As with so much NASA accomplishes, this is quite an engineering feat. MESSENGER needed to leave Earth at a sufficient speed to reach Mercury but then slow down to enjoy the view. It's a schlep. As I type this MESSENGER has already traveled 2.8 billion miles!

The speed reduction is being accomplished with a gravity assist--a maneuver used in the past to speed up spacecraft. By flying in front of a planet, some of the spacecraft's energy is absorbed, and the speedometer starts ramping down. MESSENGER will continue to brake this way until making its final approach at Mercury in 2011. Its elliptical orbit will become more stable. Even now, the spacecraft has approached within 120 miles of the surface.

If all goes well, MESSENGER will achieve six distinct scientific goals, finding out:

• Why is Mercury so dense?
• What is the geologic history of Mercury?
• What is the nature of Mercury's magnetic field?
• What is the structure of Mercury's core?
• What are the unusual materials at Mercury's poles?
• What volatiles are important at Mercury?

In the meantime the first high- res photos are being received on Earth and they are everything we could have hoped for.

From NASA:

At roughly 1:50 a.m. ET on October 7, MESSENGER's most recent images began to be received back on Earth. The spectacular image shown here is one of the first to be returned. It shows Mercury about 90 minutes after the spacecraft's closest approach. The bright crater just south of the center of the image is Kuiper, identified on images from the Mariner 10 mission in the 1970s. For most of the terrain east of Kuiper, toward the limb (edge) of the planet, the departing images are the first spacecraft views of that portion of Mercury's surface. A striking characteristic of this newly imaged area is the large pattern of rays that extend from the northern region of Mercury to regions south of Kuiper.

There are plenty more images on NASA's Messenger site.