Things are changing on Mars. For two decades, NASA has regularly launched missions to the planet, engaging in a sustained effort of robotic exploration. These missions have revealed signs of water, complex organic compounds, volcanic activity and tantalizing hints of possible life—either extinct and gone, or, perhaps, lurking in the subterranean realms of the planet to this very day.

Since the dawn of the 21st century, NASA has successfully sent eight spacecraft to Mars, to orbit or to land, with no failures. But looking into the future, a marked lack of NASA missions to the planet breaks a pattern that has persisted for decades.

“We’re taking for granted this incredible presence NASA has had for 20 years, and we’re watching that wither away,” says Casey Dreier, chief advocate and senior space policy adviser at the Planetary Society , an NGO co-founded by Carl Sagan in 1980 to advocate space science and exploration.

In the near term, the Martian landscape will see no lack of robotic activity. The InSight lander touched down on the surface of the planet last November . Just last week, NASA announced that the Opportunity rover, which had been exploring Mars for almost 15 years, has finally shut down for good . And as the 2020 launch window for Mars missions approaches, countries around the world are gearing up for interplanetary launches—the most spacecraft to fly for Mars at the same time in history .

After 2020, however, the Mars manifest is conspicuously thin. Without a mission to follow NASA’s Mars 2020 rover, many scientists are left wondering what comes next in the reconnaissance of the most accessible and hospitable world beyond our own—a planet that NASA plans to land astronauts on in another 20 short years.

**********

Mars and Earth align every 26 months for an ideal launch to the red planet, and not coincidentally, NASA has sent a spacecraft to Mars about every other year on average since 2000. The next launch window opens up in July and August of 2020, with spacecraft expected to arrive at Mars about half a year later.

In 2020, NASA is planning to send a flagship rover—an upgraded version of Curiosity—to the surface of Mars. The European Space Agency (ESA) and Roscosmos are also planning to send a rover, recently named for DNA scientist Rosalind Franklin , to Mars next year. China is planning an orbiter and rover, Japan has an orbiter and lander in the works, and the United Arab Emirates is planning its first Mars orbiter as well—all in 2020. Two more spacecraft from the Indian and Japanese space agencies are slated to follow in 2022 and 2024, respectively.

The most ambitious of these missions is NASA's Mars 2020 (which will receive an official name before launch). While the 2020 rover is still under construction, NASA has taken an important step toward tackling the goals of the mission: selecting a landing site. Jezero Crater, Mars 2020’s future home, features a now-dry river delta where steams are thought to have once flowed into a large lake bed.

“Jezero Crater has a couple of major aspects that make it very attractive,” says Michael Meyer, NASA’s Mars exploration lead scientist. “One is that you can look at it and you know it’s a delta. The geomorphology is fairly obvious.”

Meyer says that evidence from orbiters points to past river flows into the lake basin, transporting materials from all over the planet. As a result, “you have a good assemblage of minerals there.”

The geologically rich Jezero Crater makes the dried lake bed an ideal site for one of Mars 2020’s primary objectives: to cache samples and deposit them on the surface for a future mission to pick up and launch back to Earth. The problem is that no future mission for sample return currently exists—and for planetary scientists, the idea of gathering samples on Mars and leaving them there indefinitely is simply unpalatable.

“It will begin caching samples for return to the Earth,” Dreier says. “The question, though, is if we’re going to come get them.”

**********

It’s hard to overstate the value of planetary samples brought back to labs on Earth —and humanity has never returned a sample from Mars. Compared to using a spacecraft’s onboard instruments, scientists can measure samples on the ground with much higher precision, revealing such subtle clues as isotope ratios that could provide “smoking gun” evidence of life.

Without a sample return, “you’re going to find powder burns,” Meyer says. “You’re not going to find the gun.”

A sample return mission has long been a goal of planetary scientists, tracing its official origins back to at least a 2007 study titled “ An Astrobiology Strategy for the Exploration of Mars .”

“It laid out what you needed to do to find out if there was life on Mars,” Meyer says. “It essentially came down to: The next step is to do sample returns.”

To collect the samples that Mars 2020 leaves on the surface of the planet, NASA is considering multiple mission plans. The leading idea is to use a lander with a small “fetch rover” to grab the samples and then blast them into Mars orbit, where a spacecraft would snag them and fly back home.

Sample return is, “broadly within the scientific community, one of the highest priority scientific goals,” Dreier says.

However, the decision to fund such a mission, which Dreier says would likely cost about $2 or $3 billion, is made by Congress, not NASA. The current White House administration, which wields significant influence shaping the direction of NASA, is focused on human exploration of the moon rather than robotic exploration of Mars, even though sending astronauts to Mars is a stated long-term goal.

Within NASA, however, Meyer says there is some inertia toward a sample return mission—mostly in the form of feasibility studies. Optimistically, the space agency could shoot for the 2026 launch window to bring back some of the red regolith of Mars.

After 2020, if NASA does not attempt another launch to Mars until 2026, it will be the longest gap in the space agency’s missions to Mars since a hiatus between 1975 and 1992.

**********

In the coming decade, NASA may face a more fundamental problem than leaving uncollected samples on the surface of Mars. The primary telecommunications relays between Earth and Mars, Mars Odyssey and the Mars Reconnaissance Orbiter , are around 17 and 13 years old, respectively.

“We know full well that to bank on them being there 20 years from now is silly,” Meyer says. “Banking on them being there 10 years from now is more credible.”

A proposal for a new orbiting spacecraft to serve as a communications link, called the Next Mars Orbiter, was initially envisioned for a launch in 2022. However, the various and competing needs for a new Mars spacecraft have since scrapped the initial plan. Many scientists want to use the next mission to Mars for sample return, while others argue the need for a communications relay is more immediate—and a third option would morph the Next Mars Orbiter into a large-scale mission that could do both, a prospect that would require significant technological advances. In any case, Next Mars Orbiter (or whatever it becomes) seems unlikely to launch until the late 2020s.

NASA has another potential solution, however. Future NASA operations could hitchhike on missions sent by other space agencies. Institutions around the world have Mars ambitions, from countries like India and the United Arab Emirates to private companies like SpaceX.

By cooperating with international and private space institutions, NASA could affordably send CubeSats or other small-scale spacecraft. Such a mission could, in theory, work as a surveyor and science mission for one agency while doubling as a communications relay for NASA.

“We are willing to entertain a different mode,” Meyer says. “Instead of buying a spacecraft and having it do x, we actually just buy x and let someone else figure what to do with it.”

Mars 2020 is shaping up to be one of the most ambitious planetary exploration missions in history, and it could provide an unprecedented picture of the planet’s history, habitability and viability for future human exploration. But the rover will also be collecting priceless samples of Martian material, the true key to unlocking the planet’s past—and at the moment, no one knows how we are going to pick them up.