The stakes could not be higher for the US space agency's Perseverance rover.
The six-wheeled robot is fast approaching the end of its seven-month journey from Earth and will very shortly plunge into the atmosphere of Mars.
It's got to put itself down safely on the Red Planet - a task that has befuddled so many spacecraft before it.
But if Perseverance is successful, it has an amazing opportunity to find signs of past life on Mars.
Never has a science mission gone to the planet with so sophisticated a suite of instruments; never has a robot been targeted at so promising a location.
Jezero Crater, the intended touchdown zone, bears all the hallmarks in satellite imagery of once having held a giant lake. And where there's been abundant water, perhaps there's been biology as well.
Perseverance is on a path to engage the Martian atmosphere late on Thursday (GMT).
It will plunge into the rarefied air at more than 20,000km/h and it will have to reduce this to near-walking pace by the time it reaches the surface - a daunting prospect that Nasa engineers half-jokingly call the "seven minutes of terror".
The robot's protective capsule will do most of the work of scrubbing off the entry speed but a supersonic parachute and a rocket jetpack, or "Sky crane", will be needed for the last three minutes of braking and surface placement.
Much is made of the difficulty of mounting Mars missions, and it's true many have failed. But the statistics actually favour Perseverance.
Of the 14 landing attempts at the planet, eight have been successful - all of them American. Indeed, Nasa has only got it wrong once, way back in 1999.
Engineers will be following proceedings at mission control at the Jet Propulsion Laboratory in Pasadena, California.
Telemetry from the rover during its descent will be relayed by an overflying satellite - the Mars Reconnaissance Orbiter. The team will also be listening to a series of low-data tones coming back direct from the robot itself.
A signal confirming touchdown should arrive at Earth at 20:55 GMT.
With luck, we could even be looking at the first pictures of Jezero just a few minutes afterwards.