Before going on an expensive vacation most people will buy new clothes, a new camera, new luggage, or maybe even a brand new car. After all, you don’t want anything to wear out or break down while you’re away. You’d think NASA would take a similar philosophy when sending spacecraft to other planets. Billions of miles from home, the losses can be enormous if some piece of critical hardware suddenly stops working.
The truth is that, even though they look shiny and new, spacecraft are more like used cars than something right off the assembly line by the time they’re launched. Every switch and instrument has been tested, stressed, and tested again. Even as something as simple as a light bulb will have been turned on hundreds of times before it flies. Why? Because engineers know from experience that a bad light bulb will burn out after only a few hundred on/off cycles, while good light bulbs can be switched on many thousands of times before they fail.
Take the Mercury Dual Imaging System, or MDIS, on board the MESSENGER spacecraft that recently took close-up pictures of the innermost planet Mercury for the first time in 34 years. MDIS is actually two instruments, a wide-angle camera and a narrow-angle camera. Like any digital camera, the WAC and NAC focus light from an object onto a CCD (charged-coupled device) detector composed of thousands of tiny cells or pixels. Each pixel converts the light it receives into electrons that create an electrical current. A computer reads the strength of this current and records the information from each pixel to produce a digital image. Whereas you might not care about the subtle differences in pixel values that make up the freckles on your child’s face, the tiniest variations in an image taken of a distant planet might contain a wealth of information for scientists. Hidden in these pixels are clues about topography, physical characteristics of the soil, and surface chemistry. Before reading the clues, however, scientists must first know precisely how the camera performs. This requires not only subjecting it to conditions, such as temperature extremes, similar to what it will encounter in space, but taking thousands of photographs of objects on Earth with known physical properties. The response of every pixel is carefully recorded as a reference for use during the actual mission. By the time MDIS was installed on MESSENGER, it was a very used, and very well understood, camera.
Of course, all of this takes time—lots of it—which is one reason spacecraft take years to build and test, and why patience is a necessity for planetary scientists. When the MDIS cameras sent back spectacular images from MESSENGER’s first Mercury flyby in January, my thoughts were with Tom Watters, my friend and colleague at the National Air and Space Museum’s Center for Earth and Planetary Studies. Tom is a member of MESSENGER’s Science Team, and he had spent countless hours over the course of several weeks leading up to the launch helping to get MDIS ready at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
When I first saw the MDIS images in January, I was anxious to hear what Tom thought about the camera’s performance. I finally caught up with him after a hectic week spent looking at all the images. “The amount of detail you can see is amazing,” he told me. Even in areas that had been photographed in the 1970s, “We’ve found new features on the Mariner 10 side. Mariner 10 had imaged the region before, but you couldn’t make out the features because of the poor lighting geometry. They show up perfectly in the MESSENGER images.”
To an average person Mercury doesn’t look much different than the moon. The surfaces of both worlds are pretty dead, and both are pocked with impact craters. But Mercury is about 40% bigger than the moon, and seems to have undergone a very different history of crustal evolution and volcanism. Tom refers to several well-known scarps, or cliffs, that scientists have been studying since Mariner 10. These scarps suggest that the whole planet shrank a bit as it was cooling, deforming the crust like dried paint on a shrinking balloon. Determining just how much shrinkage occurred has been difficult, since until MESSENGER we’ve only had information from about half of the planet.
As scientists continue to study the new images—and they’ve only just begun—they’ll be looking for more scarps like the one pictured here. Tom hinted to me that the team has already found lots more. Once they have a complete map of these features, scientists will be able to determine more precisely how much shrinkage occurred, which will tell us a lot about the planet’s geologic history.
And none of it would be possible without all those hours spent testing some very expensive “used” equipment.
Bob Craddock is a geologist with the National Air and Space Museum’s Center for Earth and Planetary Studies.
how is air in planet mercury?
Posted by on April 14,2008 | 03:30PM