NASA's Curiosity rover is zooming toward Mars and the space agency has said the nuclear-powered rover appears on course.
Curiosity is on track "to fly through the eye of the needle" for a precise, safe landing on Sunday night, NASA officials said on Saturday.
Tension will be high when the rover plummets through the thin Martian atmosphere and attempts to set its six wheels down on the surface.
Mission control engineers at the Jet Propulsion Laboratory near Los Angeles acknowledge that delivering the one-tonne, six-wheeled rover in one piece is a highly risky proposition under the best of circumstances.
JPL's team reported on Saturday that the spacecraft and its systems were all healthy and performing flawlessly, and that weather forecasts over the landing zone on Mars were favourable, as Curiosity streaked to within 4.5 million km of its destination.
"We're on target to fly through the eye of the needle," Arthur Amador, the Mars Science Laboratory mission manager, told reporters at a briefing about 36 hours before landing time.
Touchdown is scheduled for 10:31pm on Sunday Pacific time (05:31 GMT on Monday).
In what the head of NASA's Mars programme called a "daring" operation, the space agency plans to land its largest-ever rover, weighing 900kg, by carefully lowering it to the surface on cables from what amounts to a giant jet pack in a so-called sky crane manoeuvre.
Given the delicate nature of the endeavour and the $2.5bn mission price tag, a lot is at stake.
"We're about to land a small compact car on the surface with a trunk-load of instruments," Doug McCusition told reporters.
NASA has promoted the dramatic landing in a video game and is inviting space fans across the US to gather to watch the arrival live, including on the giant screens in New York's Times Square.
The landing will be followed by orbiting satellites already deployed around Mars.
It will also be streamed live on NASA's website.
Zooming toward the surface at more than 21,000kph, or 17 times the speed of sound, after it's nearly nine-month journey, the craft carrying Curiosity will decelerate using thrusters and a parachute.
Along the way it will jettison its cruising rockets, heat shield and outer shell - going through six different vehicle configurations - before gently lowering the rover to the Martian surface like a spider on a thread.
The complicated landing manoeuvre is being employed with a rover for the first time, and has been dubbed the "seven minutes of terror" by NASA staff.
Earlier efforts - with much smaller rovers - always involved an airbag-like structure.
The new routine involves Curiosity steering itself for the final phase of landing. Due to the signal time lag between Mars and Earth (it takes about 14 minutes for a signal on Mars to reach Earth), Curiosity will execute the landing autonomously, following the half a million lines of computer code designed by Earthlings.
"Can we do this? Yeah, I think we can do this. I'm confident,'' McCuistion said on Saturday. "We have the A-plus team on this. They've done everything possible to ensure success, but that risk still exists."
One Martian year
The mission will spend at least one Martian year - nearly two Earth years - studying Mars' Gale crater, in a bid to transition from the search for water to a wider search for the presence of other ingredients necessary for life, such as carbon. It will also study minerals on the surface to get an idea what conditions were like on the planet millions of years ago.
The Gale crater is nearly 154km in diameter and features a mountain that rises some 5km above the surface. The massive feature includes layers of rock strata that will provide a virtual history of Mars' geological past.
Curiosity will make use of a range of new instruments. Armed with two cameras atop a mast, Curiosity can take three-dimensional and panoramic images, and shoot a laser into rocks to determine their component chemical elements.
A 2m long robotic arm can be extended out from the rover to examine its surroundings more closely, and a drill will allow it to take samples from inside rocks.
The area has already been studied extensively from orbiting spacecraft, and scientists hope that Curiosity will provide clues to a probable wet Martian past. The area contains clay and sulfate-rich areas, where organic compounds necessary to life could be found.
Like its predecessors, Curiosity is equipped with a series of instruments to analyse the composition of the samples. It builds on the work of past rovers, including Opportunity, one of a pair of water-hunting twin rovers that continued functioning years beyond their orginal missions.
NASA eventually hopes to send a manned mission to Mars, and robotic missions to Earth's nearest planetary neighbour have continued. Mars is the chief component of NASA's long-term deep space exploration plans.