By Patrick Byrne
A spacecraft that aims to land on a comet may also establish whether there is a thin ring of debris around the Red Planet this week.
Rosetta will use Mars' gravity to build up speed
The Rosetta probe will use Mars' gravity to pick up speed on a mission that will reach its climax in 2014.
But during a very close flyby, Rosetta will look for a scattering of dust from Mars' two moons, Phobos and Deimos.
In seven years' time, the probe will enter orbit around a comet, releasing a small lander on to its icy nucleus.
Rosetta scientist Horst Ewe Keller said a number of different observations would be made of Mars from a distance of only 250km (155 miles), using high resolution cameras and instruments.
The spacecraft would be in position at about 1800 GMT on Saturday, 24 February, to observe the martian moon Phobos appearing from behind the planet, he told BBC News.
"We will observe the event through the planet's atmosphere and we will be looking for evidence of water," said the principal investigator on Rosetta's Osiris instrument.
This observation will last for nearly two hours and will be followed by an imaging sequence of the red disc of the planet which Professor Keller said would be at a resolution 10 times better than the Hubble Space Telescope could achieve.
"The spacecraft will then look for traces of a faint ring around Mars which is believed to be made up of dust from the planet's two satellites, Phobos and Deimos, which orbit very close to the surface," explained Professor Keller, who is affiliated with the Max Planck Institute for Solar System Research.
"Theory tells us there should be a faint ring and Hubble has come close to finding the evidence.
"At the resolution of our cameras, we are hoping to see the rings themselves and take images."
Rosetta has already flown past the Earth once
Rosetta's closest approach to Mars will be made at 0153 GMT on Sunday. At this point, the spacecraft will be travelling at about 10.1 km/s (36,400km/h or 22,600mph) relative to the centre of the planet.
During the swing by, there will be a 25-minute period when Rosetta will pass into the shadow of Mars, and this will deny the probe the ability to generate power using its vast solar arrays (which span 32m from tip to tip).
To cope, the orbiter will be put in "eclipse mode", and no science observations will be done for approximately three hours around closest approach.
Either side of this "blackout", however, scientists will be busy.
Professor Ian Wright from the Open University, UK, said: "Rosetta's close vicinity to Mars will provide an excellent opportunity to take a close look at the planet.
"It also means that some of Rosetta's instruments, including the mass spectrometer built at the OU, will be able to be calibrated allowing us to check that they are operating correctly."
The mission's prime objective is to land on comet 67P/Churyumov-Gerasimenko, where instruments designed by OU scientist John Zarnecki and his team will assess the icy object's water content.
Comets are thought to be about 80% water, and Rosetta's landing vehicle will examine the amount of deuterium, a heavy form of hydrogen, present in its ices.
The investigations will help test the theory that water was brought to the Earth by comet impacts during its early development.
But Rosetta still has a lot more to do on its journey to rendezvous with the comet.
The manoeuvre around Mars this weekend is designed to increase the spacecraft's speed and to hurl it into the asteroid belt between the Red Planet and Jupiter.
Rosetta will also target two asteroids
It is in this belt and at a distance of 1,700km (1060 miles) that Rosetta will encounter in September 2008 the asteroid 2867-Steins, which is just a few kilometres in diameter.
Instruments on board Rosetta are designed to measure the mass and density of this asteroid and, also, the 100km (60 miles) diameter 21-Lutetia, which will be encountered in July 2010 on a second fly-through of the belt.
As well as transmitting images of the primordial rocks, instruments will be used to detect composition, subsurface temperatures and search for gas and dust.
Esa scientists regard these manoeuvres as highly important because so little is known about asteroids and few have been observed from such short distances.
Professor David Southwood, director of Esa's science programme, said: "Comets and asteroids are the building blocks of our Earth and the planets in the Solar System.
"Rosetta will conduct the most thorough analysis so far of three of these objects and the scientific insights we will gain into the origin of the Solar System and, possibly, of life, are more than rewarding."