Mercury or bust

Posted by Rich Talcott
on Thursday, June 07, 2007

After a weeklong stretch of off-and-on clouds, Tuesday evening dawned clear in Wisconsin. There, gleaming in the west as it has for the past several months, brilliant Venus dominated the sky. But Venus had added allure this evening: A new spacecraft was skimming just above its cloud tops.

No, I couldn't see it. As someone who has explained countless times why the Apollo lunar modules can't be seen from Earth, I'm not about to claim a spacecraft of similar size can be glimpsed across interplanetary distances. But just knowing the sophisticated robotic probe was using Venus' gravity as a slingshot to reach Mercury made the sight of Venus special this night.

The NASA spacecraft MESSENGER — short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging — flew just 209 miles above Venus' surface. The close approach changed the probe's trajectory and slowed it from 81,700 mph to 62,300 mph. The gravity assist sets up MESSENGER for its first flyby of Mercury next January.

This was the spacecraft's second flyby of Venus. Last October, however, mission operators did not make any scientific observations of the planet because it was then on the opposite side of the Sun from Earth and in a 2-week communications blackout. As Venus' gleaming countenance in Tuesday's sky confirmed, there were no such hindrances this time around.

Just like the New Horizons' flyby of Jupiter in late February, mission planners used MESSENGER's flyby of Venus to test the spacecraft's systems. The verdict: It passed with flying colors. The most important test was of the spacecraft's battery. The flight path took MESSENGER through Venus' shadow for 20 minutes, when there was no solar power. The battery successfully recharged after the eclipse.

MESSENGER turned its scientific instruments on Venus as well, making observations in conjunction with the European Space Agency's Venus Express spacecraft now orbiting Earth's "twin." MESSENGER took more than 630 images in visible and near-infrared light, and studied how the solar wind interacts with the planet. The results will start coming back to Earth later this week.

The main purpose of the flyby, however, was to redirect the probe toward Mercury. It will arrive there in January for the first of three flybys — the first such reconnaissance since Mariner 10 flew past the innermost planet more than 30 years ago. MESSENGER will return to Mercury in October 2008 and September 2009, mapping most of the planet (Mariner 10 imaged a bit less than half of Mercury) and studying its surface and atmospheric composition. MESSENGER then returns to Mercury in March 2011 to begin a year-long orbital mission. It promises to be an exciting next 5 years. And you can count on Astronomy to keep you up-to-date on all the latest findings.

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