MESSENGER, the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft launched by NASA in 2004, is scheduled to fly over Mercury today in the first visit in almost 33 years to the mysterious small planet. This will be the first of four sweeps over Mercury by the MESSENGER, which will bring the probe some 120 miles from the planet's surface.

The sophisticated yet small MESSENGER probe prepares to fly by the hemisphere completely missed by the Mariner mission in the 1970s. NASA's gadget will send some 1,200 photos starting tomorrow, on Tuesday, when it resumes communications with Earth. The spacecraft has already flown past Earth once and Venus twice as it spiraled down the solar system on the carefully calculated 4.9 billion-mile trek to Mercury.

"We really need better information on Mercury to make sure that our ideas on how the Earth and sister planets formed" are accurate, said to AP lead researcher Sean Solomon of the Carnegie Institution of Washington.

Launched on a Boeing Delta II rocket, MESSENGER lifted off from Cape Canaveral Air Force Station, Florida at 02:15:56 EDT on August 3, 2004 with the goals to determine the chemical composition of Mercury's surface, its geologic history, the nature of the planet's magnetic field, the size and state of the core, the volatile inventory at the poles, and the nature of Mercury's exosphere and magnetosphere.

However, MESSENGER's most important mission is yet to come: its Mercury orbit insertion will be on March 18, 2011, beginning a year-long orbital mission which will see a lot more data sent to Earth.

The space probe is also interesting because its navigation team is lead by KinetX, the first private company to be responsible for navigation of a NASA deep space mission. Their experts are fully responsible for determining all trajectory adjustments throughout the probe's flight through the inner solar system ensuring that MESSENGER arrives at Mercury with the proper velocity for orbit insertion.

Mercury, named after the Roman god Mercurius, is the innermost and smallest planet in the solar system and orbits the Sun every 88 days. The robotic space probe Mariner 10 was the only spacecraft so far to approach Mercury, and managed to map about 40 percent of its surface. Its mission ran between 1974 and 1975 and the probe was the first spacecraft to make use of an interplanetary "gravitational slingshot" maneuver.

"Probably the first thing that most of us want to see is what the other 55 percent of Mercury [missed by Mariner 10] looks like," said Faith Vilas, a MESSENGER scientist and director of the MMT Observatory at Mount Hopkins, Ariz.

NASA announced that it plans to release images and results from today's flyby on Jan. 30. The transmission of the images will take up to a week and will start tomorrow.

"The biggest mystery of Mercury is why it has so much heavy metal -- a core very different in size from other planets," Solomon said. "We think we can begin to unravel the mystery once we know the chemical makeup of the planet's surface."