The journey of the comet "Chury" tracked by the European Rosetta probe will come to a climax on 13th August


The comet's tail will be at its longest and erosion at its strongest when the probe arrives tomorrow to its closest distance to the Sun.

The animation is a visualization of the team's model of perihelion conditions. We start with a view of the cometary nucleus to scale, being engulfed by the blue cloud of water ions. Soon we zoom past Rosetta's typical location, marked with a white sphere. The blue-white lines are the flow lines of solar wind ions, flowing in from right and interacting with the cometary ions, then being deflected and slowed down (blue colors). After zooming out, we introduce a physical process known as charge exchange. In a charge exchange reaction electric charges are transferred between the solar wind ions and cometary particles. Animation: Markku Alho

From a year ago until now, the Rosetta probe has witnessed the awakening of the  comet 67P/Churyumov-Gerasimenko made of water ice and dust, from the formation of the first water jets to its current Mars-like interaction with the solar wind.

Now the comet is about to reach the culmination point of its journey when coming closest to the Sun on Thursday.

– This is going to be a really exciting week, Post-Doctoral Researcher Cyril Simon Wedlund and Doctoral student Markku Alho point out. They are carrying out comet research at Aalto University in Professor Esa Kallio's space research group.

– As the comet comes closest to the Sun on the 13th of August, its atmosphere will expand and its tail will be at its longest and at its most beautiful. Also, due to heat and radiation, the comet's surface erosion will be strongest. Simultaneously, many interesting phenomena will take place as a result of the icy comet ejecting gaseous jets. No-one has ever followed a comet so close for so long until now, Simon Wedlund says.

Comets are time capsules

Aalto researchers, aided by three-dimensional computer simulations, analyse the data collected by one of Rosetta's instrument, ICA, the ion composition analyser, built and managed by collaborators at the Swedish Institute of Space Physics (IRF, Sweden). The simulations are of critical importance, because no comprehensive picture about the events on and around the comet can be obtained through single-point measurements.


In the picture, the blue-white cloud presents cometary water ions, being expelled from the comet's environment. From right, the blue-white lines with arrows show the flow of solar wind ions as it impinges upon the particles emitted from the comet, getting deflected and slowed down (blue colors). The blue-yellow-red lines across the flow of solar wind show the magnetic field embedded in the solar wind and how it is bent and piled up in front of the comet. Picture: Markku Alho.

Aalto University also uses radio telescopes to observe the data sent by Rosetta.

– We listen to Rosetta and the comet's particle environment, explains Post-Doctoral Researcher Guifré Molera Calves working also in Kallio's team. Molera analyses and interprets the telescope observations performed at several sites around the world including the Metsähovi Radio observatory of Aalto University.

– The measurements help us to better understand changes created by the solar wind on the comet.


Detection of the signal transmitted by the Rosetta spacecraft on the 24th of July 2015. The signal was observed with the radio telescope of Hartbeeshoek in South Africa. Large radio telescopes, like the 14-metre radio telescope at Metsähovi, are used to track very accurately the orbit of the spacecraft in our Solar System. Picture: Guifré Molera Calves

Rosetta is the cornerstone mission of the European Space Agency (ESA). It comprises t orbiter and the lander Philae. The orbiter boasts 11 different instruments measuring several aspects of the comets: its tenuous atmosphere, its electromagnetic environment, its surface, its evolution. What kind of information is expected from such an endeavour?

– Comets can be seen as time capsules. They were created at the same time as our solar system and have remained practically unchanged up to now. They are like very very old fossils, the research of which can teach us a lot for example about the origin of the Earth's water and about the birth of our solar system, Simon Wedlund tells.


67P/Churyumov-Gerasimenko, July 2015. Image: ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0

In addition to the Rosetta climax, another comet-related highlight for space fans is taking place during this week: the Perseid meteor shower or "shooting stars". The meteor shower consists of particles torn away from the Swift-Tuttle comet which have remained in orbit along the comet's path.

Finland's bright summer is not the best time to keep track of meteor showers, but during the coming nights until the end of the week, it will be worthwhile to observe the sky after midnight in the North-East direction, at least outside cities.

– At its best, up to 60 shooting stars per hour might be seen, Simon Wedlund hints.

More information:

Professor Esa Kallio
Aalto University
School of Electrical Engineering
tel. +358 50 420 5857

Post-Doctoral Researcher Cyril Simon Wedlund
Aalto University
School of Electrical Engineering
tel. +358 50 448 1280

Research group's website

European Space Agency's (ESA) Rosetta site

The ESA-organised discussion forum with the Rosetta probe experts will kick off on Thursday, 13 August at 4 pm Finnish time, at

Metsähovi Radio observatory