MESSENGER Mission News
August 31, 2007

MESSENGER TO FEEL THE HEAT DURING FIRST "HOT" PERIAPSIS

Tomorrow, the MESSENGER probe will come within 0.33 AU of the Sun-49.67 million kilometers (or 30.86 million miles). "This is the closest we've approached the Sun during the mission so far," notes MESSENGER's Deputy Project Scientist Brian Anderson of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "It will give us an opportunity to see how the spacecraft behaves this near the Sun."

Of course, MESSENGER was designed to withstand the heat. Mercury is so close to the Sun that the probe will be exposed to up to 11 times more sunlight than it would in space near Earth. To prevent the intense heat and radiation from having catastrophic consequences, the mission has been planned carefully to ensure that the spacecraft can operate reliably in the harsh environment.

To protect against direct sunlight, the spacecraft's sunshade is pointed toward the Sun at all times so that the instruments are always shaded. The spacecraft's orbit around Mercury has been designed so that its closest approach to the planet will be away from the most Sun-baked region of the surface and so that it spends little time at low altitudes over sunlit areas. This outcome is achieved by an orbit where the periapsis (the closest point to the surface of Mercury and also the part of the orbit where the spacecraft's speed over the planet is the highest) is at a high latitude and the apoapsis (the farthest point of the orbit and also the part of the orbit where the spacecraft's speed is the lowest) is far from the surface of Mercury. In this manner, infrared radiation received by the spacecraft from Mercury's surface can be kept at safe levels.

The MESSENGER team at APL spent a good part of August preparing for this first "hot" periapsis. Solar array feathering began on August 1 as the probe's distance to the Sun continued to decrease. On that day, when the probe was 0.5 AU from the Sun, the arrays were tilted 50° back from their earlier position facing the Sun; on August 15, the tilt of the panels was increased to 65°; and on September 1, the arrays will tilt 70° past their Sun-normal position.

Next up is a Deep Space Maneuver (DSM-2), the second for the mission, on October 18. That course correction event will adjust MESSENGER's orbit in preparation for its January 14, 2008, encounter 200 kilometers (124 miles) above the surface of Mercury. According to MESSENGER Mission Design Lead Engineer Jim McAdams, the vast majority-92 percent-of DSM-2 will slow the spacecraft orbit's closest point to the Sun. "After DSM-2, the time required to complete one orbit of the Sun will be only 4.4 months," McAdams notes.

To view MESSENGER's current position, as well as its full orbital path since launch, go online to http://messenger/whereis/index.php.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as principal investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.