An Atlas 5 rocket is poised for launch from California Saturday to send a robotic lander to, the centerpiece of a $1 billion mission to map the hidden interior of the red planet by recording the faint vibrations of remote marsquakes and hammering a probe into the crust to take the temperature of its core.
In a third investigation, scientists will measure tiny changes in the InSight lander's radio signals as the planet rotates to tease out the orientation of its polar axis as it slowly wobbles, or precesses, due to the core "sloshing around" in the deep interior. From that, they hope to determine the core's size, density and composition.
"The goal of InSight is nothing less than to better understand the birth of the Earth, the birth of the planet we live on, and we're going to do that by going to Mars," said Bruce Banerdt, principal investigator of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport -- InSight -- mission.
"How we get from a ball of featureless rock into a planet that may or may not support life is a key question," he said. "And these processes that do this all happen in the first few tens of millions of years. We'd like to be able to understand what happened, and the clues to that are in the structure of the planet that gets set up in these early years."
But Earth is very active, and much of the early geologic record has been destroyed by continental drift and the churning of the mantle. As a result, "we're kind of at a loss to see, on the Earth, what's the evidence for what happened in those early years," Banerdt said.
"But we can go to Mars. Mars is a smaller planet, it is less active than the earth, and so it has retained the fingerprints of those early processes in its basic structure -- the thickness of the crust, the composition of the mantle, the size and composition of its core.
"And by mapping out these boundaries, these various different sections of the inside of the planet, we can then understand better how the planet formed and how our planet got to be the way it is where we can actually live and play and have a good time."
Since 1964, the United States has launched 22 robotic spacecraft to Mars at a cost of more than $20 billion in an evolving campaign to map out the red planet's surface, determine the role of water in its history and to search for signs of past habitability and the organic building blocks of life.
InSight, built by Lockheed Martin and based on the design of NASA's successful Phoenix lander, is the first mission dedicated solely to the interior of Mars. It is expected to operate for at least two years, or one 687-day martian year.
It all begins with launch from pad 3 at Vandenberg Air Force Base northwest of Los Angeles at 4:05 a.m. PDT (GMT-7; 7:05 a.m. EDT) Saturday, two years late because of a problem with the seismometer instrument that forced NASA to pass up the previous Mars launch window.
Forecasters predicted an 80 percent chance of heavy fog Saturday and Sunday, but mission managers had the option of waiving a visibility constraint if all other systems are "go."
All previous interplanetary missions launched from the United States took off from Florida, taking advantage of Earth's eastward rotation to gain additional velocity. InSight is the first to be launched from the West Coast, where rockets fly away to the south into polar orbits and cannot take advantage of the planet's rotation.
The move was prompted by a busy Cape Canaveral launch schedule and the Atlas 5's ability to make up for the lost velocity.
As a result, early risers in southern California and western Mexico, weather permitting, will have a chance to watch InSight's fiery departure as it takes off from Vandenberg on a southerly trajectory over the Pacific Ocean.
If all goes well, the Atlas 5's Centaur second stage will propel the spacecraft out of an initial parking orbit and onto a 171-day trajectory to Mars. Regardless of when it takes off during the 2018 Mars launch window, InSight will reach the red planet on Nov. 26, setting up what has come to be known as "seven minutes of terror."
That's how long it will take InSight to reach the surface of Mars after slamming into the thin martian atmosphere at a blistering 13,200 mph.
"That's when the project manager, and maybe the principal investigator, are completely terrorized by all the possible things Mars could throw at us," said Tom Hoffman, the InSight project manager.
"Hopefully, we won't get any surprises on our landing day, but you never know. We've done a lot of testing, a lot of analysis to make sure we've done everything we possibly can to land safely, and I believe we're going to do that."
After separating from its interplanetary cruise stage, InSight, nestled inside a flying saucer-shaped "aeroshell" and protected by a state-of-the-art heat shield, will plunge into the martian atmosphere at an altitude of about 80 miles.
NASA's Mars Reconnaissance Orbiter will be flying overhead at the time, recording telemetry from InSight for later relay to Earth. In addition, two small "cubesat" spacecraft launched along with InSight, the first such micro spacecraft to make an interplanetary voyage, will attempt to relay data back to Earth in realtime as they race past the planet in a technology demonstration.
InSight will endure deceleration forces up to 7.4 times Earth's gravity and heat shield temperatures up to 2,700 degrees Fahrenheit. Just under four minutes after entry, at an altitude of 7.5 miles and a velocity of some 928 mph, InSight's large parachute will unfurl to rapidly slow the descent.
Fifteen seconds later, now descending at 295 mph, the no-longer-needed heat shield will be jettisoned, exposing the bottom of the lander to view, and 10 seconds after that, three landing legs will deploy and lock in place.
The lander's radar will be activated to help the flight computer determine the spacecraft's altitude and rate of descent and finally, less than a mile above the surface and descending at about 136 mph, InSight will be released from the aeroshell to fall on its own.
Moments later, 12 small rocket motors will fire up, horizontal velocity will be nulled out, the lander will orient itself so its solar arrays will receive maximum sunlight on the surface and it will descend to a five-mile-per-hour touchdown in the Elysium Planitia area 4.5 degrees north of the martian equator at around 2 p.m. local time.
Fifteen minutes after touchdown, InSight's two circular solar arrays should unfurl to begin battery charging.
InSight is equipped with two primary instruments: the Seismic Experiment Interior Structure -- SEIS -- seismometer, provided by the French space agency, CNES, and the Heat Flow and Physical Properties Probe -- HP3 -- provided by the German Aerospace Agency, DLR. The two instruments cost the European space agencies about $180 million.
To collect the desired science, both instruments must be lifted from InSight's upper deck and placed on the ground near the lander using a multi-joint 7.8-foot-long robot arm. Before that happens, cameras will carefully photograph the surrounding terrain to identify the smoothest, rock-free areas.
Engineers at the Jet Propulsion Laboratory then will use an engineering mockup of the lander to repeatedly practice picking up the instruments and lowering them in place. Finally, the seismometer will be placed on the martian soil, a protective weather shield will be lowered on top of it and the heat probe will be set on the surface nearby.
Trailing data cables will link the instruments to InSight's computer and communications systems. The entire process is expected to take about 10 weeks to complete.
The seismometer "is really the heart of the InSight mission," Banerdt said. "The seismometer is what allows us to see deep into the planet, to have insight into the planet, and it uses the seismic waves that are generated by marsquakes."
Housed in a vacuum bottle, the instrument is capable of measuring movements "on the scale of an atom," he said. "The resolution limit of our seismometer, sitting in the dirt on Mars, in the weather, is about one half the radius of a hydrogen atom. So it's a very, very exquisitely precise measurement."
As seismic waves from marsquakes travel through the martian interior, they will pass through different types of rock and soil that will alter their shape.
"Scientists understand how to take the shapes of those wiggles, their frequency, their amplitude, the polarization, the timing, all kinds of properties of those waves and pull that information out and finally, after we've gotten many, many marsquakes from different directions, we can put together a three-dimensional view of the inside of Mars," Banerdt said.
The heat probe will use a spring-driven internal hammer-like device to pound its way down into the martian soil trailing a cable studded with sensitive temperature sensors. After some 10,000 hammer blows, the probe should reach a maximum depth of about 15 feet, pausing along the way to measure the thermal conductivity of the soil at different depths.
Once at its maximum depth, the sensors will measure the slight differences in temperature between the surface and the end of the cable, allowing scientists to extrapolate temperatures all the way to the core 2,000 miles below.
"And that amount of heat is tied to the geological activity of the planet," Banerdt said. "It's the heat engine of the planet that drives vulcanism, it drives tectonic activity, it drives mountain building. So all the geological processes that happen on a planet are driven by its heat engine, and we want to measure sort of the vigor of that heat engine."
InSight's third investigation does not require a separate instrument. By carefully analyzing slight changes in the radio signals from the lander as Mars rotates on its axis, scientists can determine the spacecraft's position to within a few inches, precisely locate the martian polar axis and measure how it slowly changes orientation like the motion of a spinning top.
"Over the course of a year, we can watch that north pole wobble just a little bit because of the core sloshing around inside the planet, and that will give us a very, very tight constraint on the size of that core, its density and so its composition," Banerdt said.
"That tells us the structure of Mars, the structure of Mars tells us something about the processes that put that structure together, we can put that into our models, extrapolate to the Earth and understand how the Earth formed four and a half billion years ago. And that's really the crux of the science behind this mission."