With just a day to go, NASA's InSight spacecraft aimed for a bull's-eye touchdown on Mars, zooming in like an arrow with no turning back.

InSight's journey of six months and 300 million miles (482 million kilometers) comes to a precarious grand finale Monday afternoon.

This illustration made available by NASA in 2018 shows the InSight lander drilling into the surface of Mars. InSight, short for Interior Exploration using Seismic Investigations, Geodesy and Heat Transport, is scheduled to arrive at the planet on Monday, Nov. 26, 2018. [Photo: AP]

The robotic geologist — designed to explore Mars' insides, surface to core — must go from 12,300 mph (19,800 kph) to zero in six minutes flat as it pierces the Martian atmosphere, pops out a parachute, fires its descent engines and, hopefully, lands on three legs.

It is NASA's first attempt to land on Mars in six years, and all those involved are understandably anxious.

NASA's top science mission official, Thomas Zurbuchen, confided Sunday that his stomach is already churning. The hardest thing is sitting on his hands and doing nothing, he said, except hoping and praying everything goes perfectly for InSight.

"Landing on Mars is one of the hardest single jobs that people have to do in planetary exploration," noted InSight's lead scientist, Bruce Banerdt. "It's such a difficult thing, it's such a dangerous thing that there's always a fairly uncomfortably large chance that something could go wrong."

Earth's success rate at Mars is 40 percent, counting every attempted flyby, orbital flight and landing by the U.S., Russia and other countries dating all the way back to 1960.

But the U.S. has pulled off seven successful Mars landings in the past three decades. With only one failed touchdown, it's an enviable record. No other country has managed to set and operate a spacecraft on the dusty red surface.

InSight could hand NASA its eighth win.

It's shooting for Elysium Planitia, a plain near the Martian equator that the InSight team hopes is as flat as a parking lot in Kansas with few, if any, rocks. This is no rock-collecting expedition. Instead, the stationary 800-pound (360-kilogram) lander will use its 6-foot (1.8-meter) robotic arm to place a mechanical mole and seismometer on the ground.

The self-hammering mole will burrow 16 feet (5 meters) down to measure the planet's internal heat, while the ultra-high-tech seismometer listens for possible marsquakes. Nothing like this has been attempted before at our smaller next-door neighbor, nearly 100 million miles (160 million kilometers) away.

No experiments have ever been moved robotically from the spacecraft to the actual Martian surface. No lander has dug deeper than several inches, and no seismometer has ever worked on Mars.

By examining the deepest, darkest interior of Mars — still preserved from its earliest days — scientists hope to create 3D images that could reveal how our solar system's rocky planets formed 4.5 billion years ago and why they turned out so different. One of the big questions is what made Earth so hospitable to life.

The planetary know-how gained from InSight's $1 billion, two-year operation could even spill over to rocky worlds beyond our solar system, according to Banerdt. The findings on Mars could help explain the type of conditions at these so-called exoplanets "and how they fit into the story that we're trying to figure out for how planets form," he said.

Concentrating on planetary building blocks, InSight has no life-detecting capability. That will be left for future rovers. NASA's Mars 2020 mission, for instance, will collect rocks for eventual return that could hold evidence of ancient life.