Curiosity Lands on Mars: Know What You're Watching When You're Watching '7 Minutes of Terror'

A cheat-sheet guide to the dramatic descent of the Little Rover That Could

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UPDATE August 6, 1:42am EDT: The Mars Curiosity rover has safely landed on the surface of Mars! See the scene at mission control when they got word. Spoiler alert! They were psyched.

Later today, at 10:30 p.m. West Coast time, NASA's Curiosity rover is scheduled to touch down on the surface of Mars. If it's successful, the landing will represent not just a step forward for Martian exploration; it will also represent progress for interplanetary navigation. 

But ... if it's successful. People are calling Curiosity's impending landing "seven minutes of terror" for a reason: Though there are many reasons to hope that the landing will go smoothly -- more than 2.5 billion reasons, in fact -- there is also much potential for the touch-down to go spectacularly wrong.

Here's a guide to Curiosity's trip as it's gone so far and as it's expected (and hoped) to conclude.

What is Curiosity, exactly?

The rover is basically a really expensive, and really advanced, robotic photographer. Curiosity -- full name: The Mars Science Laboratory -- is, as our Ross Andersen has put it, "a dune buggy equipped with a set of tools and instruments to shame Inspector Gadget." It carries ten instruments in total, among them two rectangular "eyes" -- the first a primary imaging camera featuring different filters and focal lengths, and the second a large, circular camera that can fire a laser that turns rock into vapor. (Another camera on the rover picks up the images from the laser-firer and interprets their composition.)

So "this rover can go around firing laser beams at rocks and other materials to find out what they're made of; I'd say that's one of its most impressive instruments," planetary scientist Michael Mischna told Andersen. And the rover can also gauge Martian weather. It can film in HD. And in 360-degree panoramas.

What does it look like? 

Curiosity, our latest emissary to the Red Planet, is slightly larger than a standard automobile: It measures ten feet long by seven feet tall at the top of its mast. That makes it substantially larger than NASA's previous Mars explorers, which have ranged in size from slightly-larger-than-a-Roomba to slightly-smaller-than-a-human. And Curiosity really does look like a dune buggy. Here's one picture: 

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So, all things considered, Curiosity is actually pretty small! How has that tiny little thing gotten to Mars? 

Curiosity has been hitched to an interplanetary spacecraft, tucked inside a protective aeroshell for the duration of the voyage to Mars. It's been traveling, ensconced in that way, for eight-and-a-half months -- and for 345 million miles.

Here it is, being launched into space by the Atlas V rocket, on November 26, 2011

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So now that it's approaching Mars, how will Curiosity descend to the Martian surface? 

This is the nail-biting part. Curiosity's approach phase will begin 45 minutes before the spacecraft penetrates the Martian atmosphere. To help with navigation, Curiosity will enter that atmosphere 2,188 miles above the center of the planet. It will then barrel into Mars's atmosphere at 13,200 miles an hour, finally making entry "in a burst of fire."

The aeroshell that has protected Curiosity in its voyage to Mars -- at 14.8 feet across, the largest ever used for a Martian mission -- will at that point act as a heat shield as the craft makes that entry. 

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Then begins the descent and landing (EDL) phase -- the true test of Curiosity's mettle. That phase begins when the craft reaches the atmosphere of Mars, roughly 81 miles above its landing site, and ends when the rover -- fingers crossed! -- touches down on Mars intact. This is the "seven minutes of terror" everyone's been talking about: During those tense minutes, the spacecraft carrying Curiosity -- powered by rockets -- will decelerate from its starting velocity (that mind-boggling 13,200 miles an hour at the top of the atmosphere) to a stationary state. The margin of error here is zero: If the craft doesn't decelerate enough, it will hurl into the Martian surface; if it decelerates too much, it'll lose momentum and crash.

And how, actually, will it land? 

Right. As the spacecraft is descending, its heat shield will be jettisoned and its backshell will drop away, allowing the craft's built-in radar system to determine its altitude and velocity. That system will leverage the friction of the Martian atmosphere to slow the spacecraft and trigger the craft's robotic steering.

And "that's one of the unique things about this mission -- the capsule isn't just a ballistic object plummeting through space," Mischna told Andersen. "It actually moves in response to precise conditions in the atmosphere."

To help ensure a gentle landing, The Mars Science Laboratory is making use of the largest parachute ever built for a planetary mission. The device, more than 165 feet in length and more nearly 51 feet across, has 80 suspension lines to keep it secured to its craft. And it's designed to survive the Mach 2.2 punishment it'll take as it hurtles through the Martian atmosphere.

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Part of NASA's challenge here was figuring out how to deal with the (relatively!) large size of Curiosity. "The gravity on Mars is about one-third of the gravity we have here on Earth, which means you don't need as much rocket power to make the spacecraft hover -- but it's still a lot of power," Mischna said. "To get something of this size to the Martian surface, you have to come up with a whole new landing system because the air bags, which we used on previous rovers, just aren't going to work. It would be like dropping a piano wrapped in bubble wrap and expecting it to land without damage. We had to completely rethink our approach to landing, but that's a good thing, because we hope this will be the first of many projects like it, eventually leading to human exploration on Mars. And if you're going to send humans to Mars, you're going to need to bring a lot of heavy equipment -- places to live, food, water, etc."

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Megan Garber is a staff writer at The Atlantic.

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