The Japanese Hayabusa spacecraft has approached within 20km (12 miles) of the asteroid it has been travelling to for more than two years.
Images show contrasting hilly and smooth regions on the asteroid
The probe will collect a sample from the surface of asteroid Itokawa and return it to Earth for analysis.
Mission scientists hope this will shed light on how these mysterious Solar System bodies formed and evolved.
Hayabusa has been using an efficient ion engine propulsion system to reach its target.
The probe will now hover where it is, gathering data on Itokawa from a distance.
The plan is for the probe to land on the asteroid, collect a sample from the surface and return it to Earth. Hayabusa's sample-return canister should parachute back to Earth in the summer of 2007.
A new image taken by the spacecraft's camera on 12 September shows the contrast between hilly regions and a smooth area on the asteroid.
Scientists at the Institute of Space and Astronautical Science (Isas), who have been monitoring the mission, said studying these surface features could be key to understanding how this asteroid formed.
The spacecraft will return a sample of the asteroid to Earth
Scientific observation of the asteroid, including sampling and topographic measurement, will be carried out over two months.
So far, the Moon remains the only extra-terrestrial body from which samples have been gathered from the surface and returned to Earth.
Asteroids are thought to consist of material left over from the formation of the Solar System 4.6 billion years ago. Most of these fragments - sometimes referred to as minor planets - orbit the Sun in a belt between Mars and Jupiter.
Hayabusa was launched on 9 May 2003 from Japan aboard an M-V-5 rocket.
Its asteroid target was named after the late Dr Hideo Itokawa, considered one of the fathers of Japan's space programme.