According to the mission team, their largest engineering challenge was designing a spacecraft that could work both near Earth and less than 0.4 astronomical units (AU) from the Sun. (One AU is the average distance from the Sun to the Earth, about 150 million kilometers or 93 million miles.)

Messenger employs a multilayered approach that the team devised to fend off the excess heat while the spacecraft is near Mercury, and yet still allow the spacecraft to remain warm when closer to home.

The main layer of protection is the ceramic-fabric sunshade, which serves as a giant parasol, ensuring that the spacecraft always keeps its cool while near the Sun.

During the initial stages of Messenger's long voyage to Mercury the spacecraft will fly backward - the sunshade side will be turned away from the Sun to allow the delicate electronics to remain at a comfortable temperature while basking in the relatively benign lighting conditions near Earth.

In this spacecraft view (top right), the sunshade has been temporarily removed to allow engineers ready access to Messenger's thermal blankets; here a blanket has been folded up, revealing one of the three main propellant tanks. The thermal blankets serve as Messenger's second line of defense against the intense solar radiation and the heat reflecting from the surface of Mercury.

On the front of the spacecraft they limit the incoming heat; on the rest of Messenger they will keep components warm during the cruise to Mercury and the portion of each Mercury orbit when the spacecraft is away from the sunlit (hot) side of the planet.

The blankets are made of many layers (about 20) of aluminized Kapton. The layers are kept from touching each other by embossed bumps. This design almost eliminates conductive heat transfer, the process in which heat flows through a solid form. Heat stays in the spacecraft when Messenger is near the Earth, but heat can't get in when Messenger is near the Sun.

Working with other parts of Messenger's guidance and control system, the Digital Solar Attitude Detectors (or Sun sensors) will help keep the sunshade pointed at the Sun. The solar arrays also pivot, yet another strategy to protect the spacecraft from overheating.

Messenger's latest 15-day launch period extends from July 30-August 13, 2004. After a swingby of Earth and multiple passes of Venus and Mercury, the spacecraft is set to begin orbiting Mercury in March 2011.

Messenger arrived in Florida after being shipped from NASA's Goddard Space Flight Center in March. Messenger is currently at the Astrotech Space Operations facilities near Kennedy Space Center where it is undergoing prelaunch testing.

Testing of the spacecraft's radio system uplink and downlinks through the Kennedy Space Center/Jet Propulsion Laboratory interface with the Deep Space Network (MIL-71) continues. Autonomy testing is also continuing. This verifies Messenger's ability to operate on its own when not in direct contact with Earth. Installation of thermal blankets continues.

On April 13, the spacecraft will be moved from its current location in the hazardous processing facility, where it has been since arrival, to an adjacent non-hazardous payload processing facility. The remainder of its final assembly and testing will be completed there. The spacecraft will return to the hazardous processing facility when ready for fueling, spin balance testing and mating to the upper stage.

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