Getting to the surface of Mars takes years of planning, engineering and science work, a successful launch, and a months-long journey of millions of miles. You only get one opportunity to touch down at a site on Mars, so it's critical to get it right. "You can't say, 'here are ten different sites, let's go to them,'" said Michael Meyer, the lead scientist for the Mars Exploration Program at NASA Headquarters in Washington. "Each mission-it's one shot. You definitely want to pick the right one."

The Community is Crucial
Meyer spoke about how NASA picks Mars landing sites on Sunday, during the annual meeting of the American Geophysical Union (AGU) in San Francisco. He said the "spectacular" involvement of the broader science community is critical in choosing sites for both robotic and human landings. Continuing the open process started with Spirit and Opportunity, NASA is holding landing site workshops for both the next rover, Mars 2020, and a future human landing site mission. (Find out how to get involved or attend a workshop: Mars 2020 Landing Sites | Human Landing Sites )

Meyer was joined at the AGU lecture by Alex Longo, a high school student who has proposed both human and robotic landing sites, and Bethany Ehlmann, a scientist from NASA's Jet Propulsion Laboratory who works on both the Mars Reconnaissance Orbiter and Curiosity, and will be part of the Mars 2020 team.

Meyer gave the AGU audience an update on landing site selection for Mars 2020. Two workshops have already narrowed suggested sites down from 54 to 21, and then from 21 to eight high priority sites. A third workshop is set for February 8-10 in Monrovia, California. Now, Meyer said, the scientific community can do more "homework," using orbital assets such as the high-resolution camera on the Mars Reconnaissance Orbiter to get more information about the site.

The community starts its search by focusing on the mission objectives and what areas might support that kind of science. For Mars 2020, scientists are looking for a place that has evidence of water - possibly an ancient shoreline - and a strong potential to be a place where life could've thrived. Mars 2020 is also set to cache samples for return to Earth later. That adds more interesting possibilities for landing sites, since there could be science targets the rover isn't set up to study on Mars that still have value when returned to Earth.

The last Mars 2020 workshop had 150 attendees and another 50 online, exchanging ideas and raising fresh questions. "It's a sincere scientific discussion about what we know about Mars, about what we think we know but have to admit that we really don't know, or things we have misconceptions about," said Meyer.

Decisions for Decades to Come
NASA's Journey to Mars calls for sending humans to orbit the Red Planet in the 2030s, with trips to the surface to follow. But NASA isn't waiting to scout potential sites. We're using the orbiters at Mars today to help refine our sense of what makes an optimal landing site, and to eventually narrow the list of candidate sites.

The first human landing sites workshop in 2015 yielded 47 landing site proposals. NASA is looking to set up a semi-permanent base, dubbed an "exploration zone," where crews can live off the land and explore up to a 60-mile (100-kilometer) radius. There will be more workshops and more reconnaissance as the list narrows down in years to come.

With humans, said Meyer, NASA is looking for a place where astronauts can make the best of the surroundings, and trying to answer some key questions. As he puts it, "Is there a place to land? Are there resources that you can make use of there, such as water so that you can make your own fuel? Are there interesting things in the area for the astronauts to explore?"

More than 400 people supported the first human landing sties workshop in 2015, with a diverse group of disciplines represented-all of which are critical for picking an ideal site for the first human base on Mars.

Selecting the Site - for 2020 or Human Exploration

Even after the process narrows it down to a small group of sites, it can be painful to pick only one. And despite all the advance work, there are always new things to learn from going there.

"You feel like you know the place already, and then when you get there," said Meyer, "it's different. It's always a great surprise."