Getting Ready for Mars: The Seven Minutes of Terror

For the past seven months, NASA’s newest Mars rover has been closing in on its final destination. As Perseverance eats up the distance and heads for the point in space that Mars will occupy on February 18, 2021, the rover has been more or less idle. Tucked safely into its aeroshell, we’ve heard little from the lonely space traveler lately, except for a single audio clip of the whirring of its cooling pumps.

Its placid journey across interplanetary space stands in marked contrast to what lies just ahead of it. Like its cousin and predecessor Curiosity, Perseverance has to successfully negotiate a gauntlet of orbital and aerodynamic challenges, and do so without any human intervention. NASA mission planners call it the Seven Minutes of Terror, since the whole process will take just over 400 seconds from the time it encounters the first wisps of the Martian atmosphere to when the rover is safely on the ground within Jezero Crater.

For that to happen, and for the two-billion-dollar mission to even have a chance at fulfilling its primary objective of searching for signs of ancient Martian life, every system on the spacecraft has to operate perfectly. It’s a complicated, high-energy ballet with high stakes, so it’s worth taking a look at the Seven Minutes of Terror, and what exactly will be happening, in detail.

Slow Down There, Buddy

To fully understand the enormity of the undertaking of landing a rover on Mars, the first thing to grasp is the scale of the problem. At the time of launch back in July, Mars and Earth were in optimal orbital positions relative to each other to keep the 480 million km crossing time to a minimum. But Perseverance was built for a very specialized mission, and therefore is headed for a very specific spot on the Red Planet.

Jezero Crater, Perseverance’s target, is an ancient lake bed about 40 km across, making it a very small target to hit on a plant the size of Mars. But that’s not the half of it. The northwest quadrant of Jezero shows evidence of a delta system, where the flow of water in the ancient river that once fed into the crater slowed and dropped its load of silt and sediment. On Earth, river deltas are highly productive biologically; planetary scientists think that if there was ever life there, the delta is the place to look. Landing near that delta system while avoiding boulder-strewn areas of the crater left mission planners aiming Perseverance at an ellipse only 7 km by 8 km.

The other problem of scale is the sheer size of Perseverance. Like the Curiosity rover that it’s based on, Perseverance is huge — about the size of a compact car. It measures three meters in length and has a mass of over 1,000 kilograms. Getting a package that size slowed down enough to land safely on the surface is a major challenge. But luckily, it’s a challenge that flight controllers have faced before, and one they conquered. Perseverance will follow nearly the same entry, descent, and landing (EDL) stages of the mission as Curiosity did back in 2012; NASA has produced a wonderful animation of the EDL phase of the mission that makes a great visual aid to the following play-by-play.

EDL starts when Perseverance sheds its supporting cruise stage, with the solar panels and maneuvering engines needed for the trip from Earth, along with two 70-kg balance masses, and first encounters the Martian atmosphere. The spacecraft will be traveling at about 20,000 km/h at that point, and will need to safely shed an enormous amount of kinetic energy in the next few minutes. The aeroshell covering the leading edge of the spacecraft will bear the brunt of this energy, turning the ablative heat shield of honeycomb aluminum and phenolic resin into a streak of plasma across the thin Martian upper atmosphere.

As on Earth, the Martian atmosphere is a turbulent place, and Perseverance will need to adjust its trajectory with continual thruster bursts to make sure it stays on track. During this guidance phase, which includes a maneuver called SUFR, or “straighten up and fly right”, the spacecraft will slow down to 1,500 km/h. It is at this point where the most dramatic part of the early EDL stage occurs: parachute deployment. The backshell of the spacecraft has a 21-m supersonic nylon and kevlar parachute tucked into it, which is deployed and inflates within 500 milliseconds. This produces over 311 kN of drag force on the spacecraft, slowing it further. If this all goes well, Perseverance  will have survived about four and a half minutes of its seven-minute thrill ride.

The Skycrane

The final 150 seconds of EDL wil be somewhat calmer than the preceding breakneck race to 11 km above the Martian surface, but there are still more dramatic events in store for the mission. After the heat shield drops off the bottom of the package at an altitude of about 10 km, downward-looking cameras will begin the process of comparing what Perseverance sees below it to a high-resolution photo of its target, stored in the rover’s memory. Called Terrain Relative Navigation, this phase of EDL was not done for Curiosity, which was one reason the landing zone for the previous rover was so much larger (20 km by 25 km). TRN should allow Perseverance a landing accuracy of about 40 meters while avoiding any large obstacles.

The TRN process only takes about 20 seconds, during which time Perseverance will still be attached to its backshell. The parachute will have bled off as much energy as it can in the thin Martian atmosphere by this point, and at an altitude of 2,000 meters and a speed of about 300 km/h, the rover, attached to its powered descent stage, will release from the backshell. After free-falling for a few seconds, the eight hydrazine-fueled thrusters ringing the descent stage will fire for about the next 30 seconds, both slowing the spacecraft further as well as maneuvering it to the landing spot selected during TRN.

Now within 20 meters of the surface, and slowed to 25 km/h of vertical speed and almost zero horizontal speed, the single most dramatic and technically complex maneuver of the whole EDL process begins: skycrane deployment. In order to prevent contamination of the landing site by descent-stage propellants and to prevent kicking up dust which could obscure cameras and damage systems on the rover, the descent stage will instead lower the 1,000-kg rover to the surface on three nylon cords. The rover’s wheels, which have been folded under its hull for the last seven months, will finally rotate into the deployed position as the descent stage gently lowers the machine to the surface. If all goes well, the rover will make a six-point touchdown at a graceful vertical speed of 2 km/h.

I’ll Fly Away

Once the rover reports touchdown, explosive “guillotine” fittings on the upper hull fire to cut the skycrane cords and the small umbilical connecting it to the descent stage. Once the ascent stage confirms that it has cleanly separated from the rover, the descent engines will throttle up and gimbal to steer the descent stage away from the landing site at a 45-degree elevation, to maximize the distance between it and the rover. The descent stage will fly until its hydrazine tanks run dry, at which point the nearly inert vehicle will crash into the Martian surface well outside of Jezero crater.

Back at the landing site, Perseverance will be undertaking some early system checks and reporting back to controllers. Once we find out about it, 11 minutes later, Perseverance will be ready to start its ground operations and its primary mission of collecting samples for a future sample-return mission. As a side benefit, the 20-odd video cameras and microphones on the rover and in the descent stage, which will be active during the EDL, will have captured the landing in great detail. It’ll be weeks before that data is uploaded back to Earth, but when it’s finally ready for viewing, it ought to be pretty spectacular.

By the time you read this, Perseverance will be only about ten days away from its date with destiny. Coordinating the fast-paced, high-stakes events of the EDL phase of the mission seems almost like an impossible feat of engineering, especially given that it must be completed autonomously. If it weren’t for the precedent of Curiosity’s successful landing, there would be ample room for skepticism that this could be pulled off at all. There will certainly be a lot of tension in Mission Control and around the world as we watch events unfold on the live stream, but with a little luck, Perseverance will be able to repeat its cousin’s success and perhaps even exceed it.

As for coverage, NASA will be live-streaming the landing on their YouTube channel, so make sure to tune in if you can. The coverage will start on February 18, 2021 at 11:15 AM Pacific Standard Time (UTC-8). We’ve created a handy time zone converter and countdown so you don’t miss the show.