INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 29, May 2012 Editorial Uranus was the first planet to be regularities. It stood to reason that discovered since ancient times. Such Le Verrier should again come up Impressum was the luck of the young musician with the idea that an unknown William Herschel in 1781. It was planet, which he swiftly called Vul- not just a coincidence, however, be- cain, was shaking Mercury. This is cause Herschel was not only a gifted now the stuff that galvanizes as- composer and organist, but also a tronomers, and it did not last long SPATIUM skilled builder of telescopes which until hasty observers claimed to Published by the he used to observe Britain’s night have seen the hypothetical planet. Association Pro ISSI sky. Later observations of Uranus re- Still, further analyses showed in all vealed strange orbital irregularities cases that the putative Vulcains that could not be explained by were mere products of imagination: Kepler’s laws. This is why the planet planet Vulcain could not be found. continued to stir up interest in the The mystery was only solved in Association Pro ISSI astronomical community and later 1915, when Albert Einstein ex- Hallerstrasse 6, CH-3012 Bern the French astronomer Urbain Le plained Mercury’s anomalous orbit Phone +41 (0)31 631 48 96 Verrier undertook to unveil its se- with his general theory of see cret. Months of complex calcula- relativity. www.issibern.ch/pro-issi.html tions - at that time the matter of a for the whole Spatium series clear mind, much paper and a sharp That may have been the first, yet pencil - brought him to the con- undoubtedly not the last of Mer- President clusion that the planet’s wobbling cury’s many secrets to be unveiled. Prof. Nicolas Thomas, orbit might be caused by an addi- However, the more scientists look University of Bern tional neighbouring yet unknown at this planet the more they get fas- planet. He quickly announced his cinated by that small, hot sphere. Layout and Publisher findings to the French Academy on Prof. Peter Wurz of the Physics Dr. Hansjörg Schlaepfer 31 August 1846; and he also sent Institute of the University of Bern, CH-6614 Brissago them to Johann Galle of the Berlin the author of the present issue of Observatory who found the wanted Spatium, is no exception. On Printing planet precisely on the indicated 30 March 2011, he presented the Stämpfli Publikationen AG spot. That marvellous success earned current status of Mercury research CH-3001 Bern Le Verrier the nickname of the man to our PRO ISSI association. We who discovered a planet with the point thank Prof. Wurz for his consent and of his pen. support in publishing his enlight- ening talk, and wish our readers a Unknown to Le Verrier, similar cal- hearty portion of that great fasci- culations were made at virtually the nation scientists get when study ing same time by a student in England; mysterious Mercury. yet, they got lost and re-found again only much later, prompting lengthy Hansjörg Schlaepfer disputes about the true finder of Brissago, May 2012 Neptune, as the newcomer was Front Cover named soon after. Anyway, spurred Craters are Mercury’s trademark so by his success, Le Verrier set out to to speak. The front page shows address a further astronomical mys- the crater Kuiper seen by NASA’s tery: he began observing Mercury, Messenger camera, interpreted by which also exhibits some orbital ir- an artist. SPATIUM 29 2 Mysterious Mercury1 by Prof. Peter Wurz, Physics Institute, University of Bern Introduction When the first astronomical tele- cury’s orbit remains a puzzle until scopes appear in the 17th century, 1915, when Albert Einstein solves Mercury is an attractive object in the problem conclusively by means the sky. Even though observation of his general theory of relativity. The solar system’s history begins turns out to be difficult due to the Einstein’s point takes into account with a huge interstellar cloud of gas glare caused by the nearby Sun, the effect of relativity as a result of and dust far out in one of the Milky Urbain Le Verrier3 notes that its or- the tremendous forces of gravity Way’s several arms. Gravitational bit slightly deviates from what is to reigning in Mercury’s quarters close instabilities cause it to collapse into be expected from classical New- to the Sun. a large flat disk. Most of the matter tonian mechanics. Trying to explain concentrates in its centre where the planet’s mysterious behaviour, A further of Mercury’s many pe- the later Sun will emerge. The re- he stipulates a further hypothetical culiarities is found by Gordon maining material forms the outer planet which by its gravitation Pettengill5 with the help of the parts of the disk, where over a causes the excessive shift of Mer- giant 304.8 m diameter Arecibo ra- period of some 50 million years cury’s perihelion4, Fig. 1. The un- dar telescope in Puerto Rico in planets form together with all the known planet even gets a name: 1965. He recognizes Mercury’s 3 : 2 other bodies populating the solar Vulcain; yet all attempts to get hold spin/orbit resonance, meaning that system2. of it are bound to fail: the suspected whilst Mercury orbits the Sun planet cannot be found, and Mer- twice, it rotates around its axis ex- Next to the Sun is Mercury, the actly three times. Such resonances smallest of all planets. It is known occur in celestial mechanics as a re- from times immemorial as it can be sult of tidal forces caused by a large seen under good conditions by the body (here the Sun) acting on a naked eye notwithstanding its small smaller companion (here Mercury). size. The Babylonians call it Babo, The latter is slightly deformed by the messenger of the Gods, an at- the larger body’s gravity field which tribute reflecting its fast orbit in turn tends to synchronize it into around the Sun, faster than any a resonant revolution mode. other planet. Civilizations change, Mercury keeps its attribute: the That is about all that is known Greeks call it Hermes and Apollo, before the first space probes visit which is reduced to Hermes when Fig. 1: While Newtonian mechanics Mercury at close quarters. In 1974 it becomes known that the planet and Kepler’s laws stipulate an ellipti- NASA’s Mariner 10 spacecraft cal orbit, Mercury orbits on a rosette-like in the morning sky next to the Sun pattern around the Sun as a consequence reaches the planet after a half year is the same as that in the evening of – amongst other things – the near Sun’s journey, followed 30 years later by sky. Roman astronomers call it powerful gravity field. The anomalous NASA’s Messenger mission. Both Mercury, the God of dealers and perihelion shift amounts to 43 per cen- programmes provide fascinating tury, which is 120 km/year. This com- thieves, a name the planet bears pares to Earth’s anomalous perihelion new insights into the secrets of down to our day. shift of only 3.8 per century. Mercury, at the same time raising 1 The present issue of Spatium reports on a lecture by Prof. Wurz for the PRO ISSI audience on 30 March 2011. 2 See Spatium no. 6: From Dust to Planets by Willy Benz, October 2000. 3 Urbain Jean Joseph Le Verrier, 1811, Saint-Lô, Manche, France – 1877 Paris, French mathematician and astronomer. 4 The term perihelion designates the point on a planetary orbit which is closest to the Sun. Correspondingly, the aphelion is the farthest point from the Sun. If the reference is not the Sun, then the terms periapsis and apoapsis are used. 5 Gordon Pettengill, 1926, Providence, Rhode Island, USA, US-American radio astronomer and planetary physicist. SPATIUM 29 3 many new questions. This is why Mysterious inner solar system billions of years the European Space Agency ESA, ago. So, to understand the Earth’s in collaboration with the Japanese Mercury history, one also has to understand Space Agency JAXA, is currently Mercury’s evolution. implementing a mission called Be- piColombo in honour of the Ital- ian space scientist Giuseppe Co- The deeper scientists look at Mer- Orbital Characteristics lombo, see text box. This mission is cury the more they get fascinated scheduled for launch in August by the small, yet unusual planet as We have already addressed some pe- 2015. As a space mission to Mer- it turns out to be an outright sto- culiarities of the planet’s orbit. Yet, cury poses uncommon, yet specific rybook telling the early history of there are many more: As the inner- fascinating challenges to engineers the solar system, including that of most planet in the solar system, and scientists, we will treat these the Earth. This property is owed to Mercury’s mean distance to the Sun programmes somewhat more in the fact that the planet has neither is a mere 0.39 AU6. This leads to a depth below. an atmosphere nor tectonic acti vities very short year on the planet: Mer- that would tend to erode any traces cury completes one orbit around of the distant past, as is the case for Sun in only 87.97 Earth days, faster instance on Earth. Rather, Mercury than any other planet in the solar preserves faithfully the marks left by system. Furthermore, Mercury’s or- violent events and processes in the bit has the largest eccentricity7 of all planets, see Fig.