After a seven-month journey, NASA's InSight probe is about to reach Mars and – if touchdown goes to plan – embark on an unprecedented mission to map the planet's interior.
The lander will aim for one of the dullest parts of the planet's dusty surface, Elysium Planitia, and a vast lava plain that the US space agency calls "the largest parking lot on Mars." The flat, rockless expanse was considered the perfect place for InSight to record tremors unleashed by "Marsquakes" and to measure heat flow in the planet's upper layers.
The $ 814m (£ 633m) mission will help scientists understand the Martian core, crust and mantle, allowing them for the first time to know how the planet formed at the dawn of the solar system 4.6bn years ago.
Landing and probing on Mars remains a tricky business for rocket scientists: only 40% of missions have succeeded. NASA is the only space agency to have pulled off a landing on the planet, most recently in 2012 when the Curiosity rover was rolled to the surface by a "sky crane". In 2016, the European Space Agency attempted to put a lander on Mars, but the Schiaparelli probe switched off its retro-rockets too soon and smashed into the ground.
The InSight lander is expected to touch at about 8pm GMT on Monday, and multiple critical steps will need to work perfectly. The spacecraft will hurtle into the thin Martian atmosphere at 13,200mph, deploy a parachute and fire 12 retro-thrusters to cushion its landing. Nasa calls the "seven minutes of terror" for the entry, descent and landing phase of its Mars missions. Confirmation of the landing will be beamed back to Earth via two experimental cube satellites that have trailed the probe to its destination.
On the surface of Mars, InSight (for Interior Exploration using Seismic Investigations) will draw on a suite of instruments to study the planet's internal structure. A seismometer deployed by a robot arm will act as an ear to the ground and listen for tremors produced when underground rock faces slip past one another along geological faultlines. Bruce Banerdt, the mission's principal scientist at NASA's Jet Propulsion Laboratory in California, expects InSight to record anything from a dozen to 100 Marsquakes of magnitude 3.5 or more over the two-year mission of Lander. The seismometer is so sensitive that it can detect vibrations smaller than the width of an atom.
Another instrument onboard the lander is a spear-like heat probe that will burrow vertically into the ground and measure the rate at which heat is escaping from the planet. The interior of Mars is still cooling off the hot days of its assembly in the solar system, and as heat leaves the planet, the surface contracts and wrinkles.
A third InSight experiment will exploit two radio antennas on the lander. Mission scientists can use these to track the lander's position with extreme precision, and thus deduce how much Mars wobbles on its axis as it orbits the sun. The amount it wobbles reflects the size of the planet's core and whether it is molten or solid.
Lori Glaze, the acting director of the NASA's planetary science division, said: "Once InSight is settled on the red planet and its instruments are deployed, it will start collecting valuable information about the structure of Mars's deep interior – information that will help us understand the the formation and evolution of all rocky planets, including the one we call home. "