Seven months after blast-off, Nasa's Mars 2020 mission will have to negotiate its shortest and most intense phase on Thursday: the "seven minutes of terror" it takes to slam the brakes and land the Perseverance rover on a narrow target on the planet's surface.
Entry, Descent, and Landing (EDL) begins when the spacecraft carrying Perseverance strikes the Martian atmosphere at nearly 20,000 kilometres per hour.
It ends around seven minutes later with the rover at rest on the surface.
Touchdown on the Jezero Crater is scheduled for 2055 GMT. Weather conditions so far appear favourable in the Martian northern hemisphere spring, but nothing is taken for granted.
"This is one of the most difficult manoeuvres that we do in this business, and almost 50% of the spacecraft that had been sent to the surface of Mars have failed," Matt Wallace, the mission's deputy project manager said.
Atmospheric entry
Ten minutes before entering the Martian atmosphere, the spacecraft sheds its cruise stage that supplied the fuel tanks, radios, and solar panels on the voyage.
It is left with just a protective aeroshell, carrying the rover and descent stage, and it fires thrusters to make sure its heat shield is forward facing.
At about 130 kilometres altitude, it careens into the atmosphere and things start to get hot: peak heating occurs about 80 seconds in when the heat shield surface reaches about 1,300°C.
Perseverance is tucked away safely in the aeroshell, only experiencing room temperature.
The craft might need to fire thrusters to stay on course as it hits air pockets.
Parachute deployment
Once the spacecraft has slowed down to less than 1,600 kilometres an hour, it's time to deploy the 21.5 meters wide supersonic parachute at an altitude of 11 kilometres.
Perseverance is deploying a new technology called Range Trigger that decides the precise moment to deploy, based on the craft's position relative to the landing site.
Asked to name the single most critical event, Nasa's EDL lead Allen Chen said: "Obviously there's a lot of concentrated risk in supersonic parachute opening."
To try out its new design, Nasa had to carry out extensive supersonic parachute testing from high altitudes here on Earth, a field of research that had been dormant since the 1970s.
Heat shield separation
Next, the spacecraft jettisons its heat shield, around 20 seconds after the parachute has been deployed. The rover is exposed to the atmosphere for the first time, and uses a landing radar to bounce signals off the surface and calculate its precise altitude.
The mission will also see another technology deployed for the first time: the "Terrain Relative Navigation" (TRN) system that uses a special camera to identify surface features and compare them to an onboard map where engineers pre-programmed the safest landing sites.
"That gives our vehicle eyes, and the ability to really see where she's going and figure out where she is," said Chen.
Powered descent
In the thin atmosphere of Mars, the parachute will only get the vehicle down to 300 kilometres per hour -- so Perseverance has to cut the chute loose, dispense with its back shell, and use rocket thrusters to ease itself down.
It does this using an eight-engined jetpack that's installed directly above the rover and fires up at around 2100 meters above the surface.
The vehicle has to tilt right away in order to avoid the falling parachute and back shell, then uses its sophisticated systems to continue its descent.
Skycrane
With 12 seconds to go, at a height of 20 meters, the rocket-powered descent stage lowers the rover down to the ground using long cables in a manoeuvre called "skycrane."
The rover locks its legs and wheels into a landing position and touches the ground at a little less than 1.2 kilometres an hour, as the descent stage flies off and makes its own controlled landing.
Perseverance is now set for its mission as Earth's fifth rover on Mars.
