Image courtesy NASA/Caltech/UCLA/MPS/DLR/IDA

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A picture of Vesta captured by Dawn as it approached the asteroid July 9.

Image courtesy NASA/Caltech/UCLA/MPS/DLR/IDA

NASA's Dawn Spacecraft to Reach Asteroid This Weekend

Robotic craft will soon settle into orbit around the large space rock Vesta.

After almost four years of interplanetary travel, NASA's Dawn spacecraft will enter into orbit around Vesta, the second largest known asteroid, very early this Saturday.

Mission managers have been steering Dawn ever closer to the 310-mile-wide (500-kilometer-wide) asteroid, and they expect the craft will be captured into orbit around Vesta at about 1 a.m. ET.

Dawn had to travel more than 1.6 billion miles (2.5 billion kilometers) to reach Vesta, which is part of the main asteroid belt, between the orbits of Mars and Jupiter. (Explore an interactive solar system.)

"This is truly an exciting mission, not only because this is the first time ever we will enter orbit around a main-belt asteroid, but because we get a chance to unlock the earliest chapters in our solar system's history," said Chris Russell, principal investigator for the Dawn mission.

Main-belt asteroids are considered to be cosmic fossils left over from the formation of the planets roughly 4.6 billion years ago. (Related: "NASA to Visit Asteroid Predicted to Hit Earth?")

After Vesta, Dawn Heads to Ceres

Dawn will initially orbit Vesta at about 1,700 miles (2,735 kilometers) from the asteroid's surface. At this distance, the mission science team expects to get its first closeup views.

The team will then begin a year-long survey, which will eventually take the spacecraft as close as 110 miles (180 kilometers) from the space rock.

Equipped with optical and spectral cameras, the robotic "geologist" will create topographic charts of Vesta's surface, map its mineralogical composition, and search for possible but as yet unknown moons.

The cameras will show Russell and his team unprecedented detail of Vesta's craggy surface, promising resolutions of up to 820 feet (250 meters) per pixel.

"The views will be amazing, about 150 times sharper than the best images [of Vesta] the Hubble Space Telescope can offer," Russell said.

After its tour of duty at Vesta wraps up in June 2012, Dawn will fly on to Ceres, the solar system's largest asteroid at nearly 621 miles (1,000 kilometers) wide, arriving in February 2015.

Although both Vesta and Ceres are main-belt asteroids, they appear to be geologically unique.

Vesta is a dry, rocky world that looks like it's been reshaped by volcanism and lava flows, similar to tectonic activity seen on Earth and the moon. Meanwhile Ceres may have a primitive, much darker surface with a softer interior, perhaps filled with water.

"By studying Vesta and Ceres we hope to really contrast the formation and evolution of these two distinct asteroids and begin to understand the nature of the most fundamental building blocks that came together and built the planets we see today," Russell said.

Vesta an Appealing Asteroid?

NASA is also looking at Dawn as a scout for possible future destinations for human missions, since the current stated goals for the space agency include trips to asteroids. (Related: "After Space Shuttle, Does U.S. Have a Future in Space?")

NASA has mainly been considering trips to so-called near-Earth asteroids, which come within 121 million miles (195 million kilometers) of the sun.

But Vesta and Ceres, while farther away, should appeal to astronauts, Russell said, since both bodies have sufficient gravity—about 3 percent of Earth's gravity—for humans to walk on their surfaces.

"And in the case of Ceres," Russell said, "it probably has water, which is a self-sustaining resource if you are going to spend significant time on the surface."