Altimetry is a term which describes a method of measuring the altitudes of things with spacecraft. It is usually performed with a laser. Lasers bounce beams of light from the surface and back to the spacecraft to measure the heights of mountains and depths of valleys on distant planets. This is even done for our own planet. The figure shown here shows altimeter results from the Mars Global Surveyor spacecraft. The red line shows the relative heights of the ground underneath the spacecraft.