In some ways, Mercury is the most difficult place to explore in the solar system. The smallest planet — its only slightly larger than our moon — it’s much closer than Jupiter and Saturn, but its proximity to the sun means that any spacecraft flown directly at it would be sucked into the star’s gravity and incinerated.
That’s why NASA’s Messenger has taken a wandering, 6 1/2-year journey to its destination, as detailed in this month’s issue of Scientific American. It flew in ever-shrinking circles around the sun, past Earth once, Venus twice and Mercury three times to slow itself to become the first probe ever to orbit the planet. If all goes according to plan, Messenger arrives Thursday.
Messenger will map the entire world — flybys by Mariner 10 in 1974 and Messenger in 2008 and 2009 captured only part of the surface — and hopefully answer questions about Mercury’s strange magnetic field and whether its craters hide any water ice.
That last discovery would be incredible, considering temperatures on the surface facing the sun can reach 800 degrees, high enough to melt zinc. But temperatures on the dark side drop to –261 degrees, and deep, shadowed craters can retain that cold. It was once thought that Mercury did not spin, but scientists now know it does rotate, and that one solar day — sunrise to sunset — lasts 176 Earth days. So Messenger’s year-long mission will only last two days on Mercury.
To survive the extremes, Messenger has a sunshade woven from ceramic fibers, while the solar panels that power it are turned at a very steep angle to avoid getting fried. Most creatively, the camera (those blue and green objects in the front of this artist’s rendering) rests on 14 ounces of paraffin, which melts in the heat, cooling the equipment, then refreezes on the dark side.
Scientists hope the Messenger mission helps answer questions about the earliest days of our corner of the universe. At the very least, the images should be spectacular — particularly of the Carolis Basin, one of the largest impact craters in the solar system, 963 miles in diameter, ringed with mountains 1.2 miles high, formed by an asteroid striking Mercury more than 3.8 billion years ago.
Caloris is named from the Latin for “heat,” a place where the sun shines at an intensity 11 times that as seen on Earth — an unblinking hell breached by a 2,000-pound spacecraft of human ingenuity.
Illustration by Don Foley, originally published in Scientific American, March 2011.