Europe's Rosetta space probe is flying past the 100km-wide Asteroid Lutetia - some 454 million km from Earth, beyond the orbit of Mars.
Lutetia is the biggest asteroid yet visited by a spacecraft.
European Space Agency (Esa) scientists are now waiting for details - the first images have already arrived on Earth.
Observations using telescopes have failed to determine Lutetia's true type. Scientists would like to know how much it 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.
AmbiguousDr Rita Schulz, Esa'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.
"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 huge distance to Rosetta means radio signals sent from the probe take 25 minutes to arrive on Earth.
When the pictures and other data are picked up at a receiving station, they are transferred to the European Spacecraft Operations Centre (Esoc) in Darmstadt, Germany.
Some distant images taken during the approach have been sent to Earth already, with close-up shots to follow.
Nearly all of the Rosetta mission's instruments were due to be switched on for a period of several hours around closest approach (1544 GMT; 1644 BST; 1744 CEST).
Multi-wavelength cameras and spectrometers, magnetic field and plasma experiments, dust instruments, a radio science experiment - all were tasked with gathering as much information as possible as the spacecraft whizzed by at the relative speed of 15km/s and a minimum distance of 3,162km.
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 asteroid it is," said Dr Schulz.
The pictures, when they are released, 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.
Before this flyby, the largest asteroid encountered by a spacecraft was Mathilde, which is a little over 50km wide.
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, Rosetta 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.
