Messenger -- which stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging -- passed a thorough four-day critical design review last week, during which a project advisory panel and NASA assessment team examined every detail of the mission and spacecraft design.

"The review was very successful," says Max R. Peterson, Messenger project manager at the Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md. "Both panels confirmed that our designs are sound and meet the mission's science and engineering requirements. We're ready to move to the next stage."

Messenger team members are building flight hardware now and will begin integrating parts on the spacecraft this November, Peterson says.

After launch and a five-year journey through the inner solar system, Messenger will orbit Mercury for one Earth year, providing the first images of the entire planet and collecting information on the composition and structure of Mercury's crust, its geologic history, the nature of its thin atmosphere and active magnetosphere, and the makeup of its core and polar materials.

While cruising to Mercury the spacecraft will fly past the planet twice -- in 2007 and 2008 -- snapping pictures and gathering data critical to planning the orbit study that begins in April 2009.

A key Messenger design element deals with the intense heat at Mercury. The sun is up to 11 times brighter than we see on Earth and surface temperatures can reach 450 degrees Celsius (about 840 degrees Fahrenheit), but Messenger's instruments will operate at room temperature behind a sunshield made of heat-resistant Nextel fabric. The spacecraft will also pass only briefly over the hottest parts of the surface, limiting exposure to reflected heat.

"The project is well on its way," says Dr. Sean C. Solomon, Messenger principal investigator from the Carnegie Institution of Washington (D.C). "Exploring the many mysteries of Mercury will help us to understand all of the terrestrial planets, including Earth. The team is eagerly looking forward to assembling and launching the spacecraft and to the first new data from the innermost planet."

In July 1999, NASA selected Messenger as the seventh mission in its innovative Discovery Program of lower-cost, highly focused space science investigations. APL manages the $286 million project for NASA's Office of Space Science and will build and operate the Messenger spacecraft.

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