About a dozen years from now, Martians might finally arrive on Earth. If they do, it will be because we brought them here.

NASA and the European Space Agency are planning an audacious mission to gather samples of rock and soil from the surface of the red planet and transport them across 34 million miles of space—giving scientists an unprecedented opportunity to study what Mars is made of and to search for evidence that the planet once harbored life. Because past missions have revealed signs of Martian lakes and river deltas, the scientists believe they may find the fossils of microscopic organisms that thrived in those lakes and rivers before the planet became the frigid desert that it is today.

Next July, the three-part mission to return samples from Mars will begin with the launch of the Mars 2020 rover. While the rover is exploring and collecting soil, NASA engineers will continue developing the technology for the other two phases of the mission—launching a rocket lifting the samples to Martian orbit, where it will rendezvous with a waiting return vehicle that will ferry the precious cargo to Earth. For each of the steps in that process, the engineers at NASA’s Jet Propulsion Laboratory are confronting a series of daunting challenges.

For starters, nobody has ever launched a rocket from the surface of another planet. This is a very different scenario from the one that brought Apollo astronauts home from the moon, just 238,900 miles away. Unlike the ascent stage of the Apollo Lunar Module, the planned Mars Ascent Vehicle (MAV) will have to free itself from a planet’s gravity, even if the pull is only 38 percent of the surface gravity of Earth. And before the ascent vehicle launches for home, it will have had to endure a gauntlet of physical punishments.

First, as a payload aboard a lander headed to Mars, the MAV will be subjected to the rough ride of a launch from Earth, followed by a six- to nine-month flight through deep space, which will culminate in a fiery entry into the atmosphere surrounding Mars, a supersonic descent, and a not-so-soft landing. After that, the craft will sit on the surface for half a Mars year (equal to a full year on Earth), exposed to dust storms, ultraviolet radiation, and temperatures as low as minus 40 degrees Fahrenheit.

Another crucial difference from the Apollo missions: There will be no humans on the spacecraft. And because it can take several minutes for a transmission to reach Mars, even remote piloting is out of the question.

“We can’t joystick it,” says Paulo Younse, an engineer at NASA’s Jet Propulsion Laboratory. “We can’t communicate with it, and we don’t have a person on board, so it’s got to be automatic.”

On February 18, 2021, the Mars 2020 rover will touch down in the 30-mile-wide Jezero Crater (pronounced “YEH-zuh-roh”), where it will collect samples and cache them in hermetically sealed tubes for later retrieval. NASA spent five years deliberating over a landing site before it settled on Jezero. Scientists believe that between 4.1 and 3.5 billion years ago the crater was filled with a lake, 820 feet deep. Perhaps more exciting are the signs of a river delta. A delta is “extremely good at preserving biosignatures, evidence of life that might have existed in the lake water, or at the interface between the sediment and the lake water, or, possibly, things that lived in the headwaters region that were swept in by the river and deposited in the delta,” said Mars 2020 project scientist Ken Farley when announcing the landing site last November. The rover will collect samples from at least five different kinds of rock, including clays and carbonates, which have high potential to preserve indicators of ancient life, whether in the form of complex organic molecules or the fossils of microbes. The search for samples will be aided by a suite of instruments, including SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), which uses spectrometers, an ultraviolet laser, and a camera to detect organic compounds. But, scientists say, this equipment will be no substitute for the more sophisticated instruments on Earth—especially when confronted with the challenging task of distinguishing signs of life from chemical activity that might mimic organic processes. “To really make the next big leap in understanding Mars as a system, we want to have samples here,” says Charles Edwards, a JPL manager for the Mars Exploration Directorate. “By getting those samples back to Earth, you can really unleash the power of all the terrestrial laboratories and answer some of the questions that we want to answer about life on Mars—whether we’re talking about extinct life or even extant life.” NASA and the European Space Agency have joined forces to plan for the later missions—not yet scheduled—that eventually will complete Mars Sample Return. After Mars 2020, the next step is to send another lander to Jezero Crater carrying a “fetch rover” and the Mars Ascent Vehicle. The rover will fetch the tubes containing the samples of rock and soil cached by Mars 2020, then load them into the MAV’s payload container, a 17-pound cylinder about the size of a volleyball. The MAV will then be raised, likely autonomously, from a horizontal to an upright launch position and will lift off to rendezvous with the third part of the mission: an Earth Return Orbiter. The demands being placed on the design for the MAV make it the riskiest part of the mission. Ashley Karp, propulsion lead and deputy manager for the ascent vehicle at JPL, says developing the propulsion system for the rocket is the toughest engineering challenge she has worked on during her seven years at the NASA facility. “We need to fit within the entry, descent, and landing system to get us to Mars, and then to be able to launch, and deliver the samples to another system as well,” Karp says. “So there are multiple interfaces at play.” The propulsion system will require fuel that can withstand the temperature extremes of Mars while also meeting the volume and weight requirements that will allow the MAV to fit inside a Mars lander: It can be no heavier than about 880 pounds and no taller than around 10 feet. Over the last two decades, NASA engineers have toyed with multiple MAV propulsion designs and have now zeroed in on two possibilities: a single-stage hybrid rocket motor and a two-stage solid-fuel rocket motor. The key advantage of solid-fuel rockets is that the technology is well-understood, Karp says. In fact, they’ve already been used on previous missions—such as Pathfinder, Spirit, and Opportunity—to land on Mars. Solid-fuel motors are less complex than motors using liquid fuels, which require a feed system as well as either a pressurization system or pumps. And since solid propellant is less corrosive and more stable than liquid fuel, it can be easily stored for long periods. Hybrid rockets—which store the oxidizer as a liquid or gas, and the fuel as a solid—are a tougher problem to solve. Engineers have been tinkering with designs ever since 1933, when the Soviet Union launched a rocket that combined liquid oxygen and a solid form of gasoline. But unlike solid rockets, where the oxidizer and fuel are already combined into a single propellant, it’s hard to safely achieve a high thrust with hybrid rockets, because the solid-fuel component doesn’t burn quickly enough when the liquid oxidizer is sprayed on separately during flight. And yet, despite being the lesser-developed technology, NASA believes the potential advantages of a hybrid rocket for a Mars mission are too numerous to ignore. Once a solid-fuel rocket is ignited, it has to stay lit. A hybrid offers more options for maneuvers since it can be throttled, shut down, and reignited in flight. NASA is optimistic about a hybrid because of a new fuel with a higher burn rate. It’s a paraffin called SP7, a waxy solid made from a mixture of saturated hydrocarbons. The oxidizer is called MON25—a liquid oxidizer that contains 25 percent mixed oxides of nitrogen. The problem with a conventional solid propellant is that the extreme temperatures on Mars could cause it to crack and possibly explode upon ignition. As such, if NASA opted for a solid-fuel rocket motor, the lander would need to devote crucial power to keeping the MAV warm. By contrast, the waxy SP7 used in a hybrid rocket motor can remain structurally sound when exposed to wide variations in temperature and the oxidizer MON25 has a freezing point of minus 67 degrees Fahrenheit, which also offers plenty of margin for the range of temperatures expected at Jezero Crater between the time the MAV lands on Mars and lifts off a full Earth-year later. In late April, the hybrid rocket passed a crucial threshold: a successful ignition at minus four degrees Fahrenheit. “It was the first demonstration that it actually worked,” says Karp. In late July, two more tests were conducted. The first tested the rocket’s rapid ignition system for a second burn as well as a new rocket nozzle, and the second tested a tweaked SP7 formulation.