WASHINGTON — With the first NASA human mission to Mars still at least two decades away, the space agency and planetary scientists have started looking for potential landing sites, a search motivated by both long-term planning requirements and urgency to take advantage of spacecraft already there.

The First Landing Site/Exploration Zone Workshop for Human Missions to the Surface of Mars, a four-day conference that started Oct. 27 at the Lunar and Planetary Institute in Houston, was designed to bring together scientists and engineers to help determine what constitutes a good landing site for human missions that will launch no earlier than the mid-2030s.

Much of the planning centers around a concept called an “exploration zone,” a region 200 kilometers in diameter that would be the site of a landing. For mission planners, an ideal exploration zone would contain regions of scientific interest as well as other areas with resources like water ice that could support multiple human expeditions there.

While the meeting was dominated by presentations about various potential exploration zones, organizers said it could be years before they chose even a shortlist of potential landing sites.

“This is the beginning of the conversation,” John Grunsfeld, NASA associate administrator for science, said during an Oct. 27 presentation at the meeting. “We shouldn’t have the expectation that we will, by chance at this event, find the first landing site.”

NASA’s current planning for human Mars expeditions, known as the Evolvable Mars Campaign, would involve multiple missions that go to the same exploration zone. That is a departure from earlier concepts that envisioned missions going to different regions of the planet. “The Evolvable Mars Campaign is looking at a single site visited by multiple crews and spending long durations of time at that site,” said Steve Hoffman of the Johnson Space Center.

NASA showed at the workshop a video of a concept it called a “Mars Surface Field Station,” featuring facilities such as habitats, banks of solar panels and propellant production plants spread across an exploration zone. Agency officials at the meeting said they assumed the infrastructure in the video had been built up over two or three human missions there.

Part of the motivation for holding the meeting now is to take advantage of spacecraft there now, such as the Mars Reconnaissance Orbiter (MRO), to study potential exploration zones before their missions end. “These things won’t last forever, so there is a real sense of urgency,” Grunsfeld said. MRO, whose high-resolution camera is frequently used to survey landing sites for robotic missions, has been in Mars orbit since 2006.

Those efforts may also drive the design for future robotic Mars missions, such as an orbiter notionally planned for launch no earlier than 2022. That mission should carry a camera with a resolution two to three times better than MRO and a synthetic aperture radar to look for subsurface water ice deposits, said Richard Zurek of the Jet Propulsion Laboratory.

Zurek said the proposed orbiter, which would also serve as a telecommunications relay for spacecraft on the surface, could make use of solar electric propulsion, a key technology NASA has identified in its Mars exploration strategies. That propulsion system would provide other benefits as well. “If you have solar electric propulsion, you also have a lot of power,” he said. “This opens up the kinds of instruments you could have.”

While scientists and engineers discussed the benefits and challenges of potential landing sites, one group of people who may care very little about what landing site NASA eventually selects may be the astronaut corps. Stan Love, a NASA astronaut with a background in planetary science, said astronauts will be primarily concerned with the safety of any landing site and the ability to carry out operations there.

“It’s the job of the crew to go there and carry out the mission,” he said. “The details of the place and the identity of the place are not really relevant to us. We don’t care where we go on Mars.”