Asteroid rendezvous nears

Dawn and Hubble images of Vesta

Already, Dawn's images of Vesta (left) are better even than the Hubble Space Telescope can obtain (right). The protoplanet Vesta is the second-most massive object in the main asteroid belt.

NASA’S DAWN SPACECRAFT is on track to begin the first extended visit to a large asteroid. The mission expects to go into orbit around Vesta on July 16 and begin gathering science data in early August.

Vesta is considered a protoplanet, or body that didn’t quite become a full-fledged planet. It lives in the main asteroid belt between Mars and Jupiter, and is thought to be the source of a large number of meteorites that fall to Earth.

After travelling nearly four years and 2.7 billion kilometres, Dawn is approximately 150,000 kilometres away from Vesta. When Vesta captures Dawn into its orbit on July 16, there will be approximately 16,000 kilometres between them. When orbit is achieved, they will be approximately 188 million kilometres away from Earth.

Artist's impression of the Dawn spacecraft

Artist's impression of the Dawn spacecraft

“The spacecraft is right on target,” said Robert Mase, Dawn project manager at NASA’s Jet Propulsion Laboratory. “We look forward to exploring this unknown world during Dawn’s one-year stay in Vesta’s orbit.”

Peeling back the layers

After Dawn enters Vesta’s orbit, engineers will need a few days to determine the exact time of capture. Unlike other missions where a dramatic, nail-biting propulsive burn results in orbit insertion around a planet, Dawn has been using its placid ion propulsion system to subtly shape its path for years to match Vesta’s orbit around the Sun.

Images from Dawn’s framing camera, taken for navigation purposes, show the slow progress toward Vesta. Made into a movie (below), they are about twice as sharp as the best images of Vesta from NASA’s Hubble Space Telescope, but the surface details Dawn will obtain are still a mystery.

“We can’t wait for Dawn to peel back the layers of time and reveal the early history of our Solar System,” said Christopher Russell, Dawn principal investigator, at UCLA.

During the initial reconnaissance orbit, at a distance of approximately 2,700 kilometres, the spacecraft will get a broad overview of Vesta with colour pictures and data in different wavelengths of reflected light.

The spacecraft will then move drop lower into a mapping orbit about 680 kilometres above the surface to systematically map the parts of Vesta’s surface illuminated by the Sun.

It will collect stereo images to see topographic highs and lows, acquire higher-resolution data to map rock types at the surface; and learn more about Vesta’s thermal properties.

Up close and personal

Dawn then will move even closer, to a low-altitude mapping orbit approximately 200 kilometres above the surface. The primary science goals of this orbit are to detect the by-products of cosmic rays hitting the surface and help scientists determine the many kinds of atoms there, and probe the protoplanet’s internal structure.

“We’ve packed our year at Vesta chock-full of science observations to help us unravel the mysteries of Vesta,” said Carol Raymond, Dawn’s deputy principal investigator at JPL.

Following a year at Vesta, the spacecraft will depart for its second destination, the dwarf planet Ceres, in July 2012.

As Dawn spirals away from Vesta, it will pause again at the high-altitude mapping orbit. Because the Sun’s angle on the surface will have progressed, scientists will be able to see previously hidden terrain while obtaining different views of surface features.

Dawn was launched in September 2007.

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Adapted from information issued by NASA / JPL-Caltech. Images courtesy NASA / JPL-Caltech / UCLA / MPS / DLR / PSI and NASA / ESA / STScI / Umd.

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