Hundreds of millions of miles away, on the frigid surface of Mars, NASA’s Perseverance rover is hard at work, diligently gathering fresh samples of Martian rock, sealing them in pristine tubes, and leaving them on the surface ready for collection. 

If they can be brought back to Earth, these would be an invaluable scientific resource: the first sample ever collected from another planet, which could answer fundamental questions about the history and habitability of Mars.

But like a child forgotten at school pickup, Perseverance may face a long, lonely wait for collection. The mission to retrieve the samples, called Mars Sample Return (MSR), has already caused NASA huge headaches with costs projected to hit $11 billion and a timetable that an independent review declared wholly unrealistic.

“The bottom line is, an $11 billion budget is too expensive, and a 2040 return date is too far away,” said NASA administrator Bill Nelson recently, announcing that the agency would be looking to make major changes, including soliciting help from the aerospace industry.

Proponents of the mission argue that it is the best chance we’ll ever have to find evidence of life beyond Earth and that samples from Mars can reveal crucial information, like how long the planet had water on its surface and when it lost its atmosphere. But critics point to the ever-ballooning budget and question whether the scientific payoff is worth the expense.

There are hopes that private industry could help, with options being floated like using the SpaceX Starship to carry the sample back from Mars. But even if that works, there are still significant challenges to address. Perhaps the biggest is launching a rocket from the Martian surface, something that has never been done before, not to mention getting a launch vehicle to rendezvous with a Starship in orbit and transfer the samples for transport back to Earth.

NASA has not shied away from acknowledging the scope of the task or the doubts raised by the public about whether that money could be better spent elsewhere. However, experts agree that sample return offers an opportunity to learn about Mars and other planets that robotic exploration can’t hope to match.

With the tremendous success of the Mars rover program, it’s reasonable to ask why carrying samples all the way back to Earth is necessary when rovers are already so capable and will only become more so in the future. The answer is simple: there’s no substitute for a well-equipped Earth lab.

The Perseverance rover has an impressive set of instruments on board, but it’s not feasible to engineer some tools to fit onto a mobile platform. Scientists want to use instruments that are room-sized, like mass spectrometers used for dating planetary materials, and ones that are the size of a city block, like particle accelerators called synchrotrons that can analyze the composition of samples down to their tiniest parts, explained Mini Wadhwa, the MSR principal scientist at NASA’s Jet Propulsion Laboratory (JPL).

The instruments need to be so big in part because the indicators that scientists are looking for in the samples will be so small. To detect organic molecules (not necessarily signs of life but the building blocks for living things) in a sample, for example, requires looking for extremely small amounts of material in samples that are already only a few grams in mass. And looking for indications of life in the structures of rock, called morphologic biomarkers, requires extremely sensitive measurements using powerful microscopes. 

“This is work that you cannot do with a rover,” said Katie Stack Morgan, a Mars research scientist at JPL. “We don’t have the instrumentation and we don’t have a way to make those measurements. Yet those are so key in the search for life and understanding how Mars as a planet — atmosphere, surface, subsurface — interacted with each other.”

Even those who have raised questions about the costs or challenges of the MSR mission have no doubts about the enormous potential value of having Mars samples on Earth.

“For the cost of Mars Sample Return, we could do a lot of wonderful robotic science,” said Michael Hecht, principal investigator of MOXIE, or the Mars Oxygen In-Situ Resource Utilization Experiment, on the Perseverance rover. “But I don’t think anyone would contest the idea that given samples here in an Earth laboratory, we could accomplish things that are leaps and bounds beyond what we can accomplish on Mars.”

For those unconvinced by the payoff, NASA may need to put in more work to persuade critics of the importance of samples to planetary science beyond Mars.

“Is the planetary science community as a whole chomping at the bit to say, yes, bring those samples home? No,” said Paul Byrne, a planetary scientist who has worked on other NASA missions but is not involved in the Mars program, adding that the problem was one of messaging and communication as much as anything else: “NASA has not done a particularly great job advocating for the scientific value of these samples.”

It’s not that planetary scientists as a group are against Mars research or that they don’t see the value of Mars samples. It’s more of a worry that MSR could take up more budget in the future.

NASA has been very explicit that MSR won’t eat up the entire budget for planetary science and that it exists on top of existing planetary research. However, the planetary science division has struggled with other budget issues, including overruns or delays for major missions like Psyche, Dragonfly, Europa Clipper, and Veritas and long-term delays and problems caused by the covid-19 pandemic.

To make MSR happen, the entire planetary science community will need to come together in support, as the astrophysics community did with the launch of the James Webb Space Telescope — another mammoth project that was hugely expensive and ran years behind schedule. “This is the first project of this scale the planetary science community has ever had to contend with,” said Byrne. “It is our Webb moment.”

Much of the justification for the huge expense of MSR comes from a single issue. There’s a very real chance that getting samples back to Earth could answer the biggest question in all of space research: did life ever exist on another planet? 

While there is almost certainly nothing alive on the surface of Mars now, the planet was potentially habitable billions of years ago, when water flowed plentifully and microbial life could have flourished. The area where the Perseverance rover is exploring, called the Jezero Crater, was once an ancient lake, and the rover has scooped up samples of the rocks there for future study.

“It could absolutely be possible that evidence for life on Mars is sitting in those tubes,” Stack Morgan said. “So if you want to answer that question, you have to bring them back.”

Hinging the justification for sample return on the possibility of finding life carries a huge risk of disappointment from the public if that isn’t found. “It is sexy to say we’re going to look for life,” Byrne said. “But it is risky unless you know you’re going to find it.” 

If you look at the history of missions like the Viking probes in the 1970s or the Allan Hills 84001 meteorite, which caused a media uproar when it was initially thought to contain evidence of Martian life in the 1990s, you can see otherwise successful research that has carried the stigma of disappointment because it didn’t find smoking gun evidence of life.

And there are plenty of reasons to advocate for Mars samples that aren’t related to searching for life. A recent meeting of the Mars Exploration Program Analysis Group (MEPAG) focused on science goals other than the search for life that could be accomplished using samples. Samples could inform our understanding of when and how the Mars climate changed from being somewhat Earth-like to the dry, cold conditions we see today; the properties of the dust that coats much of the planet; and how the inner planets were bombarded by asteroids in the early period of the Solar System.

The value of potential samples isn’t only limited to understanding Mars, either. “Many, if not all, of the things we learn from samples at Mars inform our understanding of the evolution of our Solar System and, by extension, give us better ideas of how exoplanets may form and evolve around other stars,” said Vicky Hamilton, the steering committee chair for the MEPAG group.

While the scientific community is interested in studying samples from Mars, much of the public has a stronger desire to see people stand on the surface of another planet. Human exploration of Mars has been a dream for decades, and it could be feasible within the next 50 to 100 years. (Elon Musk’s inflated claims aside, putting humans on Mars within a timescale of decades is possible, although it would require massive amounts of money.) 

NASA seems aware of the public hunger for human exploration, and the agency has positioned both its Artemis Moon program and the MSR mission as paving the way for astronauts to visit Mars. Dewayne Washington, NASA’s MSR senior communications manager, said that bringing Mars samples back to Earth would prepare the agency for human exploration by providing information on the safety of the Mars environment, developing technologies like launch and landing of large masses, and developing sterilization techniques.

Certainly, one of the biggest challenges for human exploration is safely landing on and launching from Mars, and learning how to do this with samples can only help with that goal. However, not everyone is convinced that using robots to convey samples or sending astronauts to the Moon is really helpful in furthering human exploration of Mars.

“I do wonder why people feel the need to justify these very important and valuable things in terms of being a step to something else,” Hecht said. “You wouldn’t open a bicycle factory so you could learn how to build cars.”

One suggestion for making the MSR mission more affordable is to combine it with the Artemis Moon program, such as perhaps sending samples from Mars to the planned lunar Gateway space station rather than directly back to Earth. But Hecht isn’t convinced that would solve the problem. “I don’t really see it,” he said, as including an additional rendezvous “just adds complexity.” 

Questions about the details aside, there is strong motivation to make it happen. “Even though Mars has been the focus of research for several decades now, we have truly only scratched the surface,” Stack Morgan said. “You’re not going to answer some of the most fundamental, basic questions about Mars until we get those samples back in our labs here on Earth.”

The motivation is not only because of the potential scientific value. There’s also the fact that the time, effort, and money (over $2.7 billion) has already been spent to send Perseverance to Mars to collect and seal samples and to leave them on the surface ready to be returned to Earth. 

That money “has already been spent,” Hecht pointed out, so it would be wasteful to not see the mission through: “You might find plenty of folks in the planetary science community who would grumble about the path we’ve taken, but I don’t think you would find many who would argue that, having come so far, we should pull the plug now.”

Scientists also have their eyes on the future, considering what the next big target might be after Mars. “This should be the first of these projects for the planetary community, not the last,” Byrne said. Currently, there are around 40 planetary science-related NASA missions, but scientists envision a fleet of hundreds of missions exploring not only the inner Solar System planets but also beyond to the outer planets and other targets like moons or asteroids — not only orbiting these objects but also landing on them. 

Our understanding of how planets form and develop and the range of conditions that exist on them has exploded in recent decades, looking not only at bodies within our Solar System but also exoplanets beyond. And technology is rapidly developing to allow more of these wildly ambitious plans to move within the realm of possibility. 

“What if we want to return a sample from Enceladus to Earth? Or what if we want to rove on Venus? And drill through the ice shell of Europa? Or put a fleet of spacecraft out to Neptune and bring a bit of Triton home?” Byrne said. These places may or may not be habitable, but the potential for discovery exists beyond only searching for life: “Let’s explore these worlds for the sake of understanding the universe.”