After sailing 301 million miles (548 million km) on a six-month voyage through deep space, the robotic lander InSight was due to touch down on the dusty, rock-strewn surface of the Red Planet at about 3 p.m. EST (2000 GMT).
The mission control team at NASA's Jet Propulsion Laboratory (JPL) near Los Angeles conducted a final adjustment to the InSight's flight path on Sunday to steer the spacecraft closer to its target arrival point over Mars.
"We're going to be on pins and needles," NASA Administrator Jim Bridenstine told Reuters in Pasadena. "We are going to be crossing our fingers, doing everything we can to make sure that this is a successful landing."
If all goes according to plan, InSight will hurtle through the top of the thin Martian atmosphere at 12,300 miles (19,795 km) per hour. Slowed by friction, deployment of a supersonic parachute and the firing of retro rockets, InSight will descend 77 miles through pink Martian skies to the surface in 6-1/2 minutes.
The stationary probe, launched in May from California, will then pause for 16 minutes for the dust to settle, literally, around its landing site, before disc-shaped solar panels are unfurled like wings to provide power to the spacecraft.
Engineers at JPL hope to receive real-time confirmation of the craft's arrival from data relayed by a pair of miniature satellites that were launched along with InSight and will be flying past Mars.
The JPL controllers also expect to receive a photograph of the probe's new surroundings on the flat, smooth Martian plain close to the planet's equator called the Elysium Planitia.
The smaller, 880-pound (360 kg) InSight - its name is short for Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport - marks the 21st U.S.-launched Mars mission, dating back to the Mariner fly-bys of the 1960s. Nearly two dozen other Mars missions have been sent from other nations.
InSight will spend 24 months - about one Martian year - using seismic monitoring and underground temperature readings to unlock mysteries about how Mars formed and, by extension, the origins of the Earth and other rocky planets of the inner solar system.
While Earth's tectonics and other forces have erased most evidence of its early history, much of Mars - about one-third the size of Earth - is believed to have remained largely static, creating a geologic time machine for scientists.
InSight's primary instrument is a French-built seismometer, designed to record the slightest vibrations from "marsquakes" and meteor impacts around the planet. The device, to be placed on the surface by the lander's robot arm, is so sensitive it can measure a seismic wave just one half the radius of a hydrogen atom.
Scientists expect to see a dozen to 100 marsquakes during the mission, producing data to help them deduce the depth, density and composition of the planet's core, the rocky mantle surrounding it, and the outermost layer, the crust.
The NASA Viking probes of the mid-1970s were equipped with seismometers, too, but they were bolted to the top of the landers, a design that proved largely ineffective.
Apollo missions to the moon brought seismometers to the lunar surface as well. But InSight is expected to yield the first meaningful data on planetary seismic tremors beyond Earth.
A second instrument, furnished by Germany's space agency, consists of a drill to burrow as much as 16 feet (5 meters) underground, pulling behind it a rope-like thermal probe to measure heat flowing from inside the planet.
Meanwhile, a radio transmitter will send back signals tracking Mars' subtle rotational wobble to reveal the size of the planet's core and possibly whether it remains molten.
NASA officials say it will take two to three months for the main instruments to be deployed and put into operation. (Reporting and writing by Steve Gorman in Pasadena; Additional reporting by Pavithra George in Pasadena Editing by Michael Perry and Tom Brown)
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