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.
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.
