINA, MIDAS, COSIMA and GIADA. The ROSINA resolves nitrogen from carbon monoxide and is sensitive to neutral molecules and ions. MIDAS is used to probe for physical aspects deposited on a plate made of silicon. The COSIMA performs a dust particles analysis by spectrometry of the secondary ion mass. Lastly, the GIADA conducts an analysis of the coma’s dust environment. It measures the mass, speed, momentum and cross section of each grain going into the instrument.
Conclusion and Recommendation
In conclusion, science has helped people to obtain relevant information on various topics, and astronomy has not been left behind. The Rosetta comet mission is an exploration of the comet 67P, which is a follow-up on questions that arose after the study that took place on Halley’s comet. A space probe called Rosetta with a landing module, the Philae, was developed. After the launch of the spacecraft, Rosetta made several achievements. Thus, it was the first spacecraft to pass closest to Jupiter’s orbit being powered by cells charged by the sun, the first spacecraft in Europe to go through the asteroid’s central belt, and the first probe to head to the inner Solar System hand in hand with a comet. The Rosetta comet mission is an example of the scientific expeditions and probes that scientists undertake to make new discoveries. Such probes are necessary since they add to and correct the existing knowledge. However, such expeditions require months, if not years, of preparation, and a high budget. Therefore, careful project management has to be done to ensure no glitches occur during the mission.
