Data from Gaia reveals that star paths are affected by on-going collision by smaller galaxy into Milky Way
The Gaia mission of the European Space Agency has produced extremely detailed information on the positions and movements of stars in the Milky Way near the Sun. From the structures of these positions and paths, scientists have deduced that the paths of stars are probably affected by an on-going collision between the Milky Way and a smaller galaxy. From a Finnish perspective, there is interesting new data on the objects in our solar system; this is the first time position and brightness observations have been published about 14,099 asteroids.

The European Space Agency, ESA, recently published observational data from the Gaia space mission, which has been gathered for 22 months starting at the end of July 2014. The material contains positional and brightness data on 1,692,919,135 stars, among other things.

The Gaia space telescope systematically observed objects in the sky near the so-called Lagrange L2 point, 1.5 million kilometres from Earth in the opposite direction from the Sun. The main objective of the mission is to map out our own galaxy, the Milky Way, in more detail than ever before. Gaia observed over one billion Milky Way stars with a maximum accuracy of one millionth of a degree, which will result in a three-dimensional landscape of our galaxy by year 2023.

Expecting a three-dimensional landscape of our galaxy

The new material includes colour data on 1.38 billion stars, the radial velocity of 7.2 million stars, data on 0.5 million variable stars, the surface temperature of 161 million stars, the scattering and absorption of inter-stellar dust and gas from 88 million stars, and the radius and luminosity of 77 million stars.

The computation for Gaia is made by the pan-European DPAC (Data Processing and Analysis Consortium) comprising over 300 scientists. The solar-system researchers at the University of Helsinki are part of the Gaia mission in many ways, says Professor Karri Muinonen.

“We are in charge of the daily path definition of new asteroids observed by Gaia, for example. Based on that, the follow-up observations are then set up for the telescopes on Earth”. 

The shapes of the asteroids are revealed by mathematical methods developed by Finnish researchers

In connection with the Gaia Data Release 2 on 25 April 2018, the researchers have participated in the validation of asteroid brightness observations by comparing the brightnesses computed for known asteroids to those observed by Gaia.

“The publication of this data will increase our knowledge of the rotation and forms of asteroids, soon we will be able to deduce new orbit and form data for thousands of asteroids. In the same way, the publication of the data will enable the computation of exact paths and masses of hundreds of asteroids with the help of the inversion methods developed by the scientists”.

This according to Professor Muinonen.

The accuracy of the optical system may yet outperform that of VLBI

For positioning, the Gaia mission has developed the first optical frame of reference, or set of coordinates, for remote quasars. The accuracy of this frame of reference is on a par with that of the VLBI (Very Long Baseline Interferometry) frame of reference based on the latest and best radio-signal observations, which is being developed at the moment.

When the work of Gaia is completed in a few years’ time, the accuracy of the optical system should outperform the VLBI system.

Revolutionary observations of the positions and paths of globular star groupings and dwarf galaxies

The Gaia mission has produced extremely detailed information on the positions and paths of stars in the Milky Way near the Sun. From the structures of these positions and paths, scientists have deduced that the paths of stars are probably affected by an on-going collision between the Milky Way and a smaller galaxy.

­“We have gained some quite revolutionary data on the positions and paths of globular star groupings and dwarf galaxies in space near the Milky Way,” says Karri Muinonen.

The observational data from Gaia enables us to determine the paths and model the dynamic evolution of these objects. The paths of the objects appear to be almost perpendicular to the level of the Milky Way. However, the objects are not in orbit around the centre of the Milky Way on the same level. Furthermore, the paths of the globular star groupings have been found to be much more circular than was assumed earlier.

The new material has revealed, for the first time, how the Large Magellanic Cloud rotates by exact measurements of the positions and paths of the stars in this galaxy near us. Gaia has unveiled structures never before seen both in the Large Magellanic Cloud and the Small Magellanic Cloud.

New computation service enables open analysis of spectrum data and brightness observations

On funding from ESA, Finnish scientists have cooperated with Space Systems Finland to develop the Gaia AVI (Added Value Interface) computation service enabling the open analysis online of the brightness observation data and spectrum data.

“The latest data release did not include the spectrum data from Gaia, which will later be used to gain information on the makeup of asteroids, different minerals, and whether there is water on any asteroids. This will be significant for the study of the evolution of the solar system and for future mining activities on the asteroids,” says Muinonen.  

Mining activities on the asteroids

For modelling the brightness and spectrum information that Gaia has observed on the asteroids, computation methods for electromagnetic scattering, recently developed on funding from the European Research Council, will be used. These methods will connect the spectra observed on the surface of the asteroids with the basic quantifiers of physics describing surface features for the first time.

Open Asteroid Day in June
Astronomers from the University of Helsinki will tell the public about their work in connection with the Gaia mission on Saturday, 30 June, 2018, on UN Asteroid Day, at the Helsinki Observatory. 

More details:
Gai Data Release 2, list of content

ESA press conference on the first release of Gaia data in September 2016:

The Gaia site of the European Space Agency ESA:

Contact details:
Professor Karri Muinonen, University of Helsinki and National Land Survey of Finland,, + 358 50 415 5474
Docent Mikael Granvik, University of Helsinki and Luleå University of Technology,, +358 50 521 7209; determining paths and masses of asteroids based on astrometry from Gaia
Johanna Torppa, PhD, Geological Survey of Finland,, +358 050 410 1152; determining forms and rotation of asteroids based on Gaia photometry
Antti Penttilä, PhD, University of Helsinki,, +358 50 524 0968; analysis of asteroid compositions based on Gaia spectroscopy
Minna Meriläinen-Tenhu, science communicator, @MinnaMeriTenhu, +358 50 415 0316