Since it launched on July 30 last year, a rocket bearing NASA’s Perseverance Rover has been charting a C-shaped course through inky-black space, hurtling toward Mars. If all goes according to plan, on February 18, the wheeled robot the size of a small car will complete its six-month-long, 292.5-million-mile journey and touch down safely on the Red Planet’s surface.

Should it succeed, Perseverance will be the fifth NASA rover ever to land on Mars, after Sojourner (1997), twin rovers Spirit and Opportunity (2004) and Curiosity (2012). The new rover, affectionately dubbed “Percy,” will scour the dusty, crater-strewn planet for evidence of ancient life and prepare the way for future human visitors.

But getting wheels on Mars is hard. Since countries began attempting to send spacecraft to the planet in the 1960s, just 40 percent of missions have succeeded. Some landers flew by Mars, missing the planet entirely, while others reached the planet but were destroyed on impact. “There’s always risk in any mission to another world,” says G. Scott Hubbard, a professor of aeronautics and astronautics at Stanford University. “You can’t avoid it.”

Hubbard knows this all too well: he became the first director of NASA’s Mars program in 1999, after two NASA missions to Mars had failed back-to-back. Since the turn of the century, NASA has had a perfect record with Mars missions, an accomplishment that Hubbard attributes to rigorous testing, money and patience. “[W]e didn’t try to swing and hit the home run each time,” he says. “We built up the experience, learned and incorporated new technologies as they were ready.”

NASA will livestream the February 18 landing on its Mars 2020 website beginning at 2:15 p.m. Eastern; the landing process is expected to begin around 3:38. Unlike the celebratory hugs and high-fiving in mission control after Curiosity’s landing in 2012, this year’s event will be more subdued because of the pandemic. Crew members at the Jet Propulsion Laboratory (JPL) in Pasadena, California, will be masked and limited to essential personnel to prevent the spread of the Covid-19 virus.

For the thousands of scientists who’ve helped Perseverance get to this point nerves will be at an all-time high. “It’s a bit surreal. Mars is looming pretty big in the window right now,” says Swati Mohan, the Mars 2020 guidance, navigation and controls operations lead who will serve as the mission commentator during the landing event.

Mohan says the team has done as much as they can before the landing. “It’s just a matter of executing,” she says. “Now, we just have to trust in our team and the hard work that we’ve put in until now, to see us through to the finish line.”

Before tuning into Mohan’s play-by-play of the Perseverance’s landing event, here are a few things to know that will help you follow along.

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What are the “seven minutes of terror”?

Descent to Mars
The spacecraft carrying Perseverance begins its descent to Mars in this illustration. NASA / JPL-Caltech

After six months of travel, the actual process of entry, descent and landing happens in just seven minutes. But because Mars is so far away from Earth, radio signals from Perseverance take about 11 minutes and 22 seconds to travel back to mission control. So, by the time mission control receives the signal that the rover has reached the top of Mars’ atmosphere, Perseverance will have already landed—or crashed.

Scientists call that tense entry, descent and landing period the “seven minutes of terror.” Not only is that period the riskiest part of the entire mission, but the delay in communication between Earth and Mars means that Perseverance has to land itself completely autonomously. “There’s no joysticking that we can do,” Mohan says.

As mission control receive signals that the rover has completed each key part of landing—for instance, the signal that chutes have been deployed—they’re watching the fate of the rover reveal itself. Even though the events already took place minutes ago, the experience of waiting for the next transmission is nerve-wracking, Hubbard says: “You really feel as though you’re in the moment and it’s happening right then.”

How do you land safely on Mars?

Entry Descent And Landing
This illustration shows the steps of the entry, descent and landing of Perseverance on Mars. NASA / JPL-Caltech

The first major milestone occurs at around 3:38 p.m. Eastern time, when NASA estimates it will learn that the rover capsule has detached from the rocket and its associated hardware. Ten minutes later, the capsule should breach Mars’ atmosphere and soon reach “peak heating” as it blisters down at a breathtaking 12,000 miles per hour. To begin to slow down, the vehicle should execute a sort of dance in the sky—similar to how a plane might do S-maneuvers as its pilots wait for the go ahead to land at an airport.

The rover should still be moving swiftly until its enormous parachute deploys. The chute will drastically slow the craft’s descent—but Mars’ atmosphere is thinner than Earth’s, and the rover needs an exceptionally soft landing, so eventually rockets should take over and slow the craft’s speed to just two miles per hour, or walking pace. As the craft descends slowly over a safe location, Perseverance will hopefully emerge from beneath it, suspended on bridles. As soon as the rover’s wheels touch the ground—around 3:55 p.m. Eastern on your computer’s clock—the bridles should be cut and the descent craft should fly off into space, mission accomplished.

What could go wrong?

Powered Descent
NASA’s Perseverance rover fires up its descent stage engines as it nears the Martian surface in this illustration. NASA / JPL-Caltech

Even the best-laid plans can still fail. When rockets fire nears the planet’s surface, they have the potential to stir up dust, rocks and sand that could potentially harm the craft. And a freak weather event like one of Mars’ notorious dust storms could derail things, although Grant says that this is highly unlikely—NASA has models to predict when and where these will occur.

Even a safe landing doesn’t mean the rover is entirely in the clear. When Curiosity landed in 2012, a wind sensor was partially broken. Scientists suspect that rocks flung upward by the rover’s rocket-powered descent may have damaged the sensor’s wiring and caused the minor setback.

Jessica Samuels, surface mission manager for Mars 2020 team, says that the best measure to avoid catastrophe has already taken place: “a lot, a lot, of testing,” she says. “The development program involves people from all over the world. We build different units and stress test everything.”

Only when Perseverance lands and sends its first picture back to Earth can scientists relax. “That’s when you go, Phew,” Hubbard says.

Where will Perseverance land, and why?

Jezero Crater
The landing ellipse in the Jezero Crater places the rover at the site of an ancient river delta which could harbor signs of fossilized microbial life. NASA / JPL-Caltech

Perseverance will touch down in Jezero Crater, a 28-mile-wide basin that boasts spectacular views of steep cliffs, sand dunes and boulder-strewn fields. (Geology enthusiasts can explore an interactive map of the rocky terrain on NASA’s website.) Jezero was likely formed on impact when something hit Mars’ surface billions of years ago.

Choosing a landing site for Perseverance took five years of intense discussion, says John Grant, senior geologist at the Center for Earth and Planetary studies at the Smithsonian National Air and Space Museum (NASM). Jezero stood out because of its history. Mars is too cold and dry for life to exist today, but one of the key missions of Perseverance is to hunt for signs of former life on the planet. “We know that Mars, at one point, looked much more similar to Earth than what it does now,” says Mariah Baker, a postdoctoral research fellow at NASM, “and so we are searching for those environments that may have potentially hosted life in the past.”

Billions of years ago, scientists believe that Jezero might have been a home for ancient microbial lifeforms. The site bears all the trademark signs of having once been a river delta: 3 to 3.5 billion years ago, water flowed into the crater through channels, pooled and flowed out, depositing layers of valuable sediments in its wake. If microbial life had been present in the sediment, it might have left behind biosignatures, chemical traces of its presence, Grant says.

Perseverance will be looking for these chemicals, which Hubbard calls the “fingerprints of life.”

How will this event compare to Curiosity’s landing in 2012?

Perseverance Rover
This illustration depicts NASA's Perseverance rover, which has some big upgrades when compared to the Curiosity rover of 2012, operating on the surface of Mars. NASA / JPL-Caltech

When Curiosity launched in 2012, it was the most advanced rover to be sent to Mars by a wide margin. Early on in its mission to Gale Crater, the rover had discovered chemical and mineral traces that suggest that Mars’ environment once had the conditions necessary to sustain microbial life.

Perseverance’s design looks a lot like Curiosity’s, but with some big upgrades. Most importantly for entry, descent and landing, Perseverance comes equipped with “terrain relative navigation.”

“Previous missions have primarily relied on radar to figure out where they are with respect to the ground,” Mohan says. She compares the method to moving around in the dark using just your hands to feel where you were going. Terrain relative navigation, on the other hand, “lets you open your eyes,” she says.

Perseverance carries a detailed map that identifies all the hazards in Jezero Crater—treacherous rocks, steep cliffs and more—as well as a camera to see with. As Perseverance decides where to land, the rover can look down, understand where it is with respect to the surface, refer to the map to maneuver a small distance and choose the safest possible landing spot.

Jezero Crater had been considered as a landing site for Curiosity but was dismissed because it was considered too risky. At the time, Curiosity’s team estimated they would have had an 80 to 85 percent chance of landing on Jezero safely. Thanks to terrain relative navigation, scientists estimate that Perseverance has a 99 percent chance of a secure landing.

Terrain relative navigation, Mohan says, “really made it possible to go to where the scientists wanted to go, as opposed to where the engineers deemed was most safe and feasible.”

If all goes well, what’s next on the agenda?

Perseverance Rock Sample
In this illustration, Perseverance uses its drill to core a rock sample on Mars. NASA hopes to retrieve such samples on a future mission. NASA / JPL-Caltech

After landing, Perseverance will unfurl itself, go through some internal tests and then hit the Martian surface running—or, rolling.

A few days after landing, Perseverance will hopefully send back some images of the entry, descent and landing process. The rover carries 19 cameras, and its landing craft carries four more—the largest number of cameras ever flown into a deep space mission. If all goes well, the cameras attached to the landing craft should send back high-resolution images of the parachute inflation, rover descent and other key parts of entry, descent and landing, which will help engineers refine the landing process for future missions. Hopefully, the images will also provide audiences with an “amazing” bird’s eye view of the rover landing, Samuels says.

Perseverance also carries a microphone—meaning that within days of landing, scientists and audiences will be able to hear what Mars sounds like. The microphone will record audio of entry, descent and landing, the “pop” of the rover’s laser camera in action, the sounds of the rover at work and the whoosh of the Martian wind, among other things. “Probably won’t be anything like The Martian with Matt Damon, but it’ll be something that is all new,” Hubbard says.

Crucially for future missions, Perseverance will also use its seven-foot-long robotic arm to collect samples from Mars’ surface that could contain tantalizing evidence of ancient microbial life. These will be stored in a safe site on Mars, and NASA hopes to return them to Earth on a future mission—meaning Perseverance is the opening leg on the most ambitious, interplanetary relay race yet.

Experiments on the rover will also pave the way for future Martian explorers. One of the rover’s sophisticated new instruments, the Mars Oxygen In-Situ Resource Utilization Experiment, a.k.a. MOXIE, intakes carbon dioxide from the atmosphere and turns it into oxygen.

Eventually, an instrument like MOXIE could help sustain human visitors on Mars—the next huge challenge on the horizon.

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