Three days before arriving, the lander, named Schiaparelli, after the 19th-century Italian astronomer Giovanni Schiaparelli, will separate from the orbiter. It will enter the atmosphere at 13,000 miles per hour and will quickly decelerate on its way to settling down on the surface.

The main objective of Schiaparelli is to demonstrate its landing system. (The European Space Agency’s last attempt to land on Mars — the Beagle 2 spacecraft, which accompanied the Mars Express orbiter in 2003 — failed.)

Schiaparelli carries instruments to measure Mars’s atmosphere during the height of the dust storm season. Its batteries are expected to last only two to four days.

The Trace Gas Orbiter is expected to operate much longer, until at least 2022, circling Mars at an altitude of 250 miles. Its instruments will measure gases, like methane, water vapor and nitrogen, that exist in minute quantities but that could hold important clues about the possibility of life on Mars.

Methane is the most intriguing trace gas. Sunlight and chemical reactions break up methane molecules in the atmosphere. Any methane there must have been created recently, and the two possibilities for creating methane are microbes and a geological process requiring heat and liquid water.