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Search for Comet Ion Tail Encounters - Prediction and Data Analysis

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Search for Comet Ion Tail Encounters - Prediction and Data Analysis Search for Comet Ion Tail Encounters - prediction and data analysis So¯e Spjuth 28th October 2005 1 Without knowing that your destiny is destruction, you move freely in the trajectory of yours. Towards death, you have amazed people for centuries, exploring space at new shores. A comet so small, consisting simply by ices and rocks. But carrying a history, the oldest solar system building blocks. I turn to you, when every noise has ceased in silence. I turn to you, when all I see is a world full of violence. So great is your appearance, so valuable the treasures you are bringing. When the great stars have been looked upon, I see you, the great home of my own beginning. - So¯e Abstract Rosetta is an ESA (European Space Agency) spacecraft, launched towards a comet in March 2, 2004. She will not reach the target, comet Churyumov-Gerasimenko, until 2014, but under the journey Rosetta may possibly encounter the ion tails of other comets. The chance of crossing a tail close to a comet is small, but a comet ion tail can be identi¯ed at very large distances from its nucleus. The Ulysses spacecraft crossed the tail of comet Hyakutake at a distance of 3 AU from its nucleus. There are indeed a lot of conditions that all have to be ful¯lled for the comet and spacecraft to be in exactly the right place at the right time. Here, by using two independent methods, it is shown that there were and will be several possible ion tail encounters with the Rosetta spacecraft. Possible ion tail encounters in the past have been found for other spacecrafts as well, and some of the possible corresponding signatures have been presented. CONTENTS 1 Contents 1 Introduction 4 2 Comet Orbits 7 2.1 Features of Comets . 7 2.2 The Two-Body Problem . 7 2.3 Orbital Elements . 8 3 Coordinate Systems 10 3.1 Heliocentric Ecliptic Coordinate System . 10 3.2 Heliocentric Equatorial Coordinate System . 10 3.3 Heliocentric Plane-of-Orbit Coordinate System . 10 3.4 Finding Positions . 11 3.5 Transformation of Systems . 13 3.5.1 Equatorial to Ecliptic . 13 3.5.2 Ecliptic to Plane-of-Orbit . 13 3.5.3 Plane-of-Orbit to Ecliptic . 14 4 Comet Ion Tails 16 4.1 The Solar Wind . 16 4.2 Comet - Solar Wind Interaction . 16 4.3 Ion Tail Formation . 17 4.4 Ulysses' Observation of Ion Tail . 19 5 Searching For Ion Tail Encounters 20 5.1 Method 1 . 20 5.1.1 Algorithm runrosetta.m . 21 5.1.2 Algorithm iontails.m . 22 5.2 Method 2 . 23 5.2.1 Algorithm swvel.m . 25 5.3 Resolution . 26 5.4 Accessing Orbital Data . 27 6 Rosetta encounters 28 7 Other Spacecraft Encounters 34 CONTENTS 2 7.1 Ulysses Encounters . 34 7.2 Earth Satellite Encounters . 35 7.3 Cassini Encounters . 38 8 Validation 40 9 Signatures of Ion Tail Encounters 43 9.1 What To Look For . 43 9.2 Procedure . 45 9.3 Ulysses Data . 45 9.4 Earth Data . 47 9.5 Cassini Data . 47 10 Conclusion and Discussion 54 11 Acknowledgements 56 A Results 58 A.1 Rosetta . 59 A.2 Ulysses . 72 A.3 Earth . 78 A.4 Cassini . 86 B Matlab-programs: source code 92 B.1 M1........................................ 92 B.1.1 onedayres.m . 92 B.1.2 rostoec.m . 94 B.1.3 readcomet.m . 95 B.1.4 runrosetta.m . 96 B.1.5 meananomaly.m . 97 B.1.6 eccanomaly.m . 98 B.1.7 comtoec.m . 98 B.1.8 iontails.m . 99 B.1.9 hit.m . 102 B.1.10 timeres.m . 104 B.1.11 tempel.m . 106 CONTENTS 3 B.2 M2........................................ 108 B.2.1 main.m . 108 B.2.2 coordtransf.m . 110 B.2.3 hit.m . 111 B.2.4 swvel.m . 113 B.2.5 crossing.m . 115 B.2.6 orbcom.m . 116 B.2.7 plothit.m . 118 B.2.8 cases.m . 120 B.2.9 distance.m . 122 B.2.10 readcassini.m . 126 C Comet List 128 1 INTRODUCTION 4 1 Introduction There have been several cometary missions and there are missions in progress or in prepa- ration. An armada of spacecraft (including the European Space Agency's Giotto probe and the former Soviet Union's Vega 1 and Vega 2) flew through the coma of Halley's comet in 1986 to photograph the nucleus and observed the jets of evaporating material. The Giotto mission also studied Comet P/Grigg-Skjellerup during its extended mission. The American probe Deep Space 1 flew past the nucleus of Comet Borrelly on Septem- ber 21, 2001 and con¯rmed that the characteristics of Comet Halley are common on other comets as well. The Stardust spacecraft, launched in February 1999, has already collected particles from the coma of Comet Wild 2 in January 2004, and will return the samples to Earth in a capsule in 2006. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. Deep Space 1 may have its mission extended to allow it to visit a comet. The most ambitious mission is the European Space Agency's Rosetta, which was launched in March 2004. The destination is a periodic comet known as Churyumov-Gerasimenko, which will be reached in 2014. The spacecraft will then follow the comet towards the sun and study its changes as it gets transformed by the heat of the Sun. Within 3:2 AU (1AU¼150 millions kilometers), the comet is active, evaporating gas and dust from the icy nucleus and losing mass. About one year after arrival, both the comet and spacecraft will have reached perihelion and will thereafter be travelling outwards in the solar sys- tem. The mission will then be completed. Rosetta will be the ¯rst spacecraft to orbit a comet and fly alongside to see how it is transformed by the heat of the sun (Fig. 1). The Philae lander will be released onto the surface of this mysterious cosmic iceberg to take the ¯rst images from a comet's surface and ¯nd out about the composition. On its way, Rosettta will pass the asteroid belt that lies between the orbits of Mars and Jupiter. Two asteroids, Steins and Lutetia, are selected for close flybys and will provide us with information on the mass and density that will tell us more about their composition. Earlier results have shown that comets contain complex organic molecules that are rich in carbon, hydrogen, oxygen and nitrogen, i.e. the elements that make up nucleic acids and amino acids which are essential for life (as we know it). The Rosetta orbiter and its Philae lander will help us to learn more about the formation of our Solar System around 4600 million years ago and provide clues as to how comets may have contributed to the beginning of life on Earth. Rosetta may help ¯nding out whether the occasional impacting comet provided Earth's oceans. If comets did not supply Earth's oceans then it implies something amazing about the comets themselves: if Rosetta ¯nds that they are made of extremely di®erent isotopes, it means that they may not have formed in our Solar System at all. Instead, they could be interstellar rovers captured by the Sun's gravity. No comets have yet been proven to come from interstellar space, but Rosetta will test if the relative abundances of several isotopes in the cometary matter are the same as in other bodies in our solar system. The scheduled trajectory (Fig. 2) last for 10 years, whereas most of the time she is put into hibernation. 1 INTRODUCTION 5 Figure 1: Artist view of Rosetta's rendez-vous with Comet 67P/Churyumov-Gerasimenko. Copy from http://www.alfvenlab.kth.se/res/space/hw/rosetta/. This mission is named after the Rosetta stone that was discovered in 1799. The carved inscriptions on the stone included hieroglyphics - the written language of ancient Egypt- and Greek, which was readily understood. By comparing the inscriptions on the stone, historians were able to begin deciphering the mysterious carved ¯gures. As a result scholars were at last able to piece together the history of a long-lost culture. Similarly, it is hoped that the Rosetta spacecraft will be the key to unlock the mysteries of the oldest remaining building blocks in our solar system - comets. This means that scientists will look back 4600 million years, when only asteroids and comets surrounded the sun, together with gas and dust. Sometimes in science, little surprises can have great impacts. Just ask the astronomers who stumbled across a small blip in the data collected from the ESA/NASA Ulysses spacecraft (Ulysses is a probe orbiting the Sun and aiming to study the solar wind) when it passes through supposedly empty space. The blip, it turned out, was the sig- nature from an ion tail of comet Hyakutake that was more than 500 million kilometres long, i.e. almost double the longest comet tail previously known to exist. This was a complete surprise. It indicates that the comets, in particular those that are much larger and more active than Hyakutake, may influence the conditions in interplanetary space much more than earlier thought. Scientists thought it would break up, dissipate, and it would be so diluted, you would not be able to detect it at all. But now with this kind of in situ measurement, it is clear that these tails stay identi¯able for longer distances.
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