March 02, 2021 by Lianne Frith
On February 18th 2021, the most advanced robot ever sent to Mars landed safely, following the spacecraft’s ‘seven minutes of terror’ touchdown on an especially dangerous Martian terrain. We recap the events and discuss the engineering that helped NASA prove that perseverance really does pay off.
The Perseverance rover is the heart of NASA’s $2.7 billion Mars 2020 mission. The mission aims to determine whether life has ever existed on Mars, characterise the planet’s climate and geology, and prepare for human exploration.
The Mars 2020 mission had a month-long launch window, which wouldn’t be replicated for another 26 months. During this time, Earth and Mars were in a position in their orbits that required less launch energy compared to that of preceding missions. Meanwhile, the task of liftoff was made ever-more challenging by the COVID-19 pandemic as assembly and various protocols had to be reconsidered for safety reasons.
Lifting off from Florida’s Space Complex on top of a United Launch Alliance Atlas V rocket on the 30th of July, 2020, about halfway through the launch window, the rover then travelled through space for over six months before reaching the Red Planet. During the cruise, a large solar array provided power to the rover. Meanwhile, radio antennas kept the vehicle in contact with Earth. After a much-anticipated wait, Perseverance alighted gently on an ancient lakebed inside the 28-mile-wide Jezero Crater on the 18th of February, 2021.
A graphic depiction of NASA’s Perseverance rover landing safely on Mars
Image credit: NASA
Over the decades, only 47% of Mars’s surface missions have landed successfully. However, Perseverance had additional obstacles in its way: firstly, the rover’s landing site on Jezero’s ground was the toughest terrain for any rover to navigate: it is covered with hazards, such as cliffs, sand dunes, and boulder fields for the vehicle to navigate. And secondly, its landing ellipse was around four miles squared, whereas its predecessor, Curiosity, had a comparatively manageable four by eight miles to contend with. The conditions demanded the most precise Red Planet touchdown ever for the Mars 2020 mission.
The other challenge concerned the weight of the rover. Perseverance is only a few inches longer than Curiosity was, but, with a weight of 1,025 kilograms, it is just under 14% heavier than the latter’s weight of 899 kg. Nevertheless, the rover successfully plunged into Mars’s rarefied air (very low Oxygen)—which has just ~1% of the thickness of Earth’s air—at more than 20,000 kilometres per hour. The challenge for NASA’s engineers was to then slow the 2,260 lb spacecraft down to a near-walking pace by the time it reached the Red Planet’s surface.
The definition of ‘perseverance’ means to persist to make progress or achieve success despite obstacles, and that is exactly what NASA’s Mars 2020 team has achieved in transporting its rover to Mars. While the scientific instruments on Curiosity and Perseverance are quite different, the two rovers share a similar nuclear power source and body plan. They therefore used the same strategy to land on the Red Planet, which involved allowing spacecraft parts to fall away one by one until the rover landed safely on solid Martian ground.
The Perseverance rover did, however, have various new EDL (entry, descent and landing) technologies at its disposal, which we’ll cover in the next subsections.
A photograph of Mars’s Jezero Crater superimposed with NASA’s diagram of the chain of events that occurred in the EDL (entry, descent and landing) phase of the Perseverance rover’s nearly seven-month journey to Mars.
Image credit: NASA
The aeroshell consists of two major components: the backshell and the heat shield. The backshell houses, in addition to the heat shield, additional thrusters that fire during guided entry, and a canister to fire the parachute. The latter is there to slow the vehicle down during its final approach while protecting the rover from heat levels potentially as high as 2,370°F.
Meanwhile, a next-generation aeroshell sensor package called MEDLI2 (or Mars Entry, Descent, and Landing Instrumentation 2) collects data from both the backshell and the heat shield. By measuring such temperatures and pressures and tracking performance, NASA can improve future EDL systems, reducing risks to both robotic and even human missions.
While the Perserverence’s protective capsule does most of the work of lowering the spacecraft’s entry speed, it’s the supersonic parachute that is vital for the last three minutes of braking and surface placement. Meanwhile, ‘Range Trigger’ technology is used to deploy the 20.5-metre-wide supersonic parachute at just the right moment (based on the sensors’ detection of the landing zone relative to the vehicle’s position). The technology reduces the size of the landing ellipse by over 50%, enabling the rover to land more precisely at a site where a larger ellipse would be too risky.
On separating from the aeroshell, the jetpack uses eight engines to slow the final descent and leverages a landing radar system to make last-minute decisions about the touchdown. The Terrain-Relative Navigation system takes pictures of the fast-approaching surface and then quickly compares them to its stored map of the landing site.
This means that the rover can assess the Jezero landscape, avoid the dangerous terrain (around 335 metres in diameter), and divert towards safer ground during the descent. Then, just before touchdown, the sky crane gently lowers the rover to the Martian surface on cables before flying off to intentionally crash-land a safe distance away.
A black and white photograph of Mars’s Jezero Crater. To reflect the possible route that the Mars 2020 Perseverance rover may take to find signs of ancient life on this crater, a green line has been superimposed over the photograph.
Image credit: NASA
Having successfully made it through the EDL stage, Perseverance is ready to get to work. Initial reports suggest that the power system looks good and the batteries are fully charged. After further instrument and hardware checkouts, the rover will start to hunt for signs of ancient life and collect rock samples.
The mission continues a chain of innovation that looks set to contribute greatly to both science and technology—altogether pushing the boundaries of what is possible by continual perseverance.
