NASA's MAVEN orbiter is the first spacecraft at Mars specifically designed to study the planet's upper atmosphere. Its goal is to determine how Mars lost its thick, early atmosphere to space, and in turn, its once hospitable climate.
MAVEN's elliptical orbit normally takes it from 6,200 km above Mars at its furthest point to about 150 km at its closest point, just above the upper atmosphere. However, during MAVEN's periodic "deep dip" campaigns, the spacecraft's orbit is lowered to only 125 km at closest approach, dipping into the upper atmosphere to study it in situ.
This animation depicts MAVEN maneuvering from its normal science orbit to the deep dip orbit and back. The animation is available for download in broadcast-quality HD and can be looped.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 126.96.36.199.0