As rovers get bigger and bigger, landing them gets tougher. That becomes the challenge for the engineers designing the Curiosity rover, the largest and most technologically advanced Mars rover ever built. Too big to land using the airbags that were successful on previous Mars rover missions, Curiosity calls for something new. So engineers come up with a novel system called the Sky Crane to lower the car-size rover to the Martian surface. While gasp-inducing with its system of rockets and a bungee-like umbilical cord, the Sky Crane precisely delivers Curiosity to the surface of the Red Planet before flying off to crash safely in the distance. The elegant touchdown, which allows for zero margin of error, is a seminal moment for the rover mission designed to investigate whether Mars has ever offered an environment favorable for microbial life.

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The first Earth-size planets orbiting a sun-like star outside our solar system are discovered by Kepler, a spacecraft developed by JPL and later managed by NASA's Ames Research Center designed to study exoplanets. The planets announced in this discovery, called Kepler-20e and Kepler-20f, are too close to their star to be in the so-called habitable zone where liquid water could exist on a planet's surface, but they are the smallest exoplanets ever confirmed around a star like our sun. Scientists view the discovery as an important milestone in the ultimate search for planets like Earth.

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Astronomers studying observations taken by the Wide-field Infrared Survey Explorer, or WISE, mission discover the first known "Trojan" asteroid orbiting the sun along with Earth. Trojans are asteroids that share an orbit with a planet near stable points in front of or behind the planet. Because they constantly lead or follow in the same orbit as the planet, they never can collide with it. In our solar system, Trojans also share orbits with Neptune, Mars and Jupiter. Two of Saturn's moons share orbits with Trojans. Scientists had predicted Earth should have Trojans, but they have been difficult to find because they are relatively small and appear near the sun from Earth's point of view.

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Getting in and out of orbit around a planet takes a lot of energy. But the job is easier when you're talking about a smaller object, such as a protoplanet. Taking advantage of ion propulsion, the Dawn spacecraft slips into orbit around the giant asteroid Vesta as the first of two objects it will orbit in the main asteroid belt. After spending more than a year at Vesta, Dawn will continue on to orbit the dwarf planet Ceres. If successful, it will be the first time that any spacecraft has orbited one target body after another in a single mission.

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A landing on a Utah desert marks the delivery to Earth of a sample of cometary material, the prime objective of the Stardust mission. The material is collected in 2004, when the spacecraft makes a close flyby of comet Wild-2 gathering cometary and interstellar dust in a substance called aerogel. Scientists say the samples indicate that some comets may contain materials ejected from the early sun and may have formed very differently than previously thought.

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If you want to see inside a comet, what about blasting a hole in it? That is the premise of Deep Impact, which spends 172 days and 268 million miles traveling to comet Tempel 1, where it releases an impactor that hits the comet nucleus. The collision generates a brilliant flash of light even brighter than expected, as ice and dust debris is ejected from the fresh impact crater. After flying by Earth in 2006, Deep Impact goes on to study extrasolar planets and, in 2010, conducts a flyby of comet Hartley.

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Finding and studying planets orbiting other stars has been a gradual process, advancing incrementally as observational tools have progressed. In a major advance, the infrared Spitzer Space Telescope for the first time captures the light from two known planets orbiting stars other than our sun. Previously, all confirmed extrasolar planets had been discovered indirectly, by detecting how exoplanets cause their parent stars to wobble slightly or by measuring changes in the star's brightness as planets move in front of it. In new studies, Spitzer directly observes the warm infrared glows of two previously detected "hot Jupiter" planets, designated HD 209458b and TrES-1. The achievement caps a nearly 10-year search by astronomers since exoplanets were first discovered.

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The interstellar stuff that provided the building blocks for the solar system billions of years ago was relatively homogenous – a simple cloud of interstellar gas, dust and ice. Yet the most striking feature of our solar system today is the great diversity among its planets, moons, asteroids and comets. How did that come to be? Answering that question becomes the goal of the Genesis mission, which sends a spacecraft to collect samples of the solar wind – particles that continuously stream outward from the sun – and bring them to Earth for analysis. The sample return capsule's landing in Utah turns out harder than planned when its parachute doesn't open, yet many sample collectors are successfully recovered. Researchers spend years analyzing the samples, and in 2011 announce that they show the sun and inner planets formed out of different basic materials.

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Spirit and Opportunity, two rovers capable of traveling farther than any before them, are launched to Mars where they arrive in January 2004, beginning a planned 90-day investigation of Mars' climate and water history. Exploring on opposite sides of the planet, the robotic geologists equipped with cameras, a microscopic imager and other tools far outlast their life expectancy, journeying more than 25 miles (43 kilometers) combined, and discover signs of a once wet Mars. Opportunity, still roving after eight years of exploration, also finds strong evidence that conditions on the Red Planet could have been suitable for sustaining microbial life.

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During an 11-day journey aboard the space shuttle Endeavour, the Shuttle Radar Topography Mission sweeps Earth's surface with a specialized radar to capture the first detailed topographic maps of the planet from space. The digital images are later refined and combined to create the first-ever digital elevation map of Earth. Diverse industries benefit from the data, which offer a much more accurate and standardized view of Earth than anything before it.

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Deep Space 1 is launched as a technology demonstration under NASA's New Millennium program designed to flight-test new technologies. It is the first interplanetary spacecraft to use ion propulsion, gently thrusting as it flies toward its flyby target, the asteroid 9969 Braille. In an extended mission, Deep Space 1 flies by the comet Borrelly.

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Cassini is launched as the first spacecraft designed to orbit the ringed planet Saturn, executing numerous flybys of Saturn's scientifically rich moons. Cassini's observations at Saturn's moon Enceladus, for example, reveal an icy plume shooting from the moon's surface and inspire further investigations into the possibility that life could exist there. In 2005, Cassini delivers its European-built Huygens probe equipped with six instruments to the surface of Titan to study the giant haze-enshrouded moon. The Huygens investigations along with Cassini's flybys uncover methane lakes, hydrocarbon sand dunes and an internal liquid water-ammonia ocean on Titan.

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Mars Pathfinder lands on the surface of Mars, carrying the first rover to another planet. The Mars Pathfinder mission, designed to demonstrate a low-cost method for delivering a set of science instruments to the Red Planet, is the first wheeled vehicle to be used on any other planet in the solar system and serves as the foundation for the Mars rovers of today. The mission is also notable as the first to use airbags to land on another planet. The lander, formally named the Carl Sagan Memorial Station, and the rover, named Sojourner, both outlive their design lives -- the lander by nearly three times and the rover by 12 times.

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After a five-year voyage, the Galileo spacecraft arrives at Jupiter.

Galileo is the first spacecraft to directly measure Jupiter's atmosphere with a descent probe, and conducts long-term observations of the Jovian system. After discoveries including evidence for the existence of a saltwater ocean beneath the Jovian moon Europa's icy surface, extensive volcanic processes on the moon Io and a magnetic field generated by the moon Ganymede, Galileo plunges into Jupiter's atmosphere on September 21, 2003 to prevent an unwanted impact with Europa.

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While its ultimate destination is Jupiter and its mysterious moons, the Galileo mission also becomes notable for discoveries during its journey to the gas giant. It is the first spacecraft to visit an asteroid, called Gaspra. Galileo also provides the only close observations of a comet colliding with a planet when it witnesses fragments of comet Shoemaker-Levy 9 impact Jupiter. And its flight past Venus in 1990 yields fascinating infrared images of the planet's clouds. Two years later, Galileo encounters a second asteroid, Ida, and discovers a miniature moonlet orbiting the rocky object, later given the name Dactyl.

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Twelve years after leaving Earth, Voyager 2 makes its closest approach to any planet since starting its journey, flying about 3,000 miles (4,950 kilometers) above the north pole of the planet Neptune. Voyager discovers that Neptune – the eighth and outermost planet from the sun – has the strongest winds of any planet encountered, even though it receives the least solar energy. The spacecraft's flyby of Neptune sets it on a course below the elliptic plane that will eventually take it beyond the edge of the solar system.

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After flying by the planets Jupiter and Saturn, Voyager 2 makes history as it executes the first-ever encounter of Uranus, the seventh planet from the sun. A gas giant, Uranus orbits the sun 20 times the distance of Earth’s orbit. Resembling a pale blue billiard ball, Uranus is distinctive because its axis of rotation is tilted sideways. Uranus has a retinue of 27 moons, including 10 discovered during the Voyager flyby, as well as a complex system of dark rings.

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A joint mission with the Netherlands and the United Kingdom, the Infrared Astronomical Satellite, or IRAS, provides the first infrared look at the sky from space following its launch from California's Vandenberg Air Force Base. Over just 10 months, the space telescope detects about 500,000 infrared sources, doubling the number of cataloged astronomical sources with discoveries of new asteroids and comets, and thousands of variable stars and starburst galaxies, as well as the core of our galaxy, which has never been detected. It also images disks of dust around stars that may be planetary systems in early stages of formation. IRAS paves the way for later space-based infrared observatories such as the Spitzer Space Telescope.

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As the Voyager 1 spacecraft flies by the giant planet Jupiter, it captures a picture of an active volcanic eruption on Jupiter’s moon Io. This discovery is the most surprising finding from Voyager's flyby; volcanism on Io had never been detected from ground-based telescopes or by NASA's Pioneer 10 and 11 spacecraft that previously flew by Jupiter. The volcano throws solid material upward at speeds of about 1,200 miles per hour, reaching an altitude of 100 miles.

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Taking a leap beyond the weather satellites of the 1960s and 1970s, Seasat becomes the first satellite carrying sophisticated instruments dedicated to studying Earth's oceans. Seasat carries a suite of sensors to make measurements of sea surface height, temperature, vector wind, atmospheric water vapor content, as well as the roughness of the sea surface. These sensors lay the foundation for more optimized measurements developed in ensuing decades. The payload includes the first civilian imaging radar, an instrument used later to study Earth and other planets with great success.

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JPL's Viking 1 Orbiter launches carrying a NASA lander that will become the first spacecraft to successfully land on Mars. Viking 1 and its sister mission, Viking 2, are designed to investigate the Red Planet and search for signs of life. The Viking 1 and 2 orbiters take comprehensive high-resolution images of the planet during their missions.

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Mariner 10 is the first spacecraft to pass close to Mercury, a feat that will not be repeated for more than 30 years. Flying just 437 miles (703 kilometers) above the planet, Mariner 10 goes on to perform two more encounters of Mercury that enable it to map half the globe. It also reveals a surprising magnetic field and a metallic core comprising about 80 percent of Mercury's mass.

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Mariner 10 becomes the first spacecraft to conduct multiple planet flybys as it executes encounters of Venus (2/5/74) and three passes by Mercury (3/29/74, 9/21/74 and 3/16/75). This is the first demonstration of the technique of “gravity assist,” using the gravitational pull of the target planet to bend and shape the spacecraft's flight path. At Venus, Mariner 10 produces the first clear pictures of the Venusian chevron clouds and performs other atmospheric studies.

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Mariner 9's arrival at Mars makes it the first spacecraft to orbit another planet, narrowly beating the Soviet Union's Mars 2, which reaches the Red Planet two weeks later. Mariner 9 is also the first spacecraft to make close observations of Mars' two moons, Phobos and Deimos. While in orbit, Mariner 9 maps 85 percent of the Martian surface, and collects valuable information about Mars' surface and atmosphere. Of the more than 7,000 images it transmits, some of the most significant are the first detailed views of the solar system's largest volcano and a canyon system that dwarfs the Grand Canyon.

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The first of the Surveyor missions designed to make soft landings on the moon, Surveyor 1 provides information about Earth's natural satellite that will be critical for the Apollo crewed missions. Surveyor 1 sends home 11,240 high-resolution television pictures, as well as data on the moon's bearing strength, temperatures and radar reflectivity. The soft landing is achieved using a retrorocket and a bank of three thrusters that slow the lander's speed from almost 6,000 miles per hour to just 3 miles per hour.

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After an eight-month voyage, Mariner 4 performs the first flyby of Mars, becoming the first spacecraft to take close-up photographs of another planet. The images show lunar-type impact craters, some of them touched with frost in the chill Martian evening – a disappointment to many who had hoped for a more habitable-looking environment on the Red Planet. A television camera takes 22 pictures, and radio science data provides key information about how to safely deliver future missions to the Martian surface. After passing Mars, Mariner 4 lasted about three years in solar orbit, continuing long-term studies of the solar wind and making coordinated measurements with the Mariner 5 spacecraft.

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The Ranger 7 lunar probe is the first unqualified success in the United States' early quest to explore the moon, and heralds a new era of exploration that sees dramatically more mission successes than failures. Designed chiefly to take high-resolution photographs of the moon as it descends to make an intentional crash-landing, Ranger 7 approaches the moon precisely on target and captures more than 4,300 images as it drops toward a region later named Mare Cognitum. Ranger 7 returns images of the moon from distances as close as 2,110 km – vastly closer than pictures from any previous Soviet or U.S. spacecraft.

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One hundred nine days after launch, Mariner 2 flies by Venus, becoming the first spacecraft to execute a successful encounter of another planet. It passes about 21,600 miles (34,773 kilometers) above the surface and scans the planet with infrared and microwave radiometers, revealing that Venus has an extremely hot surface. This supports a theory put forward by some scientists that Venus is the victim of a runaway "greenhouse effect"; carbon dioxide in its atmosphere acts like a blanket, trapping heat from the sun. The finding puts to rest longstanding speculation that the cloud-shrouded Venus might be a swampy world harboring life.

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Mariner 2 launches on a mission to Venus that will make it the first spacecraft to successfully fly by another planet. On its way to Venus, Mariner 2 for the first time carries out a midcourse correction to fine-tune its flight path – a technique that will become a staple of later space missions. It collects science data that for the first time confirms the existence of the solar wind – a constant flow of particles streaming outward from our sun. It also measures interplanetary dust, which turns out to be more scarce than predicted, and detects high-energy charged particles coming from the sun, several brief solar flares, and cosmic rays from outside the solar system.

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