Perseverance will enter the tenuous Martian atmosphere going more than 10,000 miles per hour, which means that as it slices through the air it will experience temperatures of nearly 4,000 degrees Fahrenheit. “The heat shield really takes the brunt of hitting the atmosphere,” says Tice of the lander’s protective structure. But if it works as it’s supposed to, inside its protective casing the rover and its sensitive equipment will never experience temperatures much warmer than a hot summer day in the desert. This is because the ablative heat shield dissipates heat as its layers are burned off.
Hitting the atmosphere will also put an enormous amount of strain on the heat shield; it will experience pressures equivalent to being sat on by a jumbo jet. To keep it from cracking, engineers at Lockheed reinforced the heat shield with 2.5 inches of honeycombed aluminum that is stiff enough to handle the stress. The atmospheric drag created by the shield serves a braking function and will slow Perseverance to a few hundred miles an hour. Once the craft is about 7 miles above the surface—the cruising altitude of a passenger jet on Earth—it will use a small explosive charge to deploy its parachute. Shortly after, the rover will jettison its heat shield. That’s when things start to get truly wild.
When the heat shield clears the spacecraft, the rover will detach itself from the parachute and experience a few seconds of freefall about a mile above the surface. It’s mounted to a so-called skycrane, a rocket powered hovercraft that will guide the rover to a soft landing. The skycrane is outfitted with eight small engines that start firing when the rover is about half a mile up to rapidly slow its fall to just a few miles per hour.
“These particular engines are actually a derivative of the original engines developed for Viking landers in the early ’70s, but we’ve made very significant upgrades,” says Fred Wilson, a propulsion expert at Aerojet Rocketdyne, the company that made the propulsion system for the Perseverance landing system. For example, the new rocket engines are capable of throttling their thrust over a huge range: They can produce anywhere from 8 to 800 pounds of thrust to keep the skycrane in the air. “To my knowledge, that is the largest throttling range of any engine that’s ever been developed and flown,” says Wilson.
During the descent, the rover will use a suite of cameras to feed images to machine-vision algorithms running on a dedicated computer. These algorithms tell the rover where it is above the Red Planet, and they can divert it to a different landing location if it’s off course. Cameras pointed at the parachute and a microphone capturing audio will also record the seven-minute ride to the surface. This means Earthlings, for the first time ever, will eventually get to see a Martian landing sequence from a rover’s point of view. (It won’t be in real time, of course; it takes 20 minutes for a signal to reach Earth from Mars, and Perseverance’s computers will be focused on more important things—like not getting destroyed.)