National Geographic News
The Rosetta spacecraft.

An artist's rendering of the Rosetta spacecraft nearing an asteroid.

Image courtesy C. Carreau

Andrew Fazekas

for National Geographic News

Published July 9, 2010

A comet-chasing probe is on course for a high-speed encounter this weekend with the largest and least understood asteroid yet visited.

At 12:10 p.m. ET on Saturday, the European Space Agency's Rosetta spacecraft is expected to skim within 1,964 miles (3,162 kilometers) of the surface of an asteroid known as 21 Lutetia.

Racing past the space rock at about 33,554 miles (54,000 kilometers) an hour, Rosetta will snap a series of pictures and collect data on the asteroid's chemical makeup, magnetic field, mass, and density.

(Related: "Japan's 'Falcon' Spacecraft Returns—Asteroid Dust On Board?")

Astronomers hope the fresh influx of data will help unlock some of the mysteries surrounding the birth of our solar system. That's because asteroids are thought to be material left over from the formation of the planets about 4.5 billion years ago.

Some of the space rocks are most likely fragments of the iron-rich cores of ancient proto-planets that shattered during impacts. Other asteroids may be near-pristine carbon-rich building blocks—examples of the raw materials that went into making today's planets.

"Estimated to be approximately a hundred kilometers [62 miles] in diameter, Lutetia was chosen [from a short list of candidates] because planetary scientists believe it may be more pristine than most asteroids visited by space probes so far," said Rita Schulz, ESA’s Rosetta project scientist.

Asteroid Encounter to Probe Solar System's "Memory"

Not all asteroids are perfect "fossils" from the early solar system. In the solar system's early days, the main asteroid belt—which exists between the orbits of Mars and Jupiter—was overcrowded.

Countless collisions and breakups have forever altered many of the early asteroids. (See "Strange 'Comet' May Be Asteroid Collision Debris.")

Still, astronomers think that some of the larger ones we see today, including Lutetia, somehow remained untouched by impacts, Schulz said.

"These type of asteroids may actually give us a glimpse of the status of their material as it was when the planets were born," Schulz said.

Lutetia in particular has perplexed astronomers, who have been unable to decipher its chemical makeup—and hence its origins—using ground-based observations.

By making its Lutetia flyby, "Rosetta is trying to look into that distant memory of that part of the formation of our solar system."

Rosetta had previously visited another asteroid, Steins, in 2008. That flyby revealed that the 3.7-mile-wide (6-kilometer-wide) asteroid was most likely a chunk from the innards of an even more ancient asteroid that shattered during an impact.

(Also see "Water Discovered on an Asteroid—A First.")

After this weekend's speedy encounter, mission controllers will put Rosetta in hibernation until it reaches its final destination in May 2014: comet Churyumov-Gerasimenko.

Rosetta will orbit the comet for at least 19 months, from May 2014 to December 2015, and will release a lander onto the comet's surface.

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