Author Topic: Forbidden planet  (Read 1612 times)

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Forbidden planet
« on: May 16, 2003, 01:05:54 AM »
This summer, an alien world will approach Earth closer than it has since Neanderthals roamed our planet, over 60,000 years ago.

"Mars seems like Earth, but Mars is also different and unique," says Cornell Professor Steve Squyres, who is also principal investigator of the Mars Exploration Rovers (MER) science package named Athena. "To understand the real planet, we must accept Mars for what it is."

This celestial body, half the size of Earth, has a surface area equivalent to all the dry land of our world. It is a place where the air pressure is as thin as it is 20 miles above Earth's surface and the average ground temperature makes Antarctica seem comparatively balmy. Deadly ultraviolet radiation from the Sun constantly bathes the planet unchecked by any natural protection as on Earth.
      Despite these harsh conditions, this world possesses signs of having been much more like our blue globe in the distant past. The numerous remains of river channels and lake beds scar the landscape among meteorite craters, including a canyon system as long as the entire United States. There are massive volcanoes, the largest of which is as wide as Arizona and twice as high as Mount Everest. Recent evidence strongly suggests that beneath the now very dry soil are massive reservoirs of water ice.
      Littered across this alien realm are the inactive remains of several automated explorers from the inhabitants of the third planet from the Sun. Soon they will be joined by a small fleet of new robotic extensions of humanity. Among them are two rovers, each with an identical package of scientific instruments from Cornell University designed to find evidence of ancient water - an important stepping stone on the scientific search for life past and present on this strange and fascinating planet.

      The "real" Mars is what astronomers have been seeking since the invention of the telescope in the early 16th century. From millions of miles away, Mars appears to be much like another Earth, with white polar caps, clouds, dark and light surface features, and dust storms. Even the planet's rotation rate and axial tilt are closer to Earth's than any other planet in the Solar System.
      It was these basic similarities between the Blue and Red Planet that once made scientists speculate that Mars was like Earth to the point of having native life forms - perhaps even an entire advanced civilization. These characteristics also made Mars one of the prime celestial targets for robot probes in the early years of the Space Age.
      When the first successful Mariner vessels flew by the Red Planet in the late 1960s, scientists were shocked at what they found from the crafts' limited surveys: A Moon-like world covered with craters, along with a very thin carbon dioxide atmosphere, only a trace of water vapor, and no signs of a planetary magnetic field. They began to seriously question whether Mars ever had any life more advanced than microbes, let alone thinking technological beings.
      But when Mariner 9 became the first spacecraft to orbit the Red Planet in 1971, a more complete picture of this alien world came to light: Mars did indeed have such Earth-like features as canyons, riverbeds, and volcanoes among the craters. Scientists became hopeful again that life dwelled on Mars.
      Thus when the twin Viking landers set down on Earth's celestial neighbor in 1976, their main focus was on finding native organisms. Instead, the Viking biology experiments reported confusing results which led most scientists to conclude the landers had found some reactive chemical properties in the soil instead of Martian microbes. They also realized to a fuller extent that Mars needed to be given more in-depth scrutiny as a planet before jumping to conclusions about any living residents there.
      Twenty-one years later, NASA began a program of renewed Mars exploration with the Mars Pathfinder mission, which deposited the first successful automated rover on the planet. Primarily designed to test technologies for future robotic space missions, Mars Pathfinder (renamed the Carl Sagan Memorial Station in honor of the late famous Cornell professor and popular science figure) lived up to its name for three months in 1997 on the rock-strewn plains of Ares Vallis, as the small Sojourner rover moved among the boulders and examined specific rocks in a rudimentary fashion. Indeed, "MER could not have happened without Mars Pathfinder," said Squyers.
      Along with Mars Pathfinder, two more orbiter missions that followed in the next few years added to the bounty of evidence that Mars was once a place which seemed much friendlier to life. But circling hundreds of miles above those intriguing landforms was not enough; the human researchers on Earth had to place instruments at those sites to once again try to discover the real Mars. They wanted to know if what appeared to be dried lake beds and rivers really were once water-filled basins and how they came into existence. Thus came into being the Mars Exploration Rovers, or MER for short.
The Eyes of Athena
After the MER project began in 1987, it underwent a number of major design changes before becoming the two rovers that now await June launches from Cape Canaveral, Florida aboard Delta 2 rockets to their ultimate destinations at Gusev Crater and Meridiani Planum (the rovers will be given actual names from the winners of a nation-wide school contest in the near future).
      The results of this long effort include the Athena science package, which Squyres - who has been fascinated with Mars since seeing the Viking Orbiter images of the Red Planet as a Cornell undergraduate student in 1977 - has been in charge of designing, building, testing, and delivering all the science instruments on the rovers prior to their space flights.
      When one takes a first look at their exterior design, the rovers appear as 400-pound boxes on six wheels covered with overhanging solar panels, a tall camera mast and antenna on top, and a thin metal arm extending from the front. Though perhaps not the most streamlined vehicles ever made, the rovers are fully equipped to be the "robot field geologists" that Squyres and the rest of the MER team call them.
      When the rovers reach Mars in January of 2004 and land at their designated sites using the same technique as Mars Pathfinder did, with a series of parachutes and airbags that will cushion the multiple impacts, they will begin their explorations of the Red Planet by scanning their new home with a sophisticated color panoramic camera, or Pancam for short.
      Pancam is the most advanced pair of electronic eyes yet placed on the Martian surface. Over three times more acute than a similar camera sent with Mars Pathfinder, Pancam approaches human 20/20 vision in terms of image clarity. As Pancam is also located five feet from the ground, the view returned from the rover will approximate what astronauts may see for themselves someday on Mars.
      "High as a 10-year-old's eye," said Jim Bell, Assistant Professor of Astronomy at Cornell and lead scientist on the Pancam, which he has spent the last seven years working on.
      Scientists will use Pancam to examine the Martian sites to see if water once shaped the areas. Pancam will also be indispensable for picking out particular rocks for in-depth study with the other Athena instruments. The rovers' mechanical eyes will help the Earth-bound scientists to guide their $800 million set of geologists across the rocky desert; under ideal conditions the rovers could cover 110 yards of ground during the Martian daytime at a speed of two inches each second - the normal pace of your average tortoise.
      Bell says that Pancam will also be used for looking at interesting objects in the sky above Mars. A special filter will allow Pancam to image the Sun to see how the light from our star becomes degraded by the dust in the Red Planet's air over time. Since the rovers will be located near the Martian equator, Pancam will be able to witness solar eclipses made by the planet's two small moons, Phobos and Deimos. These satellites will not cover the solar disk completely as Earth's Moon does during such times, though, as their distant orbits and tiny sizes allow only partial eclipses.
      Scientists will also try to image Earth and the Moon with the Pancam during several Martian evenings. If they are successful, our world may appear as a bright blue "star" with the Moon as a "bump" on the side of it. Similar attempts to image Earth with Mars Pathfinder in 1997 were always clouded out. Bell says they will probably try to view Earth later in the mission, as they naturally do not want to risk the sensitive Pancam to the bitter cold of the Martian nights during the critical early days of the rovers' explorations.
      Pancam will also spend time imaging a rather unique instrument located atop the back region of the rovers' solar panels: A combination sundial and calibration target. The rover sundial will function with much the same purpose as any sundial on Earth, displaying the local hours and seasons by the casting of a shadow from a central post. The exception is that this will be the first sundial on Mars.
      The sundial, suggested by Cornell alumni Bill Nye "the Science Guy" as an added "feature" to the existing color calibration target, will allow humanity to know the time of day and season at the location of the rovers on Mars. The shadow cast by the mast will also help scientists determine how much light is coming from the Sun and Martian sky by how bright or dark the shadow is against circles of gray, black, and white surrounding the base of the gold-topped mast. Small mirrors located at the edges of the sundial let viewers know what color the sky is throughout the day. Red, yellow, green, and blue squares at each corner of the instrument will allow scientists to determine the true colors of the Martian landscape.
      The sundial also serves as a work of art and a message to future explorers who may one day come upon the rovers. The word "Mars" is engraved in 17 different languages around the sundial base, representing 80 percent of all human languages. The words "Two Worlds - One Sun" and "Mars 2004" appear along the outside of the gray calibration circle, which also serves as part of a representation of the solar orbits of Earth and Mars (the gold ball at the top of the mast serves as the Sun). Four small golden message plaques ring the sundial base with engravings of people as stick figures have been included at the request of school children.
On the Rocks
In addition to the finest eyes of Earth, the Mars rovers also carry the best geological equipment ever sent beyond our pale blue dot.
      Not only can the rovers see in the same optical wavelengths as humans do with the Pancam, they can also view their surroundings in the other regions of the electromagnetic spectrum to determine the existence and amount of many different types of minerals.
      The Miniature Thermal Emission Spectrometer, or Mini-TES, will do a general panorama of the local rocks and soil to detect the infrared radiation given off by the surroundings. This data will help guide scientists to interesting places in tandem with the Pancam to find minerals and rocks affected by any water that might have once been present.
      Another spectrometer named Mossbauer will do the fine-detail portion of the surface investigation. Located on the rover's mechanical arm, the Mossbauer Spectrometer will be placed right against rocks and soil to look for any iron-bearing minerals. This will give clues about the early environment of the region, contributing to the primary search for ancient liquid water.
      The third spectrometer of the Athena package, the Alpha Particle X-Ray Spectrometer, or APXS, will also be placed against rocks and soil to report on their major components. An improved version of the same instrument attached to the Sojourner rover in 1997, the APXS will show the effects of weathering on the land forms and how the minerals have been shifted around due to moving water.
      Assisting in this first truly detailed examination of the Martian surface are two important tools: The RAT, or Rock Abrasion Tool, will use a small wheel to grind away at dust and outer rock layers to let the other instruments explore their interiors, which may have been long untouched by external conditions. The Microscopic Imager will give scientists extreme close-ups of the details to be found in the Martian rocks and soil to complete an all-around picture of the surface environment's history and present conditions.
A Clever Improvisation
Though the rovers are not equipped with any tools specifically designed to dig deep into the Martian soil, the project engineers did come up with a way to get down beneath that rusted surface at least a bit. By locking five of the rover's six wheels and spinning a front one in place, the rover can churn out a small area of dirt in the ground. The rover would then move away from its excavation and examine the pit in detail, or "jump in the hole" as Cornell senior research associate Robert Sullivan described it.
      Here the scientists can examine any subsurface layering and see just how well the soil holds itself together along the sides of the pit. This, along with examining the tire tracks the rovers make as they move along the surface, will provide important clues about the soil composition, all of which is important in learning whether water did indeed once flow at the site.
The Life of the Rovers
The Mars Exploration Rovers are expected to survive on the Red Planet's surface for at least 90 days, though they should continue functioning much longer than that.
      Tipping over or falling into a crevasse could end their missions prematurely, but the rovers' electronic brains are smart enough to know how to avoid these traps. If they do come across a problem they cannot handle, the robot geologists will simply halt in their tracks and radio Earth for instructions.
      One event they cannot avoid is the collection of dust on their solar panels from the Martian atmosphere. Once enough of these fine particles gather on top of the rovers, they could cut off the robots from their primary energy source, the Sun. Rather than making a complicated device to sweep off the dust, the mission planners decided to make the solar panels very large to delay the inevitable coating of soil particles.
Will Humans Follow?
The MER team members interviewed all thought the idea of humans one day following in the path of their rovers to be an exciting and scientifically rewarding prospect. As was shown in the Apollo era of manned lunar exploration, astronauts can make better determinations and gather more samples than any machine currently can. But they also recognize that a manned mission to Mars will be far more expensive than any robot probe.
      It has been foreseen, though, that the exploration of Mars and beyond will be conducted by machines and humans working together to produce the highest quality science from our explorations of the Universe.
      As Steve Squyres tells it: "I think I have the coolest job in the world. If you can't have fun building rovers and sending them to Mars, give it up."
      For more information on the Mars Exploration Rovers and the Athena science package, visit