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
