[silence] [silence] [music] Narrator: If you wanted to find out how tall mountains are on other planets, how would you do it? If you're on Earth, it's easy. You can take a picture, fly over the mountain, fly over the mountain, or you can actually go there and measure how high it is. On other planets, it's much more difficult. You might be able to estimate height using shadows, or even take 3D pictures from a satellite. But what if you wanted to know what the mountain looked like as a 3D model? To find out, NASA scientists can use a precise measuring tool called LIDAR. Mounted on a satellite orbiting high above a planet, LIDAR instruments are able to accurately measure the distance between the instrument and the landscape below using laser pulses. To make these measurements, the LIDAR instrument first sends a laser pulse down to the planet's surface. The pulse hits the ground and reflects back to the instrument, where an onboard computer measures the time it took the pulse to make its trip. That gives a precise measurement between the instrument and the ground--with respect to the planet's gravitational center. As the satellite passes over the landscape, the instrument sends out a series of regular pulses. By recording and combining these measurements, scientists can use the instrument to gradually build up a map of the height of the terrain. After many more measurements, the end result is a high-resolution 3D model that scientists can view as if they were actually on the planet, flying over the terrain. They can then study its shape in more detail, looking for clues to the relative ages of craters, the shape of valleys and landscape features, and much more. But LIDAR is far more versatile than simply measuring the shapes of mountains and craters. Earth scientists, for example, use LIDAR to measure the height and density of the Earth's forests. Others use LIDAR to study small changes in the heights of the Earth's major icecaps over time. Still other scientists use LIDAR to study the composition and structure of Earth's atmosphere, as well as the atmosphere of other planets. And they can do all that without ever having to climb a mountain. [rumbling] [rumbling, beeping] [beeping]