As NASA's Dawn spacecraft draws ever close to the asteroid Ceres, the images of the giant space rock keep getting better and better, the space agency reports.

Ceres, orbiting between Mars and Jupiter and large enough to be considered a dwarf planet, shows signs of possibly once having had an ocean, and there may still be water there, astronomers say.

The most recent pictures, with each pixel representing around 8 miles of the dwarf planet's surface, are humanity's best-ever views of Ceres, at 590 miles in diameter the largest object in the asteroid belt beyond the orbit of Mars.

A bright spot in the northern hemisphere, dubbed Ceres' "white spot," is clearly visible in the new images, although scientists still are unable to say exactly what it might be.

It is most likely a large impact crater, but better images in the coming weeks, as Dawn approaches in rendezvous with Ceres on March 6, should reveal its secret.

Ceres is the first dwarf planet to be visited by a space probe; the second will be Pluto, which will get a fly-by by the New Horizons spacecraft in July.

The latest images of Ceres were captured Feb. 4 from a distance of around 90,000 miles.

Before the dawn mission, little was known about Ceres, as even the powerful Hubble Space Telescope could only image it as a fuzzy blob in space.

The new images clearly show a surface of varying textures, covered by multitudes of impact craters including a large, well formed circular one in the dwarf planet's south polar region.

Ceres was the first asteroid ever discovered, spotted in 1801 by Italian astronomer Giuseppe Piazzi, who first thought he had discovered a new planet.

Astronomy books for 50 years afterward classified it as a planet, until the discovery of similar objects between Mars and Jupiter convinced astronomers they were looking at a new class of solar system objects.

The word asteroid, meaning "star-like," was coined by the astronomer William Herschel, who said they were difficult to distinguish from stars even with the best telescopes of his time.

When Pluto was "demoted" from full planet status to dwarf planet, Ceres was put in the same category, although it is still also considered an asteroid given its orbit in the solar system's asteroid belt.

When the Dawn spacecraft reaches Ceres it will go into a gradually decreasing orbit that will eventually bring it to within around 400 miles of the dwarf world, NASA says.