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Tuesday, March 02, 2021

Explained: What makes NASA’s Perseverance rover landing on Mars difficult?

Typically, a trip to Mars, which is about 300 million miles away, takes about seven-eight months. Perseverance was launched on July 30, 2020 during the window when Mars and Earth were the closest to each other.

By: Explained Desk | New Delhi |
February 16, 2021 2:39:19 pm
Perseverance rover uses its drill to collect a rock sample on Mars in this undated artistic conceptual illustration handout. (NASA/JPL-Caltech/Handout via Reuters)

On Thursday, NASA’s Perseverance rover is expected to land at the Jezero Crater on the Red Planet, after which it will resume work to look for signs of past life.

Landing is the shortest but one of the most significant and also difficult phases of the mission. NASA says that because it is difficult, only 40 percent of the missions ever sent to Mars by space agencies across the world have been successful because “hundreds of things” have to go right for the nail-biting drop.

To give some context to the difficulty of launching missions such as this one, there is the example of research mathematician Katherine Johnson. In the 2016 film Hidden Figures, Taraji P. Henson played Johnson who worked to figure out precise calculations that determined the trajectory of the capsule that would launch John Glenn into space in 1962.

Other missions to Mars

Another Mars mission, the UAE’s Al Amal (Hope)–the Arab world’s first such mission–entered the Martian orbit last week. However, this is an orbital mission and does not involve landing on the planet’s surface. Apart from the UAE, China also launched a Mars mission during the July-August window.

In light of such ambitious space missions, some astrobiologists have expressed concerns about possible ‘interplanetary contamination’. This means transporting Earth-based microbes to other celestial bodies and bringing extraterrestrial microbes back to Earth. The Committee on Space Research (COSPAR) lays down a ‘planetary protection policy’ that aims to limit the number of microbes sent to other planets, as well as ensuring that alien life does not cause havoc on Earth.

How does a spacecraft reach Mars?

Typically, a trip to Mars, which is about 300 million miles away, takes about seven-eight months. Perseverance was launched on July 30, 2020 during the window when Mars and Earth were the closest to each other. This window is important since the two planets orbit around the Sun at different speeds and every two years, the planets are in a position where they are the closest to each other. Space agencies look to launch their spacecraft during this window since the closer distance means using less rocket fuel.

According to an analysis done by Purdue University, the cost of solid rocket propellant is estimated at $5 per kg. However, the car-sized Perseverance rover is using a nuclear-powered system. In nearly 30 years, it will become the first rover to use domestically produced plutonium created by national laboratories in the US. The rover will be powered by a generator that will convert heat generated by the natural decay of plutonium-238 into electricity, which will keep the rover and its tools running once it lands on Mars.

What is the cost of the Perseverance mission?

NASA is estimated to spend $2.7 billion on the mission, which includes spacecraft development, launch operations and the costs of maintaining operations once it lands on Mars.

According to The Planetary Society, using plutonium-238 as fuel has driven up the cost of the mission since nuclear material is linked to elevated environmental and safety regulations. The total cost of the mission is equivalent to the amount of money Google makes in six days, or the amount of money Americans spend on their pets every 10 days or equivalent to 33 hours of running the US Department of Defense, the society says.

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Why is landing on Mars difficult?

Entry, descent and landing (EDL), is what the most intense phase of the Mars 2020 mission is called. For the Perseverance rover to successfully land on Mars, a number of things need to go right. The EDL phase begins when the rover will reach the top of the Martian atmosphere travelling at a speed of 20,000 km per hour. The challenge here for the rover is to decrease its speed from roughly 20,000 km per hour to zero and at the same time land on a narrow surface on the crater.

The EDL phase, NASA says, will end in seven minutes when the rover will be stationary on the surface of the planet. After this, the brakes need to be applied in a very “careful, creative and challenging way”.

Ten minutes before entering the atmosphere, the rover will shed its cruise stage, which consists of the solar panels, radios and fuel tanks that are used during the flight. Only the protective aeroshell, which consists of the rover and the descent stage will make the trip to the planet’s surface. Now, as the spacecraft enters the Martian surface, it will be slowed by drag, which is when the friction of a planet’s atmosphere works against the surface of a spacecraft, thereby slowing it down and lowering its orbital altitude.

While the drag slows the spacecraft down, it also heats it up and peak heating occurs about 80 seconds after the rover enters the atmosphere. But this does not affect the rover, which is inside the aeroshell at room temperature.

While it is descending through the atmosphere, the spacecraft will need to fire off small thrusters to stay on course, since it can be nudged off course because of small pockets of air with varying densities. Subsequently, the heat shield will slow down the spacecraft to about 1,600 km per hour, at which point (about 240 seconds after entry) the supersonic parachute will be deployed.

Twenty seconds after the parachute is deployed, the heat shield separates and the rover is exposed to the planet’s atmosphere for the first time. At this point, the parachute is working to further slow down the vehicle. But because Mars’s atmosphere is thin, the vehicle is still travelling towards the surface at a speed of 320 km per hour. Therefore, for a safe landing, the rover needs to abandon the parachute and make the rest of the trip using rockets, part of the descent stage, whose engines will be fired up when the rover is about 2,100 meters above the surface.

The final descent speed of the rover is about 2.7 km per hour, which is slower than what an average human can cover on foot in an hour–about 5 km. At this stage, with about 12 seconds left for touchdown on the surface at about 66 feet above the surface, the rover is lowered on a set of cables. As the rover senses that its wheels have touched the ground, it cuts the cables, which then make their independent uncontrolled landing on the surface, somewhere away from the rover.

A United Launch Alliance Atlas V rocket carrying NASA’s Mars 2020 Perseverance Rover vehicle takes off from Cape Canaveral Air Force Station in Cape Canaveral, Florida, U.S. July 30, 2020. (NASA/Joel Kowsky/Handout via Reuters)

What will the Perseverance rover do on Mars?

Perseverance will spend one Mars year (two years on Earth) on the planet during which it will explore the landing site region. The Jezero crater where it will land was once the site of an ancient river delta (scientists know this because of evidence collected during previous landed and orbital missions that point to wet conditions on the planet billions of years ago).

If Mars once harboured a warmer atmosphere enabling water to flow in its ancient past (3.5-3.8 billion years ago), and if microbial life existed on it, it is possible that it exists in “special regions” even today.

The rover is carrying with it seven instruments, which include an advanced camera system with the ability to zoom, a SuperCam, which is an instrument that will provide imaging and chemical composition analysis and a spectrometer. One of the most interesting instruments aboard the rover, however, is called MOXIE, which will produce oxygen from Martian atmospheric carbon dioxide. If this instrument is successful, then future astronauts (as of now, no human has kept foot on Mars) can use it to burn rocket fuel for returning to Earth.

The rover will also carry Ingenuity, the first helicopter to fly on Mars. This will help collect samples from the surface from locations where the rover cannot reach. Overall, the rover is designed to study signs of ancient life, collect samples that might be sent back to Earth during future missions and test new technology that might benefit future robotic and human missions to the planet.

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