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Within an exploration zone, there are typically many places where a spacecraft can touch down, meaning the issue of a safe landing as a criterion for selecting a site will likely be ignored.

NASA has begun the long process of looking for a suitable place to land humans on Mars.

Kicking off a three-day workshop in Houston on Tuesday, U.S. space agency officials asked nearly 200 scientists and engineers to help them identify particular locations on Mars that merit further study as potential landing sites. Dubbed "exploration zones" by meeting organizers, the locations are areas within a 100-kilometre radius that include features of scientific interest along with nearby resources that would help support astronauts working there over months or years.

"The plan is to send multiple crews to the exploration zone," said Ben Bussey, a meeting co-chair with NASA's human exploration directorate. "The more you can live off the land, the more achievable that is."

While a NASA-led human mission to Mars is at least two decades away, organizers say there's an urgent need to make use of spacecraft that are currently orbiting Mars to find out as much as possible about potential landing sites while those orbiters are still functioning.

The agency already has a well-honed process in place for identifying where it wants to put robotic landers and rovers, which involves winnowing down a long list of candidate sites to find one that offers the promise of rich scientific returns without too many physical hazards that might make landing a risky proposition.

The exploration-zone approach is quite different because it assumes astronauts will roam much farther than any rover has so far travelled across Mars. Within such large regions, there are typically many places where a spacecraft can touch down. That means "you can almost ignore the issue of a safe landing" as a criterion for selecting a site, Dr. Bussey said.

One issue that is not so easily ignored is planetary protection – how to avoid contaminating Mars with life from Earth.

Last month, NASA released images showing that streaks on some Martian slopes appear to darken at warmer times of the year. The changes could mean that there are reservoirs of salty water just below the planet's frigid surface from where liquid can seep out. This strengthens the possibility that Mars is a habitat for alien microbes – a living environment that would be at risk, or at least harder to detect, if exposed to Earth bacteria carried by humans.

"Current planetary-protection policies do not apply to Mars when humans get there," said meeting co-chair Rick Davis of NASA's science mission directorate. Dr. Davis said one of the aims of the workshop was to develop a future policy that protects both the crew and the Martian environment, but also "allows us to take full advantage of having creative, intuitive and adaptive humans on site at Mars to accomplish incredible science."

Locations that are rich in water could make for a tricky balancing act for such a policy. Water increases the prospects for finding life on Mars, but it would be a coveted resource for human explorers. At the workshop, participants were told about 20 million tonnes of water would be required to support a single crew over a long-duration stay, including water for growing food.

The U.S. government has not authorized a mission to Mars, but in 2010 NASA was directed to develop the technical capabilities that would allow for such a mission in the 2030s. The directive ended a period of which the space agency was focused on other goals and scientists were discouraged from pursuing Mars as a destination for human spaceflight.

"Not too many years ago, we weren't even allowed to say Mars," said former astronaut John Grunsfeld, associate administrator of the agency's science mission directorate, during his opening remarks to workshop attendees.

The policy appears to still be in effect in Canada. On Tuesday, The Globe and Mail's requests to speak with Victoria Hipkin, the Canadian Space Agency's senior program scientist for planetary exploration, about Canada's ongoing role in Mars exploration were refused without explanation. Prime-minister-designate Justin Trudeau has indicated he would remove restrictive communication policies that inhibit federal scientists from speaking with journalists when he takes office.

Canada is represented on the science team of NASA's next robotic lander on Mars, a small probe called InSight that will launch next March.

Mars One, a private foundation based in the Netherlands, has said it will place humans on Mars about a decade before NASA's plan would, but many experts doubt this claim because of the cost and the need to develop the necessary technology to enable survival on the Red Planet.

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