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Mars rover Spirit has tenaciously swept, scraped, and squeezed secrets from the forbidding surface of Mars for 6 years. Now at an impasse, up to its belly in sand, it has struggled to tilt its solar panels toward the sun and collect just enough power to survive the perilously cold Martian winter. If Spirit can make it through to spring, the feisty robot will prove it’s still in the game—by solving the mysteries of the Martian core.
Unlocking those secrets will require the guile of a veteran explorer. Like a wily old baseball pitcher who uses knuckle balls to keep winning, the aging Spirit still has a few tricks up its sleeve. It will do its next trick without moving a single mechanical muscle.
“In this case, it’s a good thing Spirit is immobile,” says principal investigator Steve Squyres. “We can track its radio signal to determine its motion through space.”
Mars is rotating around its own axis and orbiting the Sun. With the rover stationary, the radio’s only motion will be the motion of Mars. Because the scientists already know the specifics of the red planet’s orbit, they’ll be able to use Spirit’s radio signal to hone in on how the planet spins around its own axis.
“Mars wobbles, or precesses, as it spins,” says Bruce Banerdt of NASA’s Jet Propulsion Laboratory. “We’ll measure that wobble by looking at the Doppler shift of Spirit’s radio signal.”
“Mars completes an entire wobble only once every 170,000 years,” he continues. “So we’ll be measuring a very tiny motion—looking at minute changes. But these miniscule numbers speak volumes about Mars’ core.”
First, it will help scientists figure out if the core is solid or liquid. There are clues that it was molten at some time in the ancient past. A molten core is a fluid that moves and conducts electricity, so it sets up a powerful magnetic field. Researchers see remnants of that field today but are unsure how much of the core, if any, is still molten.
“If Mars’ core is solid through and through, the nature of the wobble will be subtly different from the wobble if the core is liquid,” says Squyres.
Spin a hard-boiled egg and then spin a raw egg. You’ll see a distinct difference in the way they rotate.
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Spirit’s radio signals will also reveal the precise speed of Mars’ wobble. That, in turn, will help the researchers calculate the planet’s moment of inertia, or MOI.
The moment of inertia of a spinning object—in this case, a planet—is a number that describes how easy or how hard it is to change the spin. “The MOI affects the speed at which the axis of Mars wobbles, so the wobble speed indirectly tells us the MOI,” says Banerdt.
They’ll add the MOI to what they already know about Mars—its size and mass. “Combining these three things with our understanding of how iron and rock behave inside a planet will allow us to set limits on the size and density of the Martian core. And the density will tell us what elements must be mixed with iron to make up the core.”
“This research has implications that reverberate through all kinds of basic questions about the formation of the solar system and its planets. I have to tip my hat to Spirit. It keeps coming up with new tricks.”
But first the rover has to survive the long, hard winter. Baseball great Rogers Hornsby summed it up: “People ask me what I do in winter when there’s no baseball. I’ll tell you what I do. I stare out the window and wait for spring.”
Make that Martian spring.
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NASA Hosting Events for Valentine's Night Comet Encounter – NASA Jet Propulsion Laboratory
NASA's Spirit Rover is providing a lesson to aspiring digital photographers: Spend your money on the lens, not the pixels.
Anyone who has ever agonized over whether to buy a 3-megapixel or 4-megapixel digital camera might be surprised to learn that Spirit's stunningly detailed images of Mars are made with a 1-megapixel model, a palm-sized 9-ounce marvel that would be coveted in any geek's shirt pocket.
Spirit's images are IMAX quality, mission managers say.
The word pixel is derived from the term “picture element.” A pixel is the smallest dot of information that goes into making a digital image. One megapixel is a million pixels set up in an array equal to 1,000 by 1,000.
Intuitively, more pixels means higher resolution. That's generally true on a display screen. But when capturing images, where a pixel is more properly called a sensor, the count is just one of many factors that control quality.
Seeking perfection
The technology used to make Spirit's Panoramic Camera, or Pancam, is essentially the same as what goes into a Casio or Pentax digital camera.
But the Pancam's lenses — there are two, which provides stereo imaging capability — are crafted more finely than anything you'd probably want to plunk down a Visa for. And the light-capturing chunk of silicon, called a charged coupled device, or CCD, was manufactured with no tolerance for the minor flaws that are inherent in mass-produced consumer cameras.
Perhaps most important, the sensors on Spirit's CCDs are bigger, explained Patrick Myles, director of corporate communication at the Dalsa Corporation, which built the CCDs for all of the rover's cameras (Spirit has nine altogether, including hazard avoidance cameras and a microscopic imager).
A Sony DSC-F717, with a street price of around $600, has 5.2 million sensors (or 5 megapixels) on a chip that is 8.8 by 6.6 millimeters (or .35 by .26 inches). The Pancam has just a million sensors spread across a chip that's 12 by 12 millimeters — nearly a half-inch square.
Each tiny Pancam sensor, measured in microns, is nearly four times as big as those on the Sony.
In the consumer market, which Dalsa does not target, 5-megapixel cameras often use the same size CCD as a 3-megapixel camera. More pixels are simply crammed onto the same-size chip.
“The pixels themselves get smaller,” Myles said. “This has an impact on image quality.”
Why? For one thing, smaller pixels are less light-sensitive.
Also, the lens quality might not support the additional pixels. As the receptors get smaller, a higher quality lens is needed to properly focus light onto each pixel. So where each pixel ought to capture different light information — say perhaps a subtle shading change on the subject's cheek — the same information can get spread across several pixels after passing through a lower quality lens.
20-20 vision
The Pancam was conceived at Cornell University and built at NASA's Jet Propulsion Laboratory. Dalsa, based in Waterloo, Ontario, Canada, makes cinema-quality video components and other high-end imaging devices and was called on to make the CCDs for the Pancam and the other cameras on Spirit and its twin rover, Opportunity.
“They are the world's highest performing chips in terms of light sensitivity and chip quality,” Myles said in a telephone interview earlier this week.
Overall, how does a Pancam stack up to the typical 5-megapixel camera you might purchase at Best Buy?
“There really isn't any comparison,” Myles said.
NASA officials say the camera shows what a human with 20-20 vision would see on the surface of Mars. But anyone who has zoomed in on a distant rock in one of Spirit's color pictures would have to wonder if perhaps Superman's vision might be a better comparison.
Experts argue endlessly about what the human eye can actually see, however. Comparing human vision to what a camera captures “is really up to great speculation,” Myles said.
NASA's analogy, Myles explained, is “probably a bit of marketing spin. It helps people visualize the quality.” The height and breadth of a Pancam image is roughly equal to what a person would see, taking into account peripheral vision. And the Pancam has a human perspective. It sits atop a mast on the rover, 5 feet (1.4 meters) above the surface.
Myles said the actual image quality probably exceeds human capabilities, especially after the image is processed and a computer is used to provide a zoom function.
Use your Mouse to Explore the Mars Rover Spirit 360-Degree Panorama
Image: NASA/JPL/Cornell University
Created by David Palermo of WorldVR.com
Presented by collectSPACE.com
Tricks with light
The Pancam does not make a color picture directly. Instead, it records light versus dark in shades of gray. As with other CCD cameras used in high-end astrophotography, such as on the Hubble Space Telescope, a series of filters are applied to gather multiple images that are then blended together.
In the most basic application of this process, three images are gathered of a scene, one each recording red, green and blue light. Those are then put together with special software to create a color picture.
A consumer digital camera uses a single coated filter to make the transition from photon reality to electrons and then digital information.
Additionally, the Pancam swivels 360 degrees around and 90 degrees up or down, so that individual scenes can be stitched together to create a view of the rover's entire surroundings. The pictures are expected to reveal important geologic details about rocks, and they're also used for navigation and to pick distant science targets.
Modern Ansel Adams
Spirit's pictures are said to be three times sharper than those of the 1997 Mars Pathfinder mission or the 1970s Viking landers.
The Viking missions, as well as the Voyager missions to the outer planets, used technology similar to antiquated television vacuum tubes. CCD technology was first developed in 1969, but it took decades before arrays were big enough to be useful.
Much of the research that ultimately led to today's commercial digital cameras was funded by NASA. A first major step was in developing an 800 by 800 pixel array — less than a megapixel — which is what's in the Hubble telescope.
The Pancam results so far have mission managers ecstatic. Cornell astronomer James Bell, who led the development of the camera, called the first Spirit pictures “absolutely spectacular.”
Nobody has argued with him.
In fact, Steven Squyres, a Cornell professor who directs the rover science team, called Bell “the Ansel Adams of the Space Age.”