Have rover, need payload. That’s the state of things for NASA, which is planning to launch its next rover to Mars in 2020. The rover has ambitious goals, including searching for signs of habitability and life on the Red Planet, and collecting rock samples to be stored for future return to Earth. Now, NASA is asking scientists to propose instruments that will help the spacecraft accomplish its mission.

The space agency released an “announcement of opportunity” on September 24 calling for proposals by December 23. Researchers who plan to put an instrument in the hat must file a heads-up about their plans, called a notice of intent, by October 15.

The design of the 2020 rover will hew closely to that of Curiosity, which landed on Mars in August 2012. The new vehicle will have the same basic body, called a chassis, and will use the same “sky crane” landing system to be lowered onto the surface. But the innards of the rover will be all new, featuring a suite of instruments that move beyond what Curiosity can do.

The instruments must accomplish specific goals for the rover set out in a July report by its Science Definition Team, which disbanded after the report was issued. The goals include scouting for habitable locations and looking for possible signs of past life there, such as microbial fossils and concentrations of organic material. The rover will also be tasked with digging up rock core samples and storing them for future retrieval and return to Earth by a future spacecraft, where they can be studied in laboratories with much more sophisticated instruments than anything that can be sent to Mars.

Because sample storage will take up room inside the rover, however, it won’t be able to carry instruments for analyzing dug-up samples on Mars as Curiosity does. “Curiosity has flown really high-end instruments to do its measurements on the surface of Mars,” says Jack Mustard of Brown University, who chaired the Science Definition Team. “What this coming rover will do is arguably a better job of finding materials that are interesting. It’s somewhat upgraded in its capabilities to do remote measurements. It doesn’t try to do any in situ analysis” like Curiosity’s Sample Analysis at Mars (SAM) and Chemistry and Mineralogy (CheMin) instruments do.

But that decision has angered some Mars scientists, who say the rover will have to sacrifice too much of its instrument space for caching samples. “I think if we’re going to have a Curiosity duplicate rover in 2020, it should be loaded with instruments to do in situ science,” says Robert Zubrin, co-founder and president of the Mars exploration advocacy nonprofit, The Mars Society. “This one says it’s going to have 28 kilograms of science instruments. Curiosity has 80 kilograms. They’ve reduced the science payload by a factor of three in order to have this caching function, which may not have any utility whatsoever.” Zubrin says it leaves too much up to chance to have the return of these samples rely on an unspecified mission in the future making a precision rendezvous and landing at the same spot to collect them.

The Science Definition Team members say the 2020 rover will still be able to do significant science, and it’s important to initiate Mars sample return now. “This mission I think will be on par, in terms of what we learn, with Curiosity, and hold the future prospect of being able to learn 10 times more by bringing samples back to Earth. None of us are going looking for Klingons, but we’d be thrilled if we could help find a sample that contains microbes,” says Scott Murchie at Johns Hopkins University’s Applied Physics Laboratory, who was a member of the team.

The rover’s goal of collecting samples for return follows from the conclusions of the planetary science decadal survey published by the National Research Council in 2011. It combined input from hundreds of scientists to prioritize the various science projects of interest to the planetary science community over the coming decade. Rated above all other goals, including a mission to Jupiter’s intriguing moon Europa, was the aim to begin the process of returning samples from Mars to Earth.

Initially there was some question of whether the 2020 rover would be capable of beginning the sample-return process. “We came back and we said look, ‘We think it is technically and fiscally possible to do this and to do awesome science in addition,’” Mustard says. “I’m sure some were hoping we would fail in trying to find a solution to that, but we didn’t.” Zubrin agrees that returning Mars samples is important, but said he’d prefer designing a single future mission that landed a smaller rover to collect samples together with an ascent vehicle to take those samples immediately back to Earth. “We should choose the simplest, most effective and low-risk way to get samples,” he says. “I don’t think that the 2020 mission is a sample-return mission, I think it’s a rover mission and it should be maximized for its effectiveness as a rover mission and not try to impersonate a sample-return mission.”

Opponents of that idea say the 2020 rover will have the tools to hunt for more valuable samples than a smaller, less well-outfitted rover that landed with its own return ride. For now, the question of how and when the 2020 rover’s samples will make it back to Earth is being left up in the air. “We just said, ‘Create the cache. Figure out what you’re going to do later to bring it back,’” Mustard says.

The scientists are hoping the rover will find interesting enough samples to motivate Congress and NASA to swiftly fund a mission to return the samples home. “Mars has had a way of surprising us in terms of how friendly to life it was in the distant past,” Murchie says. “If we go to a site that is carefully chosen based on the data, I would expect we’d have a very good chance of finding something on the ground that will be a high priority to bring back.”

NASA has left open the possibility that a human crew could retrieve the samples. In this way, the 2020 rover mission may lay the groundwork for manned Mars exploration in the future, which is one of the explicit goals of the mission set by NASA Headquarters. And the rover will set the stage for humans in other ways as well. It will test technology for using resources found in the environment and atmosphere of Mars—a project called in situ resource utilization (ISRU). At least one of the instruments selected to fly on the rover must test ways to use resources on Mars such as carbon dioxide to make oxygen. For human missions, oxygen would be useful for life support and as rocket fuel for the trip home. “That’s the first step—you need to cut the tether between Earth and your spacecraft,” Mustard says. Others say the rover should be more ambitious and, for instance, make use of the soil as well as the atmosphere for harvesting resources.

Overall, however, there is a lot of leeway for different types of instruments. It will probably have a color stereo imager and likely a spectrometer to go on its upright mast, and the arm might have instruments for probing the surface chemistry and mineralogy, predicts Ray Arvidson of Washington University in Saint Louis, who is deputy principal investigator for the Opportunity rover currently operating on Mars as well as a participating scientist on Curiosity's mission. Arvidson is working on a proposal for an instrument called a Raman spectrometer that would enable the Mars 2020 rover to study the mineral content of the Martian surface.

Although many of the big players in Mars rover research will likely be involved with the new rover, there’s always a chance for a new scientist with an out-of-the-box idea to become a principal investigator on one of the mission’s instruments, Arvidson says. “It depends on what comes in from the community and what the new innovative ideas are. This all comes down to cost and mass and power, and how technically ready the instrumentation will be. I’m hoping there’ll be some new folks that show up who have innovative ideas that fit into those constraints.”

Some instruments may end up being similar to those on Curiosity, whereas others will likely be departures. Competition is likely to be fierce. “I expect there will be multiple submissions for certain measurement types, and there might be submissions that go beyond the scope of what we recommend,” Mustard says.