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Let’s go to Mars! It sounds exciting, and it is. But where exactly are we going to go? After all, Mars is a planet. Although it’s smaller than Earth, it’s huge. And, it’s fantastically far away. So, landing a spacecraft there that’s barely the size of a teacher’s desk is no small thing. But landing it in a safe place that’s still an interesting place makes the missions even more complicated. Let’s look at how we choose landing sites.

They say the most important thing about the place you live is where your house or apartment is. Location, location, location is what people who buy and sell houses on Earth say. It’s the most important thing. On Mars, it’s the same, but there are no close-up, glossy pictures to see. There are no previous owners to interview. And, you sure can’t go to the landing site and look it over before you move in or send a spacecraft to land there. But, we do have lots of facts about places on Mars, lots of data. We have camera images, maps, and all kind of information about the rocks, soil, winds, and dust.

There is not an ideal or perfect place to land. Instead, the whole business of picking a landing site is a compromise. Engineers and scientists argue and argue (discuss) about the best places to pick. We want to learn as much as possible, but we don’t want to crash land or have the rover end up on a landscape that’s so rough or steep that we can’t drive it anywhere for a look around. We call this part of “mission safety.”

We’re going to use a parachute to help slow down the spacecraft. It is specially designed to work in the very thin Martian atmosphere. The parachute actually travels at supersonic speeds, so it needs as much drag as possible to slow the spacecraft's descent. The closer we get to the ground, the thicker the Martian air. So, we might want to choose a low lying area. But not all low places are safe. There are boulders everywhere, and in some places, wind howls along the surface. So we “disqualify” areas that we think have high winds.

Video Screenshot 4 We look for sites that are smooth enough for the air bags to bounce and for the petals of the spacecraft’s flower-like body to open. And of course, the rover has to be able to rove without getting stuck. Engineers look where the rocks are few and the terrain is flat. It has to be a big area too. We’re sending these things across a few millions of kilometers of space at enormous speeds. So, we need some room for error, some room to miss a little bit.

Try this: shine a flashlight straight down on a desk or the floor. Now, tilt it. The light goes from a circle to an oval shape. Mathematically, it’s an ellipse. We call it the “error ellipse.” The size of an error ellipse varies with each landing site, but it's approximately 18 kilometers (11 miles) wide and 113 kilometers (70 miles) long. That’s the shape of the target bulls-eye we’re shooting for with our Mars Exploration Rover missions. How flat or even are spaces like that here on Earth? Well, Mars is a rocky old planet. It’s a tough challenge.

To make sure we get as much Sun as possible, we shoot for the equator of Mars. Just as tropical places are warm and sunny here on Earth, they’re sunny there on Mars, maybe not warm, but sunny. Sunlight drives our recharger systems and keeps the instruments working smoothly. So, we’ll land near the Martian equator.

To study Mars, scientists want to study rocks. They need to take the rover to interesting places, where it appears liquid water once flowed. Since we’re sending two rovers, we want two sites that are different enough to test theories. One site might help us see evidence of standing water from long ago. Another site might reveal that water once boiled up from below the surface of Mars. This is called hydrothermal activity. Scientists have found that hydrothermal systems support life here on Earth.

Scientists have already spent years in meetings and reviews. They’ve made countless observations, done all kinds of analysis, and thought about this long and hard. More information keeps coming in from the Mars Global Surveyor and from the Mars Odyssey. We started out with 185 sites. That got reduced to 26, then to four, with two backups, then the “final four”. Soon, just two sites will be chosen. The Mars team will be looking for safety, long exploration time, and science. We want to learn more about the conditions that existed on Mars, when water pooled in giant lakes and flowed through gullies, channels, and canyons on this distant world.

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