PART I What Do We Know About Comets?

(1) Earliest man wondered at the objects he saw in the heavens above, particularly
about those that we call comets. These would appear temporarily, shine brightly, and then disappear. Myths, predictions (often terrible) and explanations were developed to explain these unknown phenomena. In fact, curious and sometimes bizarre explanations are still given and unusual and catastrophic events linked to their appearance ¹. (line 5)

(2) Comets, unlike the other small bodies in the solar system, have been known since antiquity. In 240 ВС the Chinese recorded Halley's appearance. It is one of the brightest, most visible comets and makes a " regular" appearance every 76 years. Being a periodic comet (one whose orbital period is less than 200 years), it has been the source of theorizing and speculation by both serious stargazers and common folk alike. (line10)

(3) What is a comet? How many are there? Where do they come from? Until the

Twentieth century, little definitive was known about their paths, their composition, or 15 about their activities and " life". Indeed, it was during Halley's last appearance, in 1986, that a gigantic step was taken in understanding the nature of comets and a number of then-held hypotheses were confirmed, or discarded. Today we know quite a bit, although there are still holes in our knowledge. But even these may soon be filled. (line16)

(4) Comets are celestial bodies orbiting the sun. Their orbits are eccentric, highly elliptical paths that bring them very close to the Sun and then swing them away deeply into space, often beyond the orbit of Pluto ³. They contain a mixture of non-volatile grains or dust and frozen gases. Comet structures are diverse and very dynamic, but they all consist of a fuzzy " head" (comprising a frozen nucleus of dust and other materials, resembling a dirty snowball, surrounded by a cloud of diffuse material, called a coma) and a " tail" that tends to grow and be brighter as the comet approaches the Sun. Comets are invisible except when they are near the Sun; then some of the frozen materials vaporize. It is these gases and other granular particles that one sees in the tail. (line 25)

(5) The current, generally accepted view is that comets are the " leftovers" from the formation of planets and are made up of the original material comprising the Solar System. There may be billions of comets orbiting the Sun in the so-called Oort cloud ⁴, far beyond the planet Pluto. Occasionally the Oort cloud is disturbed and a comet escapes into the inner part of the Solar System. But there are questions, such as: how active or changeable the Oort cloud is and might it contain " new" comets that have come in from interstellar space. (line 31)

(6) A 1995 count catalogues 878 comets whose orbits have been calculated, at least roughly. Of these, 184 are periodic; some of the others may also be periodic, but not enough is known about their orbits. (line 34)

(7) About a dozen comets pass the Sun each year, but a truly brilliant one comes into public view about once a decade. Since Comet Halley in 1986, a number of other comets have been observed; three major ones will be mentioned. Comet Hyakutake in 1996 was described by astronomers as one of the grandest comets of the millennium. The Shoemaker-Levy 9, discovered only in 1993, violated the Roche limit ⁵ by passing too close to Jupiter and broke into 21 fragments. For the first time, astronomers observed the collision of two extraterrestrial bodies from ground-based telescopes and from several spacecraft. For six days in July 1994, the fragments burned up in Jupiter's atmosphere or impacted on its surface, creating craters and lines. Comet Hale-Bopp, discovered in 1995, was visible to the naked eye from July 1996 through October 1997. It provided a great deal of data, but is remembered because of its role in the suicide of a small cult ⁶. (line 45)

(8) The large amount of images and data gathered from the Halley Comet probes, particularly from the ESA's 1986 Giotto, contributed greatly to our understanding of comets. Giotto, in one of the most extraordinary achievements of recent astronomy, managed to photograph Comet Halley as it spit out jets of dust and vapour at rates as high as fifty tons a second ⁷. But there were few images with close-up views of the surface of the nucleus ⁸. The then-new information established its seeming composition (ice, snow, dust), colours, shape, and other superficial features. Still to be answered are questions about a comet's mass and density, what it is made of, and whether it has any internal structure. (line 53)

(9) The last-mentioned is of particular interest. Dr. Mayo Greenberg, Professor of Astrophysics at Leiden University, Holland, has long argued that comets could сану organic contents that could land on Earth in the form of dust. In his opinion, the nucleus has a fairly open structure— the inner core containing complex organic materials; the outer shell being made of ice, water, and sulphur dioxide. New evidence seems to indicate that comet dust contains some complex carbon compounds. One scientist has gone even further, stating that cosmic rays could turn simple carbon compounds into biological molecules. Thus, if interstellar dust holds bacteria from other planets—a massive if they might survive to infect the Earth. (line 61)

PART II. The Rosetta Mission ⁹

(10) One of the most ambitious projects of cometary exploration was the Rosetta mission. It established a series of firsts; it was: the first spacecraft ever planned that will have to go into orbit around a comet; the first to observe the changes in comet activity, the first to carry a small Lander that will descend onto a comet's icy nucleus to determine its properties. The prime scientific objectives are to study the origin of comets, the relationship between cometary and interstellar material and its implications with regard to the Solar System. (line 67)

(11) Rosetta is a robotic spacecraft of the European Space Agency (ESA) on a mission to study comets. It consists of two main elements: the Rosetta space probe and the Philae Lander. Both the probe and the Lander carry a large complement of scientific experiments designed to complete the most detailed study of a comet ever attempted. (line 71)

(12) Rosetta was set to be launched on January 12, 2003 to rendezvous with the comet 46P/Wirtanen in 2011. However, this plan was abandoned after a failure of the planned launch vehicle Ariane 5 on December 11, 2002. A new plan was formed to target the comet 67P/Churyumov–Gerasimenko, with launch on February 26, 2004 and rendezvous in 2014. The larger mass and the resulting increased impact velocity made modification of the landing gear necessary. After two cancelled launch attempts, Rosetta was launched on March 2, 2004 at 7: 17 GMT. Besides the changes made to launch time and target, the mission profile remains almost identical. (line 79)

(13) Rosetta is scheduled to arrive at Comet by mid 2014 and go into its orbit. It will collect detailed information about comet nucleus and its environment and it will survey it, seeking a suitable landing site, once that is selected, the Rosetta Lander will separate from the spacecraft and slowly descend a few kilometers to the surface of the icy nucleus. It is expected that the data gathered by the Lander (close-up pictures, samples of the primordial ices and gases, and probes below the surface of the " cosmic iceball") will greatly change our understanding of comets. But even more exciting for some scientists is what the data will reveal about the most primitive building blocks of the Solar System. (line 87)

(14) To gain enough orbit energy to reach its target, one Mars and two Earth gravity assists¹¹ will be required. It will take the spacecraft eight years to reach the comet. The long mission duration requires that there be extendedhibernation periods. On route, it will pass close to two asteroids, Š teins and Lutetia, and gather information about them as well. (line 91)

(15) The spacecraft has already performed two successful asteroid flyby missions on its way to the comet. In 2007, Rosetta performed a Mars swing-by (flyby), and returned images. The craft completed its fly-by of asteroid 2867 Š teins in September 2008 and of 21 Lutetia in July 2010, and is presently in " hibernation" mode and on-target for its final destination. The spacecraft will remain in this state until 20 January 2014 when the hibernation exit sequence will be initiated. (line 97)

(16) If this project succeeds, then we will probably know just about everything there
is to know about comets. Our scientific knowledge will be gained at the expense of
the mystery, myth, and conjecture that mankind has projected onto these exciting heavenly bodies, these volatile travellers in space. We will also know a lot more about the universe in its earlier stages of formation. (line 102)

This is the planned timeline for the mission after its launch:

• First Earth flyby (March 4, 2005)
• Mars flyby (February 25, 2007)
• Second Earth flyby (November 13, 2007)
• Flyby of asteroid 2867 Š teins (September 5, 2008)
• Third Earth flyby (November 13, 2009)
• Flyby of asteroid 21 Lutetia (July 10, 2010)
• Deep-space hibernation (June 2011 - January 2014)
• Comet approach (January–May 2014)
• Comet mapping / Characterisation (August 2014)
• Landing on the comet (November 2014)
• Escorting the comet around the Sun (November 2014 - December 2015)
• End of mission (December 2015)

Rosetta' s current location can be found on the ESA website.^[7]