MAVEN is the first spacecraft at Mars specifically designed to study the planet’s upper atmosphere. Its goal is to determine how Mars lost its water and its thick, early atmosphere to space.
MAVEN’s elliptical orbit normally takes it from 6,200 km (3,852.5 mi) above Mars at its farthest point (apoapsis) to about 150 km (93.2 mi) at its closest point (periapsis). However, during MAVEN’s periodic “deep dip” campaigns, the spacecraft’s orbit is lowered to only 125 km (77.7 mi) at closest approach, dipping into the well-mixed portion of Mars’ upper atmosphere to study it in situ. [More at link, including video]