The comet-bound Rosetta spacecraft will use its powerful instruments to see and sniff asteroid Lutetia Saturday, taking advantage of a fortuitous flyby of the perplexing object, which could be a chunk of primordial rock from the ancient solar system.

Artist's concept of Rosetta visiting an asteroid. Credit: ESA/AOES Medialab

Scientists don't know what to expect from Lutetia, which will be the largest asteroid ever visited by a spacecraft. Lutetia is inside the main asteroid belt between Mars and Jupiter.

The size of Lutetia is even up for debate. The best estimate is the round-shaped asteroid is about 60 miles across, but other data points to an elongated shape with a peak diameter of 83 miles.

Scientists also don't know the chemical make-up of Lutetia. The best guess is Lutetia is a C-type asteroid, meaning it has stayed relatively untouched through most of the violent 4.6-billion-year history of the solar system.

C-type asteroids are dark and rich in carbon and organic molecules. Scientists believe they are leftover relics from the formation of the solar system.

"If it does turn out to be a C-type, which we all hope, then we have a large object which is rather pristine showing us what the solar system was like shortly after the planets formed," said Rita Schulz, Rosetta's project scientist at the European Space Agency.

But some measurements from telescopes on Earth and in space suggest Lutetia could harbor metals, a signature of an M-type asteroid. Schulz said metallic M-type asteroids formed from rock from the interior of a larger body after massive collisions fractured the parent object.

"It can't be, at the same time, a C-type and an M-type asteroid because they are so different that it is not possible," Schulz said. "This is a riddle that we can solve only by visiting this object because the indications from all the observations we have right now are not conclusive enough that anyone would dare to say this is for sure a C-type asteroid."

Rosetta should answer all of these questions Saturday.

ESA reports Rosetta is right on course for the flyby, and engineers don't expect to fire the craft's thrusters to correct the trajectory.

Built to study a comet, Rosetta will only have about two hours to get the best views of Lutetia as the probe speeds along at a relative velocity of approximately 33,500 mph.

Rosetta will approach within 1,965 miles of the asteroid at 1544:56 GMT (11:44 a.m. EDT) Saturday. But the asteroid is 280 million miles from Earth, meaning it will take radio signals more than 25 minutes to travel from Rosetta back to Earth.

The flyby will occur at 1610:17 GMT (12:10 p.m. EDT) as observed from Earth.

Saturday's encounter is the second asteroid flyby of Rosetta's decade-long sojourn through the solar system. The robotic probe flew past the much smaller asteroid Steins in 2008.

Rosetta launched in 2004 on a $1.2 billion mission to explore comet Churyumov-Gerasimenko. The spacecraft will enter orbit around the periodic comet in the summer of 2014 and stay with Churyumov-Gerasimenko until the end of 2015. The probe also carries a small lander named Philae to drop on the comet's surface.

Rosetta captured this image of asteroid Steins in 2008. Credit: ESA

Mission managers assembled a list of possible asteroid flyby opportunities after Rosetta's launch to give researchers bonus science on the way to the craft's ultimate destination.

"Steins and Lutetia was regarded as the best possible combination and was selected," said Gerhard Schwehm, Rosetta's project manager. "Lutetia is actually the prime asteroid target, and therefore Rosetta will achieve a major milestone on Saturday. The asteroid chapter can be closed and we will concentrate on the comet."

If everything goes as planned, Rosetta's visible camera will return images of Lutetia to Earth later Saturday night. Schulz plans to present the pictures as early as 2100 GMT (5 p.m. EDT).

"We will have pictures showing craters, valleys and all kinds of features on the surface," Schulz said.

Rosetta's mineral-mapping spectrometers will determine the surface composition of Lutetia, and the probe's gas analyzers will try to detect an exosphere, or ultra-thin atmosphere, around the asteroid.

Other sensors will measure the magnetic field around Lutetia and the asteroid's interaction with the solar wind.

"We hope to find some gases, or maybe ice and water, and we will look at the density of the asteroid," Schulz said in an interview Tuesday.

Rosetta's findings should close the book on Lutetia's chemical make-up by observing the asteroid's ratio of metallic and carbon compounds.

"After the flyby, we will know for sure what class of asteroid this is," Schulz said. "This is an accomplishment because asteroid scientists have now started calling it an X-type, meaning we have no clue what it is."

Lutetia was discovered in 1852 and named for an ancient Roman city on the site of present-day Paris.

If Lutetia is confirmed as a carbon-rich object, it would only be the second C-type asteroid ever visited by a spacecraft.

NASA's NEAR Shoemaker asteroid probe flew by asteroid Mathilde in June 1997, the closest look scientists have of such a pristine object.

"Lutetia has always been our main asteroid target because we believe this will provide us with the most precious information about how the planets have formed, and what the status of the material was like during the period of planet formation," Schulz said.

Studying asteroids is one way researchers learn about the early history of the solar system, when small objects commonly collided with each other to gradually build large planets and moons.

"What's left in the asteroid belt could not accrete into a bigger planet and had to stay small," Schulz said. "The material in the asteroid belt has not been altered as much as the planets have. These are interesting bodies, for sure. Because they are very diverse, we have to look at them all to understand better how the asteroid belt evolved."