The European Space Agency’s (ESA) Mars Express is set to perform its closest flyby of Phobos tonight. ESA hopes that the data collected by the spacecraft will help solve the origin of Mars’ largest moon.

Mars Express is currently on a mission to undertake a series of 12 flybys of Phobos, each time using its suite of instruments to take measurements of the moon to help determine how its mass is distributed. At 20:55 GMT (21:55 CET) tonight, the spacecraft will skim the surface of Phobos at just 67 kilometres and, using precise radio tracking, will allow scientists at ESA to explore the inside of this mysterious moon.

With Mars Express entering such a close range, tracking teams on the ground will be able to record extremely accurate measurements. This will be achieved by switching off all data signals from the spacecraft and listening for a ‘carrier signal’ – an underlying frequency that is modulated to transmit information.

Tiny gravitational pulls of the moon on the spacecraft, equal to just one part in a trillion, could modulate the carrier signal by changing its frequency. These variations are caused by the Doppler effect – a change in the wavelength of radiation emitted by an object as it moves closer (shorter wavelengths) or further way (longer wavelengths) from an observer.

Originally, Mars Express was planned to pass Phobos at 50 kilometres altitude, but the decision was made to extend this to 67 kilometres during a rehearsal last week when a manoeuvre at the lower height caused the spacecraft to pass behind Phobos as seen from Earth, complicating data analysis.

So far, each flyby the spacecraft has performed during this campaign has revealed new information about Phobos. As well as the tracking experiment, the MARSIS radar (MaRS for Mars Radio Science) has already been probing the moon's interior with radar beams and ASPERA (Analyser of Space Plasma and Energetic Atoms) is studying the way charged particles from the Sun interact with Phobos' surface. Other instruments will focus on characterising the surface composition and temperature characteristics of the moon, while the High Resolution Stereo Camera (HRSC) will concentrate on imaging the proposed landing site for the Russian Phobos-Grunt mission.

Mars Express is set to complete seven more flybys of the moon and analysis of the science results can be expected in the coming weeks and months. Follow updates on the ESA blog as the flybys take place.