After six months of flight, the lander component of the probe will detach itself from the cruise stage and head into the atmosphere. The lander component initially looks a fair bit like the re-entry capsule used in the 1960s and 1970s for the Apollo moon missions -- sort of conical, with a smooth and flat bottom. That bottom is a crucial heat shield that is designed to protect the probe as it passes through the thin Martian atmosphere.
The landing is a devilishly difficult feat. The landing capsule has to batter its way through the atmosphere. It will fly through the Martian air at an initial speed of 12,300 mph, and it must hit the atmosphere at an angle of precisely 12 degrees. Any shallower, and the probe will bounce off into deep space. Any steeper, and the probe will burn itself up in a spectacular and fiery death. The probe will first touch the atmosphere six minutes and 45 seconds before landing. During this phase, it will experience acceleration 12 times that of the Earth's gravity. Were the probe a 150-pound human, during the flaming descent, it would weigh nearly a ton.
About 3½ minutes after the probe hits the atmosphere, a parachute will deploy, slowing down the probe even more. Fifteen seconds later, explosives will blow the heat shield off, exposing the actual InSight probe hidden inside. Ten seconds after the heat shield falls away, the probe will extend its legs, much like an airplane extends its wheels before touching down.
The probe will fall for an additional two minutes attached to the parachute and protected by its conical shell. About 45 seconds before InSight lands, it will drop out of the shell and fall toward the surface. As soon as it leaves the shell, its landing rockets will ignite.
The actual InSight probe looks a little bit like the Apollo moon lander, with three legs to support it and a boxy top. The rockets will slow it further and stop any remaining horizontal motion. Then, about 15 seconds before touchdown, the InSight probe will descend at a speed of 8 feet per second, before hopefully touching down gently on the Martian surface.
The entire landing sequence will take about seven minutes to occur. A radio signal from Mars to Earth currently takes about eight minutes and seven seconds to get here. So the complete landing process will take place before we find out if it was successful. It will be done automatically, entirely by the probe itself. For the scientists and engineers who designed InSight, this is called "seven minutes of terror."
And they are right to be worried. Mars is a graveyard of failed probes. There have been 44 attempts by various national space agencies to land on Mars. Eighteen have been successful. Twenty-three have not. Three have achieved orbit but failed at a landing.
So, what does InSight hope to achieve? Well, as it happens, a lot. But it's different than the intrepid Curiosity probe, which NASA landed in 2012. InSight will not move around. Instead, it will stay put and tell us of the interior of Mars.
One thing it will do is emit radio waves that we can monitor on Earth. By making careful measurements of how the frequency of the radio waves change, we will be able to measure the degree to which Mars wobbles as it rotates. That will tell us something about the core of the planet, specifically its makeup and information on the degree to which it is molten.
InSight will also deploy a seismometer to listen for marsquakes (like earthquakes, but Martian-style) and for impacts of meteors on the planet. Information gleaned from the waves the seismometer detects will tell us more about the planet's interior.
The third thing InSight will do is to dig below the planet's surface. Using a jackhammer, the probe will drill down 5 meters (16 feet) into the planet and, basically, it will take the planet's temperature.
There are many reasons this is interesting. Taking the temperature at that depth will allow planetary scientists to determine how much heat is escaping from Mars. More broadly, this measurement will allow a clear determination of the temperature of the planet much closer to the core.
This information will tell us a lot about how Mars formed, which, in turn, will add to the information of how rocky planets, including our own Earth, typically develop.
And if you're more of an explorer kind of person and not so interested in Martian geology, it will also tell us how warm the planet is at modest depths, which will tell us if there is any chance of liquid water on the planet. Perhaps obviously, if the Martian subsurface is warm enough, any buried water will be in liquid form and not ice. Finding liquid water would be the key discovery that would make Martian exploration relatively easy. A relatively recent possible discovery of aburied Martian lake was promising, but the data was not conclusive. Knowing that the ground is warm would be very comforting to possible future explorers.
Exploring the solar system is the first step toward exploring the stars. The InSight probe will give us --well -- insight into whether this is something that humanity will achieve in the foreseeable future.
And maybe Elon Musk's bet on him getting to Mars will become true.