Water-ice clouds, polar ice and other geographic features can be seen in this full-disk image of Mars from 2011. NASA's Mars Curiosity Rover touched down on the planet on August 6. Take a look at stunning photographs of Mars over the years. Check out images from the Mars rover Curiosity.
This image was captured in 1976 by Viking 2, one of two probes sent to investigate the surface of Mars for the first time. NASA's Viking landers blazed the trail for future missions to Mars.
The Valles Marineris rift system on Mars is 10 times longer, five times deeper and 20 times wider than the Grand Canyon. This composite image was made from NASA's Mars Odyssey spacecraft, which launched in 2001.
The Nili Fossae region of Mars is one of the largest exposures of clay minerals discovered by the OMEGA spectrometer on Mars Express Orbiter. This image was taken in 2007 as part of a campaign to examine more than two dozen potential landing sites for NASA's new Mars rover, Curiosity, also known as the NASA Mars Science Laboratory.
NASA's Mars Phoenix Lander descends to the surface of Mars in May 2008. Fewer than half of the Mars missions have made successful landings.
Phoenix's robotic arm scoops up a sample on June 10, 2008, the 16th Martian day after landing. The lander's solar panel is seen in the lower left.
In 2006, NASA's Mars Exploration Rover Spirit captured a 360-degree view known as the McMurdo panorama. The images were taken at the time of year when Mars is farthest from the sun and dust storms are less frequent.
The European Space Agency's Mars Express captured this view of Valles Marineris in 2004. The area shows mesas and cliffs as well as features that indicate erosion from flowing water.
This view is a vertical projection that combines more than 500 exposures taken by Phoenix in 2008. The black circle on the spacecraft is where the camera itself is mounted.
A portion of the west rim of the Endeavour Crater sweeps southward in this view from NASA's Mars Exploration Rover Opportunity in 2011. The crater is 22 kilometers (13.7 miles) across.
A photo captured by NASA's Mars Global Surveyor in 2000 offers evidence that the planet may have been a land of lakes in its earliest period, with layers of Earth-like sedimentary rock that could harbor the fossils of any ancient Martian life.
A U.S. flag and a DVD containing a message for future explorers of Mars, science fiction stories and art about the planet, and the names of 250,000 people sit on the deck of Phoenix in 2008.
A rock outcrop dubbed Longhorn and the sweeping plains of the Gusev Crater are seen in a 2004 image taken by the Mars Exploration Rover Spirit.
Although it is 45 kilometers (28 miles) wide, countless layers of ice and dust have all but buried the Udzha Crater on Mars. The crater lies near the edge of the northern polar cap. This image was taken by NASA's Mars Odyssey Orbiter in 2010.
NASA's Opportunity examines rocks inside an alcove called Duck Bay in the western portion of the Victoria Crater in 2007.
Pictured is a series of troughs and layered mesas in the Gorgonum Chaos region of Mars in 2008. This photo was taken by Mars Orbiter Camera on the Mars Global Surveyor.
An image captured in 2008 by NASA's Mars Reconnaissance Orbiter shows at least four Martian avalanches, or debris falls, taking place. Material, likely including fine-grained ice and dust and possibly large blocks, detached from a towering cliff and cascaded to the gentler slopes below.
This 2008 image spans the floor of Ius Chasma's southern trench in the western region of Valles Marineris, the solar system's largest canyon. Ius Chasma is believed to have been shaped by a process called sapping, in which water seeped from the layers of the cliffs and evaporated before it reached the canyon floor.
Pictured is the Martian landscape at Meridiani Planum, where the Mars Exploration Rover Opportunity successfully landed in 2004. This is one of the first images beamed back to Earth from the rover shortly after it touched down.
An image from the Mars Global Surveyor in 2000 shows potential evidence of massive sedimentary deposits in the western Arabia Terra impact crater on the surface of Mars.
The Mars Reconnaissance Orbiter captures a dust devil blowing across the Martian surface east of the Hellas impact basin in 2007. Dust devils form when the temperature of the atmosphere near the ground is much warmer than that above. The diameter of this dust devil is about 200 meters (650 feet).
Soft soil is exposed when the wheels of NASA's Mars Exploration Rover Spirit dig into a patch of ground dubbed Troy in 2009.
An image from NASA's Mars Reconnaissance Orbiter shows the floor of the Antoniadi Crater in 2009.
The larger of Mars' two moons, Phobos, is seen in 2008 from the Mars Reconnaissance Orbiter.
Earth and the moon are seen in 2007 from the Mars Reconnaissance Orbiter. At the time the image was taken, Earth was 142 million kilometers (88 million miles) from Mars.
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STORY HIGHLIGHTS
- NASA has sent four rovers to Mars so far, dating back to 1997
- NASA's budget for Mars exploration is slated to decrease, however
- The near future could be akin to the '80s when there were no Mars missions, official says
- Europe is planning to launch the first part of the ExoMars mission for 2016
(CNN) -- The much-celebrated Mars rover Curiosity is headed for Mount Sharp, where it will help scientists explore the question of life on Mars as it climbs up and up.
Meanwhile, however, NASA's budget for planetary exploration is slated to go down, down, down.
Scientists are basking in the success of Curiosity's stunning landing on August 6, proving that a complicated system involving a parachute and a sky crane can safely deliver a 2,000-pound vehicle to Mars. The $2.6 billion Curiosity will spend years roaming the planet, snapping photos and gathering scientific data.
NASA: 'Impressive' Curiosity landing only 1.5 miles off

The 2,000-pound Mars rover Curiosity made its landing on Mars on August 6, 2012, and has been sending back fascinating images and data ever since. Mars once had conditions favorable for microbial life, NASA scientists announced Tuesday, March 12, 2013. One piece of evidence for that conclusion comes from this area of the Martian surface, nicknamed "Sheepbed." It shows veins of sediments that scientist believe were deposited under water and was an environment once hospitable to life. See more images sent by the rover:
The rock on the left, called Wopmay, was discovered by the rover Opportunity, which arrived in 2004 on a different part of Mars. Iron-bearing sulfates indicate that this rock was once in acidic waters. On the right are rocks from Yellowknife Bay, where rover Curiosity is situated. These newly discovered rocks are suggestive of water with a neutral pH, which is hospitable to life formation.
NASA's Curiosity rover shows the first sample of powdered rock extracted by the rover's drill. In subsequent steps, the sample will be sieved to be analyzed. The image was taken by Curiosity's mast camera on Wednesday, February 20.
The rover drilled this hole, in a rock that's part of a flat outcrop researchers named "John Klein," during its first sample drilling on Mars on February 8.
The latest self-portrait of the rover combines dozens of images taken by the rover's Mars Hand Lens Imager (MAHLI) on February 3.
NASA's Mars rover Curiosity has taken its first set of nighttime photos, including this image of Martian rock illuminated by ultraviolet lights. Curiosity used the camera on its robotic arm, the Mars Hand Lens Imager, to capture the images on January 22. The rover began beaming back pictures of Mars' surface after arriving on the planet in August 2012.
Another nighttime image includes this rock called Sayunei in the Yellowknife Bay area of Mars' Gale Crater. Curiosity's front-left wheel had scraped the rock to inspect for fresh, dust-free materials in an area where drilling for rock soon will begin.
Other night photos includes this image of the calibration target for the Mars Hand Lens Imager camera at the end of the rover's robotic arm. For scale, a penny on the calibration target is three-fourths of an inch in diameter.
A view of what NASA describes as "veined, flat-lying rock" selected as the first drilling site for the Mars rover taken on January 10.
Curiosity used a dust-removal tool for the first time to clean this patch of rock on the Martian surface on January 6.
The rover captured this mosaic of images of winding rocks known as the Snake River on December 20.
A view of the shallow depression known as "Yellowknife Bay," taken by the rover on December 12.
The Mars rover Curiosity recorded this view from its left navigation camera after an 83-foot eastward drive on Sunday, November 18. The view is toward "Yellowknife Bay" in the "Glenelg" area of Gale Crater.
Three "bite marks" made by the rover's scoop can be seen in the soil on Mars surface on October 15.
The robotic arm on NASA's Mars rover Curiosity delivered a sample of Martian soil to the rover's observation tray for the first time on October 16.
This image shows part of the small pit or bite created when NASA's Mars rover Curiosity collected its second scoop of Martian soil on October 15. The rover team determined that the bright particle near the center of the image was native to Mars, and not debris from the rover's landing.
This image shows what the rover team has determined to be a piece of debris from the spacecraft, possibly shed during the landing. The image was taken on October 11.
The rover's scoop contains larger soil particles that were too big to filter through a sample-processing sieve. After a full-scoop sample had been vibrated over the sieve, this portion was returned to the scoop for inspection by the rover's mast camera on October 10.
This 360-degree panorama shows the area where the rover will spend about three weeks collecting scoopfuls of soil for examination. The photo comprises images taken from the rover's navigation camera on October 5.
An area of windblown sand and dust downhill from a cluster of dark rocks has been selected as the likely location for the first use of the scoop on the arm of NASA's Mars rover Curiosity.
Curiosity cut a wheel scuff mark into a wind-formed ripple at the "Rocknest" site on October 3, to give researchers a better opportunity to examine the particle-size distribution of the material forming the ripple.
NASA's Curiosity rover found evidence for what scientists believe was an ancient, flowing stream on Mars at a few sites, including the rock outcrop pictured here. The key evidence for the ancient stream comes from the size and rounded shape of the gravel in and around the bedrock, according to the Jet Propulsion Laboratory/Caltech science team. The rounded shape leads the science team to conclude they were transported by a vigorous flow of water. The grains are too large to have been moved by wind.
This photos shows an up-close look at an outcrop that also shows evidence of flowing water, according to the JPL/Caltech science team. The outcrop's characteristics are consistent with rock that was formed by the deposition of water and is composed of many smaller rounded rocks cemented together. Water transport is the only process capable of producing the rounded shape of conglomerate rock of this size.
Curiosity completed its longest drive to date on September 26. The rover moved about 160 feet east toward the area known as "Glenelg." The rover has now moved about a quarter-mile from its landing site.
This image shows the robotic arm of NASA's Mars rover Curiosity with the first rock touched by an instrument on the arm. The photo was taken by the rover's right navigation camera.
This image combines photographs taken by the rover's Mars Hand Lens Imager at three distances from the first Martian rock that NASA's Curiosity rover touched with its arm. The images reveal that the target rock has a relatively smooth, gray surface with some glinty facets reflecting sunlight and reddish dust collecting in recesses in the rock.
This rock will be the first target for Curiosity's contact instruments. Located on a turret at the end of the rover's arm, the contact instruments include the Alpha Particle X-Ray Spectrometer for reading a target's elemental composition and the Mars Hand Lens Imager for close-up imaging.
Researchers used the Curiosity rover's mast camera, on September 7, to take a photo of the Alpha Particle X-Ray Spectrometer. The image was used to see if it had been caked in dust during the landing.
Researchers also used the mast camera to examine the Mars Hand Lens Imager (MAHLI) on the rover to inspect its dust cover and check that its LED lights were fucntional. In this image, taken on September 7, the MAHLI is in the center of the screen with its LED on. The main purpose of Curiosity's MAHLI camera is to acquire close-up, high-resolution views of rocks and soil from the Martian surface.
This is the open inlet where powdered rock and soil samples will be funneled down for analysis. The image is made up of eight photos taken on September 11, by MAHLI and is used to check that the instrument is operating correctly.
This is the calibration target for the MAHLI. This image, taken on September 9, shows that the surface of the calibration target is covered with a layor of dust as a result of the landing. The calibration target includes color references, a metric bar graphic, a penny for scale comparison, and a stair-step pattern for depth calibration.
This view of the three left wheels of NASA's Mars rover Curiosity combines two images that were taken by the rover's Mars Hand Lens Imager on Sunday, September 9, the 34th Martian day of Curiosity's work on Mars. In the distance is the lower slope of Mount Sharp.
This view of the lower front and underbelly areas of NASA's Mars rover Curiosity was taken by the rover's Mars Hand Lens Imager on Sunday. Also visible are the hazard avoidance cameras on the front of the rover.
The penny in this image is part of a camera calibration target on NASA's Mars rover Curiosity. The image was taken by the Mars Hand Lens Imager camera on Sunday.
The rover captured this mosiac of a rock feature called 'Snake River" on December 20.
The reclosable dust cover on Curiosity's Mars Hand Lens Imager was opened for the first time on Saturday, September 8, enabling MAHLI to take this image.
Curiosity rover used a camera located on its arm to obtain this self-portrait on Friday, September 7. The image of the top of Curiosity's Remote Sensing Mast, showing the Mastcam and Chemcam cameras, was taken by the Mars Hand Lens Imager. The angle of the frame reflects the position of the MAHLI camera on the arm when the image was taken. The image was acquired while MAHLI's clear dust cover was closed.
The left eye of the Mast Camera on NASA's Mars rover Curiosity took this image of the rover's arm on Wednesday, September 5.
Sub-image one of three shows the rover and its tracks after a few short drives. Tracking the tracks will provide information on how the surface changes as dust is deposited and eroded.
Sub-image two shows the parachute and backshell, now in color. The outer band of the parachute has a reddish color.
Sub-image three shows the descent stage crash site, now in color, and several distant spots (blue in enhanced color) downrange that are probably the result of distant secondary impacts that disturbed the surface dust.
An image released Monday, August 27, was taken with Curiosity rover's 100-millimeter mast camera, NASA says. The image shows Mount Sharp on the Martian surface. NASA says the rover will go to this area.
The Mars rover Curiosity moved about 15 feet forward and then reversed about 8 feet during its first test drive on Wednesday, August 22. The rover's tracks can be seen in the right portion of this panorama taken by the rover's navigation camera.
NASA tested the steering on its Mars rover Curiosity on Tuesday, August 21. Drivers wiggled the wheels in place at the landing site on Mars.
Curiosity moved its robot arm on Monday, August 20, for the first time since it landed on Mars. "It worked just as we planned," said JPL engineer Louise Jandura in a NASA press release. This picture shows the 7-foot-long (2.1-meter-long) arm holding a camera, a drill, a spectrometer, a scoop and other tools. The arm will undergo weeks of tests before it starts digging.
With the addition of four high-resolution Navigation Camera, or Navcam, images, taken on August 18, Curiosity's 360-degree landing-site panorama now includes the highest point on Mount Sharp visible from the rover. Mount Sharp's peak is obscured from the rover's landing site by this highest visible point.
This composite image, with magnified insets, depicts the first laser test by the Chemistry and Camera, or ChemCam, instrument aboard NASA's Curiosity Mars rover. The composite incorporates a Navigation Camera image taken prior to the test, with insets taken by the camera in ChemCam. The circular insert highlights the rock before the laser test. The square inset is further magnified and processed to show the difference between images taken before and after the laser interrogation of the rock.
An updated self-portrait of the Mars rover Curiosity, showing more of the rover's deck. This image is a mosiac compiled from images taken from the navigation camera. The wall of Gale Crater, the rover's landing site, can be seen at the top of the image.
This image shows what will be the rover's first target with it's chemistry and camera (ChemCam) instrument. The ChemCam will fire a laser at the rock, indicated by the black circle. The laser will cause the rock to emit plasma, a glowing, ionized gas. The rover will then analyze the plasma to determine the chemical composition of the rock.
This is a close-up of the rock that will be the ChemCam's first target.
This image, cropped from a larger panorama, shows an area, near the rover's rear left wheel, where the surface material was blown away by the descent-stage rockets.
This image, with a portion of the rover in the corner, shows the wall of Gale Crater running across the horizon at the top of the image.
This image, taken from the rover's mast camera, looks south of the landing site toward Mount Sharp.
This partial mosaic from the Curiosity rover shows Mars' environment around the rover's landing site on Gale Crater. NASA says the pictured landscape resembles portions of the U.S. Southwest. The high-resolution mosaic includes 130 images, but not all the images have been returned by the rover to Earth. The blackened areas of the mosaic are the parts that haven't been transmitted yet. See more on this panaroma on NASA's site.
In this portion of the larger mosaic from the previous frame, the crater wall can be seen north of the landing site, or behind the rover. NASA says water erosion is believed to have created a network of valleys, which enter Gale Crater from the outside here.
In this portion of the larger mosaic from the previous frame, the crater wall can be seen north of the landing site, or behind the rover. NASA says water erosion is believed to have created a network of valleys, which enter Gale Crater from the outside here.
Two blast marks from the descent stage's rockets can be seen in the center of this image. Also seen is Curiosity's left side. This picture is a mosaic of images taken by the rover's navigation cameras.
A color image from NASA's Curiosity rover shows the pebble-covered surface of Mars. This panorama mosaic was made of 130 images of 144 by 144 pixels each. Selected full frames from this panorama, which are 1,200 by 1,200 pixels each, are expected to be transmitted to Earth later.
A panoramic photograph shows the Curiosity rover's surroundings at its landing site inside Gale Crater. The rim of Gale Crater can be seen to the left, and the base of Mount Sharp is to the center-right.
A partial view of a 360-degree color panorama of the Curiosity rover's landing site on Gale Crater. The panorama comes from low-resolution versions of images taken Thursday, August 9, with a 34-millimeter mast camera. Cameras mounted on Curiosity's remote sensing mast have beamed back fresh images of the site.
NASA's Curiosity rover took this self-portrait using a camera on its newly deployed mast.
A close-up view of an area at the NASA Curiosity landing site where the soil was blown away by the thrusters during the rover's descent on August 6. The excavation of the soil reveals probable bedrock outcrop, which shows the shallow depth of the soil in this area.
This color full-resolution image showing the heat shield of NASA's Curiosity rover was obtained during descent to the surface of Mars on Monday, August 13. The image was obtained by the Mars Descent Imager instrument known as MARDI and shows the 15-foot diameter heat shield when it was about 50 feet from the spacecraft.
This first image taken by the Navigation cameras on Curiosity shows the rover's shadow on the surface of Mars.
The color image captured by NASA's Mars rover Curiosity on Tuesday, August 7, has been rendered about 10% transparent so that scientists can see how it matches the simulated terrain in the background.
This image comparison shows a view through a Hazard-Avoidance camera on NASA's Curiosity rover before and after the clear dust cover was removed. Both images were taken by a camera at the front of the rover. Mount Sharp, the mission's ultimate destination, looms ahead.
The four main pieces of hardware that arrived on Mars with NASA's Curiosity rover were spotted by NASA's Mars Reconnaissance Orbiter. The High-Resolution Imaging Science Experiment camera captured this image about 24 hours after landing.
This image is a 3-D view in front of NASA's Curiosity rover. The anaglyph was made from a stereo pair of Hazard-Avoidance Cameras on the front of the rover. Mount Sharp, a peak that is about 3.4 miles high, is visible rising above the terrain, though in one "eye" a box on the rover holding the drill bits obscures the view.
This view of the landscape to the north of NASA's Mars rover Curiosity was acquired by the Mars Hand Lens Imager on Monday afternoon on the first day after landing.
This view of the landscape to the north of NASA's Mars rover Curiosity was acquired by the Mars Hand Lens Imager on Monday afternoon, the first day after landing.
This is one of the first pictures taken by Curiosity after it landed. It shows the rover's shadow on the Martian soil.
Another of the first images taken by the rover. The clear dust cover that protected the camera during landing has popped open. Part of the spring that released the dust cover can be seen at the bottom right, near the rover's wheel.
This image shows Curiosity's main science target, Mount Sharp. The rover's shadow can be seen in the foreground. The dark bands in the distances are dunes.
Another of the first images beamed back from NASA's Curiosity rover on August 6 is the shadow cast by the rover on the surface of Mars.
NASA's Mars Curiosity Rover, shown in this artist's rendering, touched down on the planet on August 6.
Photos: Mars rover Curiosity
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Photos: Mars rover Curiosity
Compare the Mars rovers
Given the budget constraints facing the space agency, however, there are limits on what the rover, and NASA, will be able to do on the surface of the Red Planet. Although astronauts brought back thousands of moon rocks during the Apollo Mission, there's never been a sample of Martian material returned to Earth. Such a mission is considered a priority, so scientists can do more detailed chemical analyses.
But it may not happen anytime soon.
"We're optimistic, given the success of our program, but we're anxious, too," said Richard Zurek, chief scientist for the Mars Program Office at NASA's Jet Propulsion Laboratory in Pasadena, California. "Like all of us, we're anxious about our country's ability to be able to support and do these kinds of things."
NASA's budget for Mars exploration is slated to take a huge hit in 2013, dropping from $587 million to $361 million. It will then further decline to $228 million in 2014 and $189 million in 2015, rising slightly in 2016 before sloping upward to $503 million in 2017.
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Researchers landed Curiosity on Gale Crater, which is 96 miles across and may have once hosted a lake. Mount Sharp, in the middle of the crater, is composed of hundreds of rock layers accumulated over time. The rover will climb a small portion of the 3-mile-high mountain, testing different layers to search for organic molecules that could indicate the presence of life on the barren planet.
"We've demonstrated that we've got a landing system that worked. And it worked well," Zurek said, referring to Curiosity's dramatic touchdown. "The question now is: What will we use this for? Or will we have to step back from that because those kinds of missions -- of putting something that takes a metric ton down to the surface -- they're just too expensive for our future?"
The early days
Curiosity is the latest in a long trajectory of missions that have allowed humanity to study Mars more extensively than any other planet apart from our own. A series of visits from Earth-made spacecrafts has taught us that Mars used to be a warmer, wetter place, perhaps with liquid water and even life.
Decades of research have led scientists to understand Martian history and pinpoint places on the planet where liquid water may have once flowed -- targets for future investigation, if money allows.
But getting to Mars didn't happen on the first try.
The USSR initiated a number of failed attempts to go to Mars in the early 1960s, including Sputnik 24, a lander that never left Earth's orbit. The United States also started out with bad luck: a failed flyby attempt by Mariner 3 in 1964.
Two blast marks from the descent stage's rockets can be seen in the center of this images. Also seen is Curiosity's left side. This image is a mosiac of images taken by the rover's naviagtion camers.
Earthlings' first successful landing on Mars happened in May 1971 with the Soviet Union's Mars 3 lander. It failed after sending 20 seconds of video data to the orbiter, however.
NASA claimed a big success the same year: Mariner 9 marked the first time a U.S. spacecraft had orbited a planet other than our own. This orbiter discovered river and channel-like features on Mars, and took the first high-resolution images of Mars' moons Phobos and Deimos.
Hopes were high for NASA's Viking mission, launched in 1975, which included two landers equipped to search for tiny organisms. The orbiters mapped the surface of the planet, while the landers monitored the weather and sent back color panoramic views of Mars.
"It was extraordinary engineering achievement. A huge amount of science came out," said Scott Hubbard, former head of NASA's Mars program and author of the book "Exploring Mars: Chronicles from a Decade of Discovery."
Mars rover indulges readers' Curiosity
But the expectation of evidence of life -- namely, that the lander's arm would be able to put material into its chemistry kit and see organic molecules -- fell through. These molecules wouldn't prove that life existed, but they would be a signal that life was once possible there. There just wasn't conclusive evidence that Mars had these molecules.
Then came almost two decades of inactivity. NASA didn't send any other spacecraft to Mars until 1993 with the failed Mars Observer, followed by the successful orbiter Mars Global Surveyor in 1996.
Why the break? The space-shuttle program was one reason, said Zurek. NASA's priority at the time was manned vehicles, and budgets were tight. The Challenger disaster of 1986 also set NASA back.
Without the Mars rock, we might not have Curiosity today.
Scott Hubbard
Zurek believes we may be in a phase like that today, with not enough money to go around for all the programs NASA wants to develop. Also, the priority is once again in a new project: The Space Launch System, which will be the largest and most powerful rocket ever built.
Exploring Mars' surface
A game-changer was the discovery of a meteorite that appears to have come from Mars, found in Allan Hills, Antarctica, in 1984. Some scientists said they thought they saw evidence of tiny fossils that looked like dried-out life from Mars embedded in the meteorite.
No one knew where the meteorite came from, but it ignited a huge amount of interest in returning to Mars, Hubbard said. It also stressed the need to return samples from Mars to Earth, so they could be fully analyzed by multiple scientists.
"Without the Mars rock, we might not have Curiosity today," said Hubbard, whose job as the "first Mars czar" was to plan the next decade of exploration.
The first wheeled vehicle to land on Mars was during NASA's 1997 Mars Pathfinder mission, which consisted of a stationary lander and a surface rover called Sojourner. The rover was a mere 23 pounds and about the size of a large microwave oven. The mission demonstrated airbag technology in landing the rover without damage.
Mars panorama shows Curiosity's prime target
Japan got in on the Mars action in 1998 with the Nozomi orbiter, but the spacecraft failed to communicate.
At NASA, Hubbard's goals included understanding Mars as a system, looking for potentially habitable environments, examining whether the planet was habitable in the past and preparing to return samples from Mars to Earth.
"The sequence of landers was strategically planned to have ever-increasing capability to go forward and do more," he said.
NASA's 2001 Mars Odyssey mission helped relay communications to Earth from the missions that would follow.
Europe entered the Mars race in 2003 with the Mars Express Orbiter and the Beagle 2 lander. Mars Express is still operating, but the lander never communicated from the surface.
Before Curiosity, NASA's big rover project was the Mars Exploration Rover Mission: two golf-cart-sized rovers called Spirit and Opportunity. They arrived on Mars in January 2004. Each rover was only supposed to operate for 90 days, but both far exceeded that time frame.
Spirit lasted for several years before it lost one of its wheels. As it hobbled along, it broke through the crust of the surface and got stuck in an area called Troy.
During its escape, another of Spirit's wheels failed. But while stuck, Spirit had a breakthrough: it found soil rich in sulfates, which are a component of steam. This suggests there may have once been conditions on Mars able to support life.
Signs of water?
As long as Curiosity has healthy mobility, it shouldn't find itself stuck in this kind of situation, said John Callas, project manager of the mission. The newest rover also uses brushless motors, which don't have the wear mechanism of the motors in Spirit and Opportunity.
Spirit stopped communicating in 2010. Opportunity, meanwhile, still chugs along and sent back its 100,000th picture in July. The rover also found a vein of the mineral gypsum, which indicates water may have once flowed on Mars.
His other car is on Mars
The Mars Reconnaissance Orbiter, launched in 2005, conducted analyses that helped scientists determine where Curiosity should land. The orbiter also found in 2008 a new kind of mineral, called opaline silica, which suggested that liquid water had been on Mars as recently as 2 billion years ago. The theory is that volcanic activity or meteorite impact on Mars created materials that liquid water would have altered, and the result is opaline silica.
MRO camera imagery also suggested that briny water may flow near Mars' equator in its southern hemisphere. These surface features appear to fade in the winter and reutrn in the spring. The possible liquid flows are extremely small and narrow.
Further clues to water on Mars came from the Phoenix Mars Lander, which arrived on Mars in May 2008. The lander studied the thin Martian atmosphere, consisting of 95% carbon dioxide. Isotopes of carbon and oxygen can tell a lot about whether water existed on the planet.
Data from Phoenix suggested that the Martian surface has interacted with liquid water, and even in modern times. The data also indicated that the planet had volcanic activity as recently as several million years ago. The chemical signature seen in the carbon dioxide points toward liquid water that was primarily at temperatures near freezing.
Mars is most Earth-like planet we have found, for certain.
James Wray
Also, the lander's robotic arm camera discovered possible evidence of ice at its landing site.
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MAVEN and beyond
The next Mars mission is the Mars Atmosphere and Volatile Evolution Mission, or MAVEN, which is scheduled to launch late next year. The instruments for it are already built, Zurek said.
Evidence gained through previous missions suggests that early in its history -- perhaps as much as 4 billion years ago -- Mars was a wetter place. Where did that water go?
It could be that water is frozen into the planet's surface, as observations from Phoenix suggested. But the water could have also been lost to space. Mars doesn't have a global magnetic field like the Earth does, so charged particles emitted from the sun -- the solar wind -- affect Mars in a much bigger way. The current theory is that the solar wind "stole" most of Mars' atmosphere, so radiation from the sun quickly boils any remaining liquid water.
MAVEN will measure the interaction of solar wind with current atmosphere, Zurek said. Using models of the sun's history, scientists may be able to estimate how much water the planet could have lost.
"MAVEN is the next mission. We don't want it to be the last mission in Mars exploration for a similar period, like in the '80s where there were no Mars missions, and everything was sort of on hold," Zurek said.
Meanwhile, Europe plans to launch the ExoMars Orbiter and a ground-based probe in 2016. The probe, called the Entry, Descent and Landing Demonstrator Module, isn't expected to last long, and is meant to be a demonstration of landing technologies. It will perform limited scientific experiments during its brief life.
European space officials and NASA were going to work together on a trace gas orbiter, but NASA pulled out because of budgetary reasons. Instead, the European Space Agency is working with Russia's Roscosmos. The plan is to launch the ExoMars rover in 2018, carrying European and Russian instruments.
Future questions
Exploration of the moon has been more extensive than Mars in many respects. In addition to fly-bys and orbiters, six Apollo missions sent humans to the moon and brought them back between 1963 and 1972.
The moon is closer to Earth than Mars, but also has a less diverse terrain. The interior of the moon is still mysterious, which is why NASA sent twin lunar orbiters called Gravity Recovery And Interior Laboratory, or GRAIL, to investigate. But consider that while sending an SUV-sized rover to Mars is a celebrated achievement, astronauts drove a "moon buggy" 40 years ago.
We haven't figured out how to send people to Mars yet. The planet is cold and dry, with dust storms that can circle the entire planet and an atmosphere that would be toxic to humans.
"Mars is the most Earth-like planet we have found, for certain," says James Wray, assistant professor at Georgia Institute of Technology and member of the Curiosity science team. "That's not saying as much as you would like it to be saying."
NASA still has a lot of questions about Mars. Has the planet hosted life? Does it host life now, in a place we have yet to discover? Has the climate on Mars changed in a way we can understand? And could Mars be a near-term destination for astronauts?
To send a mission to Mars that would bring back samples, there are a few fundamental engineering questions that need to be answered, says Hubbard: Can scientists pinpoint where they want to go and have a spacecraft land there? Can we collect a soil sample? Can we re-enter the Earth's atmosphere?
Science has proven the answers to the first three questions are "yes." But a fourth question -- can we safely load the Mars sample onto a spacecraft that's returning to Earth? -- is trickier. A rocket would have to be carried to Mars, sit there for a year, blast off and then rendezvous with another spacecraft.
Such an endeavor would require millions of dollars of research and development work. But NASA's projected budget for Mars exploration has "now gotten to the point that it's so low that you could not contemplate doing what I just described," Hubbard said.
The next step after MAVEN remains unclear. There are three other sites that had been considered for Curiosity that could be worth exploring with a surface-based spacecraft. Budget permitting, Hubbard also would eventually like to land another rover on Mars, because a stationary device with a six-foot arm isn't nearly as good as exploring the varied terrains of the planet.
"Mars is far more diverse than anybody thought," he said. "Roving is the way to go. Even though it's more difficult, that's what you need."
NASA officials are hopeful the current dire budgetary situation won't last too long.
Congress has signaled that money will be put back into planetary sciences, said Hubbard, who believes scientists will continue to be vocal about the value of Mars exploration.
"I think this [space science] community is going to make some noise," he said.
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