Could life exist on Jupiter's moon Europa?

NASA just moved a step closer to answering that longstanding question. The space agency announced Monday that it would move forward with its long-anticipated Europa Clipper mission, which aims to place a robotic spacecraft in orbit around an icy moon that's considered one of the likeliest places in our solar system to find alien life.

"We are all excited about the decision that moves the Europa Clipper mission one key step closer to unlocking the mysteries of this ocean world," Thomas Zurbuchen, associate administrator for the science mission directorate at NASA headquarters in Washington, said in a statement.

Plans call for the van-sized spacecraft at the heart of the $4.25 billion mission to be designed, built and tested primarily at NASA's Jet Propulsion Laboratory in Pasadena, California. The craft is expected to be ready for launch as early as 2023; it will be propelled into space from Cape Canaveral, possibly by NASA's giant Space Launch System (SLS) rocket. The journey to Europa will take about three years — possibly longer if another rocket is used.

Europa, which was discovered by Galileo in 1610, is about 1,940 miles in diameter — or roughly 90 percent of the size of Earth's natural satellite. It circles Jupiter at a distance of about 417,000 miles above the surface of the gas giant, which circles the sun at a distance of about 500 million miles.

A vast ocean believed to exist under Europa's cracked, icy crust and the recent discovery of sodium chloride (table salt) on its surface suggests that the subsurface ocean may be more like Earth's salty oceans than scientists had realized. Scientists think Europa's ocean may be home to microbial life.

The Europa Clipper spacecraft will be equipped with nine scientific instruments, including cameras, chemical and magnetic sensors and ice-penetrating radar. The spacecraft will use these instruments in an attempt to confirm the existence of the subsurface ocean and determine whether it might be habitable.

"What we want to understand is whether Europa has the potential for life," said project scientist Robert Pappalardo.

In addition to water, life as we know it requires carbon, nitrogen and other molecules as well as a source of energy. "Life is like a little battery, and we need to understand if the plus and minus of the battery are there," Pappalardo said. "We're pretty sure the stuff on the surface can serve as fuel for life if it can get into the ocean."

If the data it collects indicates that Europa is habitable, Pappalardo said, the Europa Clipper mission might be followed by a mission to land a spacecraft on the surface. A lander might be able to confirm the existence of life on Europa — though, given the time needed to design and build a lander and get it to the distant moon, the answer might not come until the 2040s.

Pappalardo said more than a thousand people have been working to develop the Europa Clipper mission, whose origins can be traced to the late 1990s. With so many people having worked for so long just to reach this point, he called NASA's decision to move forward a "big deal."