The hardest part of getting to Mars is sticking the landing. On the 26th of November, 2018, NASA’s InSight spacecraft will plunge into the thin Martian atmosphere at over 14,000 miles an hour. An alien fireball over the red plains. InSIght will pop a supersonic parachute, drop its heat shield and prepare to touch down. In the past, Martian rovers have bounced and rolled to a stop inside huge airbags, or floated to the surface hanging from an elaborate sky crane. But InSIght will touch down on rockets, “Just the way God intended,” in the words of the mission’s lead scientist. If all goes well, and it arrives in one piece, the lander will unfurl its solar panels and get to work. InSIght won’t send back any dramatic images. Its mission is to look down, into the geological heart of the planet. A seismometer will listen for vibrations and rumbles, marsquakes and meteorite impacts. This detector is so sensitive it can sense movements smaller than the width of an atom, or feel the faint tidal groundswell from the gravity of the tiny moon Phobos. A second device with a pointed spike will hammer itself 15 feet into the ground, to measure heat flowing up from the planet’s core. Mars today is a craggy, frigid desert, but once it was wet and warm. What InSight learns could give us hints about the structure of other rocky planets: scarred Mercury, clouded Venus, and our own dynamic Earth, which is too large and too active to have kept the traces of its early formation. And perhaps it will even tell us about the billions of rocky worlds orbiting other stars. Over the past two decades, three generations of rovers have toured Mars. They have scraped and poked, scooped and sampled, dusted and drilled into Mars’s surface. InSight will stay in one place, but dig deeper and listen longer. It may provide clues to why Mars was once habitable, why and how it changed, and what that might mean for Earth. For our old sibling Mars, Earth represents the road not taken. The road to life, blue water and green mountains. Home to a species now curious enough to send machines to investigate and interrogate nature’s choices. To understand how we got lucky with a garden world.