A long-distance trip ends as this week begins. An unmanned NASA spacecraft called InSight lands Monday afternoon on Mars after a six-month flight from Vandenburg Air Force Base in central California. America's first Mars mission since 2012 starts a two-year study of the planet’s core. Instruments will analyze the surface and core of the 4.5 billion-year-old planet. Radio signal data about soil, minerals, past quakes and other findings will be relayed by two briefcase-sized satellites that are part of the mission and transmitted 91 million miles to scientists at the Jet Propulsion Laboratory in Pasadena, Calif.

NASA's last landing on Mars took place six years ago, when the Curiosity Rover – which still operates -- reached the Red Planet. It and earlier visits were aimed at understanding Mars' surface and atmosphere by studying its canyons, volcanoes, rocks and soil. "But the signatures of the planet's formation can only be found by sensing and studying its vital signs far below the surface," NASA says. In addition to a camera, a seismometer (underground vibration detector) and a robotic arm more than seven feet long, the three-legged robot craft has a heat probe that will hammer itself 16 feet below the Martian surface — much deeper than other spacecraft have drilled. Underground mapping and data gathering “will help us understand the formation and evolution of all rocky planets, including the one we call home,” says Lori Glaze, NASA’s acting director of planetary science. Sue Smrekar, the mission's deputy principal investigator in California, says: "InSight scientists can't wait to explore the heart of Mars."

America is the only nation whose spacecraft have survived a Mars touchdown. The thin atmosphere – just 1 percent of Earth's – means that there's little friction to slow a craft as it descends. "Landing on Mars is hard. It takes skill, focus and years of preparation," says Thomas Zurbuchen, an associate administrator at space agency headquarters in Washington, D.C.

NASA specialist says: "There's nerves and excitement, of course, and as we get closer that starts to amplify. . . . Certainly hearts will be pounding as we get to the top of the [Martian] atmosphere and the landing starts, but it's just part of the job." – Rob Grover, landing team leader

Four rocky planets: Mercury, Venus, Earth, and Mars have solid cores and were formed about 4.5 billion years ago.

Long-term goal: NASA hopes to send a manned mission to Mars in a decade or two.