Nasa rover bearing down on Mars

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The US space agency’s Perseverance rover is now just three weeks from arriving at Mars.

The robot’s current distance to the Red Planet is still some 4.5 million km (3 million miles), but that gap is closing at a rapid rate.

The biggest, most sophisticated vehicle ever sent to land on another planet, the Nasa robot is being targeted at a near-equatorial crater called Jezero.

Touchdown is expected shortly before 2100 GMT on Thursday 18 February.

To get down, the Nasa rover will have to survive what engineers call the “seven minutes of terror” – the time it takes to get from the top of the atmosphere to the surface.

The “terror” is a reference to the daunting challenge that is inherent in trying to reduce an entry speed of 20,000km/h to something like walking pace at the moment of “wheels down”.

“When the scientists look at our landing site, Jezero Crater, they see the scientific promise of everything: the remains of an ancient river flowing in and flowing out of this crater and think that’s the place to go to look for signs of past life. But when I look at Jezero, I see danger,” says Allen Chen, the engineer who leads the Entry, Descent and Landing (EDL) effort for Perseverance.

“There’s danger everywhere. There’s this 60-80m-tall cliff that cuts right through the middle of our landing site. If you look to the west, there are craters that the rover can’t get out of even if we were to land successfully in one of them. And if you look to the east, there are large rocks that our rover would be very unhappy about if we put down on them,” he told BBC News.

Fortunately, Perseverance has some tried and tested technologies that should ensure it reaches a safe point on the surface. Among them is the famous “Skycrane” jet pack that successfully landed Nasa’s previous rover, Curiosity, eight years ago.

There are even some additions designed to improve reliability. The parachute system that slows the atmospheric descent from super- to sub-sonic speeds now has something called “range trigger”. This more precisely times the opening of the parachute to bring the rover closer to its notional bulls-eye.

Unlike Curiosity which just opened the chute when it reached a pre-determined velocity, Perseverance will check its surroundings first before issuing the command.

Allied to this is Terrain Relative Navigation. Perseverance will be examining the ground below and checking it against satellite imagery of the crater to better gauge its position.

It’s like you or I looking out the window of our car and then looking back at a map to see where we are, says Chen.

“That’s what we’re asking Perseverance to do on her own, to figure out where she is, and then fly to known safe spots that are nearby.”

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