The US space agency says its Dawn probe should go into orbit around the asteroid Vesta early on Saturday (GMT).
The robotic satellite will be spending a year at the 530km-wide body before moving on to the "dwarf planet" Ceres.New pictures on Dawn's approach to Vesta show the giant rock in unprecedented detail.
The asteroid looks like a punctured football, the result of a colossal collision sometime in its past that knocked off its south polar region.
Vesta was discovered in 1807, the fourth asteroid to be identified in the great belt of rocky debris orbiting between Mars and Jupiter.
At the time, its great scale meant it was designated as another planet but it later lost this status as researchers learnt more about the diversity of objects in the Solar System.
Close but careful Dawn's encounter is occurring about 188 million km (117 million miles) from Earth.
The probe is propelled by an ion engine and engineers have put the spacecraft on a course to be captured in the gravitational field of Vesta.
They expect to hear confirmation from the satellite on Saturday that it is safely circling the rock.
Initially, Dawn will be about 16,000km (9,900 miles) from the asteroid, but this distance will be reduced over time.
Mission scientists hope to get within 200km of the surface but the team do not intend to take any unnecessary risks.
"We would like to get as low as possible but if we crash Dawn, Nasa would understandably be very angry at us," Principal Investigator Chris Russell told BBC News.
Asteroids can tell us about the earliest days of the Solar System. These wandering rocks are often described as the rubble that was left over after the planets proper had formed.
Vesta and Ceres should make for interesting subjects. They are both evolved bodies - objects that have heated up and started to separate into distinct layers.
Surface detail "We think that Vesta has a metal core in the centre - an iron core - and then silicate rock around it," explained Dr Russell.
"And then, sometime in its history, it got banged on the bottom and a lot of material was liberated. Some of this material gets pulled into the Earth's atmosphere. One in 20 meteorites seen to fall to Earth has been identified with Vesta," he added.
Ceres, which, at 950km in diameter, is by far the largest and most massive body in the asteroid belt, probably did not evolve as much as Vesta.
Scientists think it likely that it retains a lot of water, perhaps in a band of ice deep below the surface.
Dawn's quest at Vesta over the coming months will be to map the asteroid's surface. The probe carries instruments to detect the mineral and elemental abundances in its rocks. It will be looking for evidence of geological processes such as mountain building and rifting. The team is keen to understand how Vesta's surface has been remodelled over time by impacts and even lava flows.
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