"No one has before made measurements when a comet passes so close by a planet," says Associate Professor Mats Holmström at the Swedish Institute of Space Physics in Kiruna, Sweden.

The comet Siding Spring will pass by Mars at a distance of only 140,000 km, about a third of the distance from the Earth to the Moon, by way of comparison. The outer parts of the comet's thin atmosphere will collide at high speed (56 km/sec) with the atmosphere of Mars.

"We expect that gas and dust from the comet will impact on the Martian atmosphere, which will be temporarily heated and will expand," says Mats Holmström. "We should be able to see that with our instrument."

The Swedish Institute of Space Physics (IRF) has Principal Investigator responsibility for the satellite instrument ASPERA-3 which is an international collaboration with participants from some 15 research groups from about 10 countries. ASPERA-3 on board the measures how charged particles from the sun, the so-called solar wind, influences the atmosphere of Mars. Mars Express was lauched by the European Space Agency (ESA) and has been orbiting Mars since 2003.