To see Gaia begin its journey into space after so many years of preparation was a very special moment for me. My happiest moment was to see Gaia actually leave from the launch pad in French Guiana. That was an extraordinary experience. I am looking forward to the moment that astronomers all over the world will use the beautiful data set we are providing for new scientific findings. It is December 19, 2013. In the South American sky the glow from the rocket with the space observatory Gaia slowly disappears over the launch site of the European Space Agency in French Guyana. The goal of the journey is a region around the so-called Lagrange point L2 – 1.5 million km from Earth. In this region the attraction of Earth and Sun are balanced in a way that an object follows our home planet, without feeling any forces on its orbit around the Sun. Unimpaired by the Earth and the Moon he satellite can efficiently investigate that part of the sky which points away from the glaring Sun. An observatory such as Gaia can therefore gradually observe each part of the sky during its orbit around the Sun. Gaia is equipped with two identical reflecting telescopes, each of them with a main mirror of 145 by 45 cm in size. They simultaneously observe two regions of the sky about 107° apart. A complete strip of the sky is scanned as Gaia rotates around its axis every six hours. Over 100 CCD detectors receive the light of the registered stars on about 1 billion pixels. Apart from the images of the stars, photometric and spectroscopic data are also taken. But what is actually the main task of Gaia? Gaia is a satellite launched into space in December 2013 and its aim is to create the most
accurate map ever of our galaxy, the Milky Way. During the next five years Gaia will survey the sky many times and observe all objects brighter than magnitude 20. This will mainly comprise stars of our Milky Milky Way, but also other astronomical objects. The exact number of stars is not certain but should amount to about one billion. The final goal of our mission is to produce a highly precise astrometric catalogue of all these observed objects. The objective of the Gaia mission is therefore the determination of the positions and motions of stars in space – what is known as astrometry. These measurements are required to survey our galactic home, the Milky Way. Like millions of other galaxies our own galaxy consists of about 200 billion stars, many star clusters, luminious gas and dark dust clouds. From the gas and dust new stars and planets are born- including our solar system almost five billion years ago. The rotating, disc-shaped structure of our galaxy has about 100,000 light years in diameter and is typical for large spiral galaxies. But in order to find detailed answers to many questions about the structure, dynamics, the past and future of our galaxy, the interplay of the gravitational forces of stars, gas and dust and the mysterious dark matter, it is important to measure the positions and motions of
the stars and their physical properties. However, the most important ingridient for our understanding of the stars in the Milky Way is the determination of their distances, the more distance measurements and more distant
such data can still be reliable taken the better. And Gaia is to measure about one billion stars: We think that these one billion stars form a representative sample of the whole population of our Milky Way galaxy. These five position and motion variables for each star – plus the radial velocity – provide a six-dimensional map from which quite elementary properties of the structure and dynamics can be derived. This information will teach us much – mostly about our Milky Way – that is this system of 100 to 200 billion stars to which our Sun belongs. We will also learn about the history, the structure, the formation and the behaviour of this system. Additionally, we will learn a lot about the stars themselves – about the individual stars, their evolution, their internal structure and about many, many other questions in astronomy. For example, the question how many spiral arms our galaxy has and how its components move in relation to each other. In order to obtain these data from the measurements of Gaia, AGIS is needed. AGIS stands for Astrometric Global Iterative Solution. With the help of AGIS the positions, the annual motion of a star in the sky, and the so-called parallax – a measure of the distance – are determined. This is done for 1,000 million stars with unprecedented precision. However, AGIS is not an instrument, but a mathematical procedure that performs this task. AGIS is the way that we put together the billions of pieces of information
sent down from the satellite into this map of the Galaxy. So you could think of it like a giant jigsaw puzzle with hundreds of
billions of pieces that have to be put together very accurately before you
can see the whole picture. The assembly of this gigantic puzzle is a c puzzle is a complex process that needs to be performed over many years, n parallel with the ongoing return of data from Gaia. Therefore regular meetings and conferences of the scientists working on AGIS – like this one in Heidelberg – are necessary. AGIS is a huge computer program – mathematical method – which can derive the proper motions, positions, distances of the observed 1,000 million stars from the pre-processed aw data. It is a very complicated process which in particular needs the precise modeling of all movements of the Gaia satellite over a period of at least five years. Moreover, the geometry of Gaia’s instruments and the telescopes must be determined very accurately. Gaia opens a completely new era of quantity and quantity in a field of
astronomy which always was extremely challenging: The determination of the distances of stars. Even over very historical periods of time the constellations do not seem to change. However, the so-called fixed stars actually move in space with velocities of several kilometers per second. This is not detectable without extremely accurate measurements, because even the nearest star is already about four light-years away. This corresponds to a distance of about 40 trillion kilometers. We are in the year 1838: Only now – more than 200 years after the beginning of astronomical observations with telescopes Friedrich Wilhelm Bessel successfully determined the distance of a sof a star for the first time: 61 Cygni in the constellation Cygnus. For his measurement, he uses the so-called parallax. The parallax refers to the apparent shift of a remote object against a background when an observer changes his position. It is a perspective effect. The larger the baseline and the closer the object is the more pronounced the parallax effect. Even a relatively near star exhibits only a very small parallax but ever since Bessel’s time it is possible to measure the displacement when the star is observed at various times during the course of one year. The diameter of the Earth’s orbit orbit around the Sun – 300 million kilometers – is in this case the maximum possible base line. Nevertheless, the angles to be measured are extremely small and a few decades ago, the maximum range of this method was about 1000 light years. A significant improvement did not occur until 1989, when the astrometry satellite Hipparcos was launched to measure 100,000 stars. Gaia will measure one billion stars with incredibly high precision. But what does “precision” really mean in this context? This is not so easy to imagine for a layman. The positional accuracy is typically 20 micro arc seconds. With the same accuracy we also determine the annual motion and the parallax – the measure of the distance. The smallest angle that we can determine corresponds to the size of a coin on the Moon – a one Euro coin as seen from Earth. This means that if there would be a light source – a strong flashlight – on the moon and it were moved by two or three centimeters, Gaia would in principle be able to determine this shift with its high measurement accuracy. Achieving this almost unbelievable accuracy is extremely complex and at the edge of what is technically feasible, because: Firstly, there are extreme requirements on the spacecraft and the instruments on board. For example these have to rotate very very uniformly – the whole spacecraft turns once in six hours – and very, very smoothly. Secondly, the temperature inside the spacecraft and in particular that of the telescopes must be precisely constant, changing less than about one thousandth of a degree over many days. To illustrate this a bit: Over the full five years we need to determine the orientation of the instrument – which is three meters in size – within every millisecond with an accuracy of a few atomic diameters. This also holds for the position of the mirrors relative to each other and the individual CCD detectors of Gaia’s camera. Even extremely exotic effects must not be ignored: For example, the relativistic light deflection is important in our solar system due to the presence of the Sun and the large planets and must be considered. That is, we must consider the deflection of a light beam in the presence of the gravitational field of objects that arises because their masses bend the spacetime. Normally this effect is too small to be considered for measurements, given the small masses of the objects in our solar system. But for the precise measurements taken by Gaia the situation is very different and extremely challenging. We are basically trying to do something a
hundred times better than has ever been done before and that
means that we don’t really know exactly how material behaves at that accuracy A challenge for technology, as well as for the participating scientists. They will try, also with their their meeting in Heidelberg, to achieve their ambitious goals up to the end of Gaia’s measurements in four years time. The first results that we have seen here today and yesterday, are in line with our expectations. This is all very satisfying. The results are still far away from the ones that we want to reach at the end of the mission, but they are in the range that we can expect at this moment, given our current knowledge of the instruments and the spacecraft. And we are seeing that the measurement accuracy – measuring accuracy that needs to be calibrated afterwards – actually is about as precise as was previously specified and to which we aspire. These 20 micro-arcseconds will be achieved. do room do %uh it them do good

Gaia – with impressions from the AGIS Meeting in Heidelberg (now also with closed captions)
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