The U.S. space agency spacecraft Messenger swooped within 199-kilometers of the planet Mercury, Monday, in preparation for a permanent orbit, beginning in 2011. Messenger, which is operated by remote control from Earth, will soon begin beaming data back which scientists hope will answer questions about the planet closest to the Sun. Jessica Berman reports.

The unmanned spacecraft was launched by NASA in August, 2004. Messenger, stands for Mercury Surface, Space Environment, Geochemistry and Ranging.

Eric Finnegan is systems engineer for NASA's Messenger Mission. Finnegan says Messenger made a nighttime approach toward Mercury at nearly 26,000 kilometers per hour. The planet's gravitational pull slowed the spacecraft by 8,000 kilometers per hour for its eventual descent into orbit around Mercury in 2011.

Image of Mercury taken by Messenger during its approach between January 9 and January 13, 2008

Finnegan says Messenger sent back preliminary photos of the approach. He says the spacecraft will begin beam back more detailed imagery and data within the next day or two.

"We're very, very excited. We had a very successful flyby today," he said. "We'll still still be waiting for the next day or so to get down imagery but our initial indications flying by the planet using just radio beacon information looks good."

Planetary scientists hope the $450 million mission will help them answer key questions about Mercury, a tiny planet with temperatures that swing between 315 degrees in the daytime to minus 180 degrees at night.

Astronomers say the planet is heavily cratered and has a large iron core.

"Mercury is really an oddball," said Louis Friedman who heads the Planetary Society. "It is a very dense solar system object. It's very small. It's in toward the Sun. It's only about the size of the Earth's moon, a little larger. And, as such, how did it form? And however it formed is going to tell us something about planet formation."

Friedman says the Milky Way is like a jigsaw puzzle; he says only when all the pieces are in place, and all the planets are explored, will astronomers understand how the galaxy was formed.

"Mercury is a very, very dense and very heavy object, and therefore getting a really good handle on that density and its mass and associated size will help us in trying to determine what happened at the time of planetary formation," he said. "How did it reach that size, how did it evolve and how did assume its final orbit?"

Messenger is about halfway through its journey to put it in permanent orbit around Mercury in 2011.

But, until then, there will be another flyby this October and again in September 2009.