A version of this article appears in the May 26 edition of Aviation Week & Space Technology.

This image, a mosaic collected by the Mast Camera on the Curiosity Mars rover in December 2012, is historic. It shows what the mission science team titled “A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars” in their peer-reviewed paper published in Science a year later. Water once pooled here, for hundreds to tens of thousands of years, and where there is water, there can be life.

“We have found a habitable environment that is so benign and supportive of life that if this water had been around and you had been on the planet, you would have been able to drink it,” says John Grotzinger, project scientist on the mission (AW&ST March 18, 2013, p. 11).

Grotzinger was crowing just a little, because his mission had achieved its primary objective only four months after landing on Mars. Curiosity is not equipped to look for life, but the implications of its finding that Mars was indeed habitable are profound.

Scientists believe that if there is life in the Solar System, they will find it in the next 25 years. Missions underway, planned or under study to Mars, Enceladus, Europa and other potential watery places around the Sun should be able to settle the age-old question “Are we alone?” in that timeframe, essentially by following the exploration approach that has been followed at Mars for almost 40 years.

“We went there; we identified places that looked like they could have supported life; we sent the mission to one of them . . . and essentially we’ve already met our prime mission objective in identifying a place on Mars that we are pretty sure could have supported life,” says Michael Meyer, lead scientist on NASA’s Mars exploration program.

Meyer and Nadine Barlow, a professor of physics and astronomy at Northern Arizona University, were clear when they were asked at a May 14 Brookings Institution symposium what the big space-science discovery of the next 25 years will be.

“If we continue our space exploration of Mars and some of these icy worlds in the outer Solar System, we’ll have gone a long way toward being able to answer the question whether or not life has ever existed on these other worlds or exists there today,” said Barlow.

Meyer says the orbiters and landers sent to the red planet since Viking imaged a cold, dry, apparently dead planet in 1976 have taken a more holistic look, with Curiosity’s conclusion about habitation the logical outcome.

“Basically we have gotten more and more specific about what do we really know about the planet, because you can’t understand life unless you understand the environment,” said Meyer. “That has really been a revelation to planetary scientists.”

Water is considered a prerequisite for life as we know it. NASA chose Gale Crater as the landing site for Curiosity after pursuing a “follow-the-water” approach on Mars, looking for places from orbit that looked like they might have been formed by water, and continuing the pursuit of water with landers and rovers on the surface. Other spacecraft, including the Cassini probe and the Hubble Space Telescope, have found reasons to believe there is liquid water below the frozen surfaces of Saturn’s moon Enceladus and Jupiter’s moon Europa, and there appears to be some sort of liquid moving around beneath the hydrocarbon-rich surface of Titan, Saturn’s largest moon.

Meyer cautioned that protecting extraterrestrial bodies from contamination by Earthly microbes carried there on scientific spacecraft will continue to be a problem, even though the early concerns about planetary protection are moot. He says that when the original planetary-protection standard requiring sterilization of orbiters expected to crash into their target planets within 50 years was adopted, the space-exploration community expected humans to be on Mars and other bodies after a half-century.

We are not there yet, so Earth remains the only planet known to support life. Scientists believe discoveries of extreme conditions here and the continuing identification of extrasolar planets in the “habitable zone” where water should exist as a liquid, suggest life may be common in the universe. In the next 25 years, new instruments and better observation techniques may put some of those extrasolar planets in the same category as Curiosity has placed Mars.

“By then I think we’ll actually be able to figure out whether there are habitable planets around other stars in our galaxy,” said Meyer. “The degree to which we think they are habitable and what we know about them is quite a stretch, because looking from a distance is not the same as actually being there.”