PASADENA, Calif. — Mars is set to get its latest visitor Sunday night when NASA's new robotic rover, named Curiosity, attempts to land there. Mars has been a prime target for space exploration for decades, in part because its climate 3.5 billion years ago is believed to have been warm and wet, like early Earth. Here are five other key points:

_About the color: It's called the red planet because the landscape is stained rusty-red by the iron-rich dust.

_Quick weight loss: Its gravity is only 38 percent that of Earth. So if you weigh 150 pounds on Earth, you would weigh 57 pounds on Mars.

_Hot and cold: Mars' temperatures can range from 80 degrees at its equator to -199 degrees at its poles.

_The air is different: Mars' atmosphere is mostly carbon dioxide with traces of nitrogen and argon. Earth's atmosphere is a mixture of nitrogen, oxygen and other gases.

_Longer days: They last 40 minutes longer than a day on Earth.

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  • Liftoff

    Viking 1 launches from NASA's Kennedy Space Center on August 20, 1975, bound for Mars. A twin spacecraft, Viking 2, followed about three weeks later.

  • The Mission

    Each Viking spacecraft had two parts--an orbiter (top left) and lander (bottom left). After orbiting Mars and scouting for landing sites, the orbiter and lander would separate. Then the lander, protected from intense hear by an "aeroshell," would parachute to a safe landing (right).

  • In Mars Orbit

    This image from June 29, 1976, shows a 30 mile wide swath of Chryse Planitia dominated by Belz Crater. It's known as a "rampant crater" because of the raised ridge around the inner layer of ejecta, material thrown out from a volcano or meteor impact.

  • Landing

    Viking 1 touched down on July 20, 1976, seven years to the day after the first moon landing. Just minutes later, the lander took this photograph, the first picture ever taken in the surface of Mars.

  • Stars And Stripes

    At left, the American flag is seen on the Viking 1 lander with the bicentennial symbol and Viking symbol below. At right, the six foot long rock known as "Big Joe" looms about 25 feet from the lander.

  • First Color Image

    This is the first color image of the surface of Mars, snapped by Viking 1 the day after landing. The rocky wasteland, covered by iron oxide, at last provided an image to match the nickname "red planet."

  • In The Trenches

    Viking 1's sampling arm created a number of deep trenches in the red planet's soil as part of surface composition and biology experiments.

  • The "Face"

    Meanwhile, the Viking 1 Orbiter continued to snap intriguing photos of the surface, like this photo from the Cydonia region that showed what many thought looked like a human face.

  • The View From Orbit

    A Viking 1 Orbiter image from September 1976 shows debris flows east of the Hellas region. The image is about 174 miles across and the debris flows extend up to 12 miles from the source.

  • Red Planet

    A global mosaic from 102 Viking 1 Orbiter images from February 1980 shows a full Martian hemisphere. The view represents what you would see from a spacecraft about 1500 miles high.

  • Volcanic Trio

    A color mosaic from Viking 1's Orbiter shows the eastern Tharsis region. At left, from top to bottom, are the three 15 mile high volcanic shields, Ascraeus Mons, Pavonis Mons, and Arsia Mons.

  • Olympus Mons

    A color mosaic from Viking 1 shows the massive Olympus Mons volcano. The largest volcano in the solar system, Olympus Mons is about the same size (in area) as the state of Arizona, nearly 375 miles in diameter and 16 miles high. A crater 50 miles wide sits atop the summit.

  • Chandor Chasma

    A color mosaic from both Viking Orbiters shows a part of Valles Marineris known as Chandor Chasma. The walls and floor show evidence of erosion. The Viking 2 Lander ended communications on April 11, 1980, and the Viking 1 Lander on November 13, 1982, after transmitting over 1400 images of the two sites. The Viking 2 Orbiter was powered down on July 25, 1978 after 706 orbits, and the Viking 1 Orbiter was powered down on August 17, 1980, after over 1400 orbits.