Flying the freeway to Mars, the robotic probe InSight nears the end of its 301-million-mile cruise with nary a hitch and hardly a hiccup.
But looming just ahead is the exit ramp—the Martian atmosphere.
“There’s a classic term for it,” says Rob Grover of the Jet Propulsion Laboratory in Pasadena, California. “The seven minutes of terror.”
That’s approximately the time InSight takes to land, a spooky 70-mile descent from the top of the atmosphere down to the ground. “There is very little room for things to go wrong,” says Grover.
Yet hundreds of things must go right, all without NASA’s backseat driving; during landing, there’s no joysticking.
“We can’t fly the vehicle in ourselves. The flight computer on board has to do it on its own. Everything has to work perfectly by itself.” And for those seven minutes, “our hearts will be pounding.”
InSight lands November 26, the Monday after Thanksgiving, at 11:47 AM Pacific Time (2:47 PM Eastern).
Before the clock starts, the cruise stage—its delivery done—detaches from the capsule containing the lander. Then the capsule—just before reaching the atmosphere—points itself, “tilting down 12 degrees,” says Grover.
NASA’s leeway is minuscule, only “plus or minus a quarter of a degree.” Too shallow an angle, and the spacecraft skips off the atmosphere. Too steep, and it burns up.
And now the terrifying part. InSight thunders in at 12,300 miles per hour—almost three-and-a-half miles per second. Friction roasts it. The temperature on the heat shield hits 2,700 degrees Fahrenheit.
Friction also brakes it; within two minutes, the speed of the spacecraft slows by more than 90 percent. Yet it’s still going 1,000 miles per hour.
At seven miles up—commercial airliners fly about that high—the parachute opens. Within 15 seconds, the heat shield jettisons. For the first time, the lander is exposed to Martian air.
Another 10 seconds, and the three legs deploy. One mile above the ground, the lander falls from the backshell. Descent engines turn on. Touchdown velocity is 5 miles per hour.
Much could happen. The parachute might not open properly. The falling heat shield could graze the lander. Descent engines may not shut off. A large surface rock could sit in the way. One of the legs might not release and lock.
Those scenarios, though unlikely, are not implausible. Any of them could cause an erratic landing. Right now, atmospheric dust is minimal; weather at the landing site appears normal.
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