Image caption Rosetta will reveal the true nature of Lutetia - including its shape

The Asteroid Lutetia will become the largest space rock to be visited by a probe when the European Rosetta mission flies past it on Saturday (GMT).

The 100km-wide mass of rock will be the last encounter in Rosetta's tour of the Solar System before it sweeps out to meet up with a comet in 2014.

Lutetia is a fascinating target - observations using telescopes have failed to determine its true type.

Scientists would like to know how much the asteroid has changed through time.

This will tell them to what extent it contains pristine materials left over from the formation of the Solar System some 4.6 billion years ago.

Fast rendezvous

Dr Rita Schulz, the European Space Agency's Rosetta project scientist, said the giant rock was something of an enigma.

"Lutetia is interesting not just because it will be the biggest asteroid ever visited by a spacecraft, but because it is so ambiguous," she told BBC News.

"We have observed Lutetia from the ground and also with the Hubble Space Telescope for a while to get some characteristics already.

Image caption Two million km and closing: Lutetia appears as a small dot to Rosetta

"We know pretty well the range of the size; we have some idea about its rotation and we have some ideas about the rotational axis. But we don't really know much," she conceded.

The Rosetta spacecraft will make its closest approach to Lutetia at 1544 GMT (1644 BST; 1744 CEST). The minimum separation distance will be 3,162km.

The encounter itself is taking place some 454 million km from Earth.

Nearly all of the Rosetta mission's instruments will be switched on for the flyby.

Multi-wavelength cameras and spectrometers, magnetic field and plasma experiments, dust instruments, a radio science experiment - all will try to gather as much information as possible as the spacecraft whizzes by at the relative speed of 54,000km/h.

The Rosetta observations will fix Lutetia's composition and refine estimates for its mass and density. The probe will search for signs of water-ice and any hint of an ultra-tenuous atmosphere (or exosphere).

ASTEROID 21 LUTETIA Image caption Models built from telescope data suggest a slight elongation for Lutetia It was discovered in 1852 by Hermann Goldschmidt in Paris

Lutetia was the name of a pre-Roman town on the site of Paris

The designation "21" refers to the order of asteroid discoveries

Models built from telescope data hint at a slight elongation (above)

And if Rosetta captures any dust in the flyby, the probe will be able to study it in detail thanks to the onboard atomic force microscope - the most distant deployment yet for such a high-resolution science instrument.

Earth-based telescopes have had great difficulty in classifying Lutetia.

Some observations have suggested it is a very primitive body, little changed since its formation (a so-called C-type asteroid).

Other measurements, though, have also spied what appear to be metals in its surface, indicating the rock might have undergone a greater degree of evolution (M-type asteroid). Lutetia might even be the fragmented remains of a much larger asteroid smashed apart in a great collision.

Rosetta will attempt to resolve these issues once and for all.

"After Rosetta has flown by Lutetia, this will be one of the best observed and characterised asteroids that is there at all, and for sure we will know what type of asteroid it is," said Dr Schulz.

Picture wait

The most eagerly awaited data, however, is likely simply to be the visual imagery of Lutetia.

These will show the degree to which gravity has been able to pull this colossal rock into a roughly spherical shape. Also immediately apparent will be the hills and valleys on its surface, the boulder fields, and the craters left by impacts from other, smaller asteroids.

ROSETTA'S SOLAR SYSTEM ODYSSEY The spacecraft launched on an Ariane rocket on 2 March, 2004

A number of planetary passes then helped build up momentum

Rosetta made its first Earth swingby on 4 March, 2005

This was followed by a Mars swingby on 25 February, 2007

A second Earth pass occurred on 13 November, 2007

The 2867 Steins asteroid was visited on 5 September, 2008

A third and final Earth swingby happened on 13 November, 2009

The flyby of 21 Lutetia is all set for 10 July, 2010

Rosetta will arrive at its target comet in May 2014

Its Philae lander will go down to the surface in November 2014

The probe will study the comet as it rounds the Sun

Rosetta's mission is due to end in December 2015 Follow Esa's Rosetta blog

These pictures will be obtained by Rosetta's Osiris (Optical, Spectroscopic, and Infrared Remote Imaging System) camera system. The images should start arriving back on Earth at about 1800 GMT (1900 BST; 2000 CEST).

They will need to be processed so the world will not get to view them until about 2100 GMT (2200 BST; 2300 CEST).

Before this flyby, the largest asteroid encountered by a spacecraft was Mathilde, which is a little more than 50km across.

Mathilde was visited by the US space agency's (Nasa) Near-Shoemaker probe in 1997.

Rosetta itself has already made one close asteroid flyby, of the Steins rock in 2008.

Once this latest pass is complete, the probe will head out to its meeting with the Comet Churyumov-Gerasimenko, set for the May of 2014.

The probe will go into orbit around the 4km-wide ball of ice and dust and even place a small lander called Philae on its surface.

Asteroids are the object of keen interest currently. The Japanese Hayabusa mission has recently returned from the Itokawa space rock, and next year the US Dawn mission will go into orbit around Asteroid Vesta.

The American President Barack Obama says Nasa should also have the goal of trying to send astronauts to an asteroid sometime in the 2020s.

Jonathan.Amos-INTERNET@bbc.co.u