Yesterday the MESSENGER spacecraft successfully completed the first of four "hot seasons" expected to occur during its one-year primary mission in orbit about Mercury. During these hot seasons, the Sun-facing side of the probe's sunshade can reach temperatures as high as 350 degrees C.

These hot conditions are the result of two concurrent circumstances, says MESSENGER Mission Systems Engineer Eric Finnegan, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

"Mercury is in an eccentric orbit, and its distance from the Sun varies over 88 days, from 43,689,229 miles to 28,816,300 miles," he explains. "On May 13, Mercury began heading closer to the Sun in its orbit. The planet reached its closest distance from the Sun on June 12."

The second contributor to this heat is the geometry of MESSENGER's orbit relative to the hot dayside of Mercury. The spacecraft is in a highly eccentric orbit around the planet, approaching to within 310 miles of the surface every 12 hours.

"During this hot period, the closest point of approach of the spacecraft to Mercury's surface occurs on the sunlit side of the planet, so for almost one hour per orbit the spacecraft must pass between the Sun on one side and the hot dayside surface of the planet on the other," Finnegan says.

"To add further extremes, this season is also when the spacecraft passes over the nightside of the planet at high elevations and experiences the longest solar eclipses of the mission. During this period, when eclipses last as long as 62 minutes per orbit, the solar arrays are not illuminated and the spacecraft must derive its power from its internal battery."

High temperatures are always a risk to mechanical and electronic systems, and the geometry of this portion of the orbit severely constrains the ability of the spacecraft to cool itself by radiating heat to cold space. MESSENGER engineers have taken several steps to ensure that the spacecraft remains safe.

"We rotated the solar arrays off the Sun through some of the hottest points so they do not have a view to either the Sun or the hot, dayside surface of the planet," Finnegan says.

"We are power cycling some of the more sensitive instruments to reduce their internal heat dissipation. In a manner similar to the treatment of the solar arrays, we are also adjusting the attitude of the spacecraft to keep some of the more sensitive parts of the spacecraft from seeing the hottest parts of the planet's surface."

All of the instruments have been operating during this period. Finnegan says that there have been times during each orbit when instruments are turned off, however, mostly to conserve power during eclipses.

These conditions are expected to recur approximately every 88 days (i.e., the time it takes Mercury to orbit the Sun). MESSENGER can therefore look forward to three more hot seasons during the course of its primary mission.